From Wikipedia, the free encyclopedia
Some examples of behavioral ecology
An African elephant crossing a river
Behavioral ecology is the study of the
evolutionary basis for
animal behavior due to
ecological pressures. Behavioral ecology emerged from
ethology after
Niko Tinbergen outlined
four questions to address when studying animal behaviors that are the
proximate causes,
ontogeny,
survival value, and
phylogeny of behavior.
If an organism has a trait that provides a selective advantage (i.e., has adaptive significance) in its environment, then
natural selection
favors it. Adaptive significance refers to the expression of a trait
that affects fitness, measured by an individual's reproductive success.
Adaptive traits are those that produce more copies of the individual's
genes in future generations. Maladaptive traits are those that leave
fewer. For example, if a bird that can call more loudly attracts more
mates, then a loud call is an adaptive trait for that species because a
louder bird mates more frequently than less loud birds—thus sending more
loud-calling genes into future generations.
Individuals are always in
competition
with others for limited resources, including food, territories, and
mates. Conflict occurs between predators and prey, between rivals for
mates, between siblings, mates, and even between parents and offspring.
Competing for resources
The
value of a social behavior depends in part on the social behavior of an
animal's neighbors. For example, the more likely a rival male is to
back down from a threat, the more value a male gets out of making the
threat. The more likely, however, that a rival will attack if
threatened, the less useful it is to threaten other males. When a
population exhibits a number of interacting social behaviors such as
this, it can evolve a stable pattern of behaviors known as an
evolutionarily stable strategy (or ESS). This term, derived from economic
game theory, became prominent after
John Maynard Smith (1982) recognized the possible application of the concept of a
Nash equilibrium to model the evolution of behavioral strategies.
Evolutionarily stable strategy
In short,
evolutionary game theory asserts that only
strategies
that, when common in the population, cannot be "invaded" by any
alternative (mutant) strategy is an ESS, and thus maintained in the
population. In other words, at equilibrium every player should play the
best strategic response to each other. When the game is two player and
symmetric, each player should play the strategy that provides the
response best for it.
Therefore, the ESS is considered the evolutionary end point
subsequent to the interactions. As the fitness conveyed by a strategy is
influenced by what other individuals are doing (the relative frequency
of each strategy in the population), behavior can be governed not only
by optimality but the frequencies of strategies adopted by others and
are therefore frequency dependent (
frequency dependence).
Behavioral evolution is therefore influenced by both the physical environment and interactions between other individuals.
An example of how changes in geography can make a strategy
susceptible to alternative strategies is the parasitization of the
African honey bee,
A. m. scutellata.
Resource defense
The
term economic defendability was first introduced by Jerram Brown in
1964. Economic defendability states that defense of a resource have
costs, such as energy expenditure or risk of injury, as well as benefits
of priority access to the resource.
Territorial behavior arises when benefits are greater than the costs.
Studies of the
golden-winged sunbird
have validated the concept of economic defendability. Comparing the
energetic costs a sunbird expends in a day to the extra nectar gained by
defending a territory, researchers showed that birds only became
territorial when they were making a net energetic profit.
When resources are at low density, the gains from excluding others may
not be sufficient to pay for the cost of territorial defense. In
contrast, when resource availability is high, there may be so many
intruders that the defender would have no time to make use of the
resources made available by defense.
Sometimes the economics of resource competition favors shared defense. An example is the feeding territories of the
white wagtail.
The white wagtails feed on insects washed up by the river onto the
bank, which acts as a renewing food supply. If any intruders harvested
their territory then the prey would quickly become depleted, but
sometimes territory owners tolerate a second bird, known as a satellite.
The two sharers would then move out of phase with one another,
resulting in decreased feeding rate but also increased defense,
illustrating advantages of group living.
Ideal free distribution
One of the major models used to predict the distribution of competing
individuals among resource patches is the ideal free distribution
model. Within this model, resource patches can be of variable quality,
and there is no limit to the number of individuals that can occupy and
extract resources from a particular patch. Competition within a
particular patch means that the benefit each individual receives from
exploiting a patch decreases logarithmically with increasing number of
competitors sharing that resource patch. The model predicts that
individuals will initially flock to higher-quality patches until the
costs of crowding bring the benefits of exploiting them in line with the
benefits of being the only individual on the lesser-quality resource
patch. After this point has been reached, individuals will alternate
between exploiting the higher-quality patches and the lower-quality
patches in such a way that the average benefit for all individuals in
both patches is the same. This model is ideal in that individuals
have complete information about the quality of a resource patch and the
number of individuals currently exploiting it, and free in that individuals are freely able to choose which resource patch to exploit.
An experiment by Manfred Malinski in 1979 demonstrated that feeding behavior in
three-spined sticklebacks
follows an ideal free distribution. Six fish were placed in a tank, and
food items were dropped into opposite ends of the tank at different
rates. The rate of food deposition at one end was set at twice that of
the other end, and the fish distributed themselves with four individuals
at the faster-depositing end and two individuals at the
slower-depositing end. In this way, the average feeding rate was the
same for all of the fish in the tank.
Mating strategies and tactics
As with any
competition
of resources, species across the animal kingdom may also engage in
competitions for mating. If one considers mates or potentials mates as a
resource, these sexual partners can be randomly distributed among
resource pools within a given environment. Following the ideal free
distribution model, suitors distribute themselves among the potential
mates in an effort to maximize their chances or the number of potential
matings. For all competitors, males of a species in most cases, there
are variations in both the strategies and tactics used to obtain
matings. Strategies generally refer to the genetically determined
behaviors that can be described as
conditional.
Tactics refer to the subset of behaviors within a given genetic
strategy. Thus it is not difficult for a great many variations in
mating strategies to exist in a given environment or species.
An experiment conducted by Anthony Arak, where playback of synthetic calls from male
natterjack toads
was used to manipulate behavior of the males in a chorus, the
difference between strategies and tactics is clear. While small and
immature, male natterjack toads adopted a satellite tactic to parasitize
larger males. Though large males on average still retained greater
reproductive success, smaller males were able to intercept matings. When
the large males of the chorus were removed, smaller males adopted a
calling behavior, no longer competing against the loud calls of larger
males. When smaller males got larger and their calls more competitive,
then they started calling and competing directly for mates.
Sexual selection
Mate choice by resources
In many sexually reproducing species, such as
mammals,
birds, and
amphibians,
females are able to bear offspring for a certain time period, during
which the males are free to mate with other available females, and
therefore can father many more offspring to pass on their genes. The
fundamental difference between male and female reproduction mechanisms
determines the different strategies each sex employs to maximize their
reproductive success.
For males, their reproductive success is limited by access to females,
while females are limited by their access to resources. In this sense,
females can be much choosier than males because they have to bet on the
resources provided by the males to ensure reproductive success.
Resources usually include nest sites, food and protection. In some cases, the males provide all of them (e.g.
sedge warblers).
The females dwell in their chosen males’ territories for access to
these resources. The males gain ownership to the territories through
male-male competition
that often involves physical aggression. Only the largest and strongest
males manage to defend the best quality nest sites. Females choose
males by inspecting the quality of different territories or by looking
at some male traits that can indicate the quality of resources. One example of this is with the grayling butterfly (
Hipparchia semele),
where males engage in complex flight patterns to decide who defends a
particular territory. The female grayling butterfly chooses a male based
on the most optimal location for
oviposition. Sometimes, males leave after mating. The only resource that a male provides is a
nuptial gift, such as protection or food.
The female can evaluate the quality of the protection or food provided
by the male so as to decide whether to mate or not or how long she is
willing to copulate.
Mate choice by genes
When
males' only contribution to offspring is their sperm, females are
particularly choosy. With this high level of female choice, sexual
ornaments are seen in males, where the ornaments reflect the male's
social status. Two hypotheses have been proposed to conceptualize the
genetic benefits from female mate choice.
First, the good genes hypothesis suggests that female choice is
for higher genetic quality and that this preference is favored because
it increases fitness of the offspring. This includes
Zahavi's handicap hypothesis and
Hamilton and Zuk's host and parasite arms race.
Zahavi's handicap hypothesis was proposed within the context of looking
at elaborate male sexual displays. He suggested that females favor
ornamented traits because they are handicaps and are indicators of the
male's genetic quality. Since these ornamented traits are hazards, the
male's survival must be indicative of his high genetic quality in other
areas. In this way, the degree that a male expresses his sexual display
indicates to the female his genetic quality.
Zuk and Hamilton proposed a hypothesis after observing disease as a
powerful selective pressure on a rabbit population. They suggested that
sexual displays were indicators of resistance of disease on a genetic
level.
Such 'choosiness' from the female individuals can be seen in wasp species too, especially among
Polistes dominula
wasps. The females tend to prefer males with smaller, more elliptically
shaped spots than those with larger and more irregularly shaped spots.
Those males would have reproductive superiority over males with
irregular spots.
Fisher's hypothesis of runaway sexual selection suggests that
female preference is genetically correlated with male traits and that
the preference co-evolves with the evolution of that trait, thus the
preference is under indirect selection.
Fisher suggests that female preference began because the trait
indicated the male’s quality. The female preference spread, so that the
females’ offspring now benefited from the higher quality from specific
trait but also greater attractiveness to mates. Eventually, the trait
only represents attractiveness to mates, and no longer represents
increased survival.
An example of mate choice by genes is seen in the cichlid fish
Tropheus moorii where males provide no parental care. An experiment found that a female
T. moorii is more likely to choose a mate with the same color morph as her own.
In another experiment, females have been shown to share preferences for
the same males when given two to choose from, meaning some males get to
reproduce more often than others.
Sensory bias
The
sensory bias hypothesis states that the preference for a trait evolves
in a non-mating context, and is then exploited by one sex to obtain more
mating opportunities. The competitive sex evolves traits that exploit a
pre-existing bias that the choosy sex already possesses. This mechanism
is thought to explain remarkable trait differences in closely related
species because it produces a divergence in signaling systems, which
leads to reproductive isolation.
Sensory bias has been demonstrated in
guppies,
freshwater fish from
Trinidad and Tobago.
In this mating system, female guppies prefer to mate with males with
more orange body coloration. However, outside of a mating context, both
sexes prefer animate orange objects, which suggests that preference
originally evolved in another context, like foraging.
Orange fruits are a rare treat that fall into streams where the guppies
live. The ability to find these fruits quickly is an adaptive quality
that has evolved outside of a mating context. Sometime after the
affinity for orange objects arose, male guppies exploited this
preference by incorporating large orange spots to attract females.
Another example of sensory exploitation is in the water mite
Neumania papillator, an
ambush predator that hunts
copepods (small crustaceans) passing by in the water column. When hunting,
N. papillator
adopts a characteristic stance termed the 'net stance' - their first
four legs are held out into the water column, with their four hind legs
resting on aquatic vegetation; this allows them to detect vibrational
stimuli produced by swimming prey and use this to orient towards and
clutch at prey. During courtship, males actively search for females - if a male finds a female, he slowly circles around the female whilst trembling his first and second leg near her. Male leg trembling causes females (who were in the 'net stance') to orient towards often clutch the male. This did not damage the male or deter further courtship; the male then deposited
spermatophores
and began to vigorously fan and jerk his fourth pair of legs over the
spermatophore, generating a current of water that passed over the
spermatophores and towards the female. Sperm packet uptake by the female would sometimes follow.
Heather Proctor hypothesized that the vibrations trembling male legs
made were done to mimic the vibrations that females detect from swimming
prey - this would trigger the female prey-detection responses causing
females to orient and then clutch at males, mediating courtship.
If this was true and males were exploiting female predation responses,
then hungry females should be more receptive to male trembling – Proctor
found that unfed captive females did orient and clutch at males
significantly more than fed captive females did, consistent with the
sensory exploitation hypothesis.
Other examples for the sensory bias mechanism include traits in
auklets,
wolf spiders, and
manakins. Further experimental work is required to reach a fuller understanding of the prevalence and mechanisms of sensory bias.
Sexual conflict
Sexual conflict, in some form or another, may very well be inherent in the ways most animals reproduce.
Females invest more in offspring prior to mating, due to the
differences in gametes in species that exhibit anisogamy, and often
invest more in offspring after mating.
This unequal investment leads, on one hand, to intense competition
between males for mates and, on the other hand, to females choosing
among males for better access to resources and good genes. Because of
differences in mating goals, males and females may have very different
preferred outcomes to mating.
Sexual conflict occurs whenever the preferred outcome of mating
is different for the male and female. This difference, in theory,
should lead to each sex evolving adaptations that bias the outcome of
reproduction towards its own interests. This sexual competition leads
to sexually antagonistic coevolution between males and females,
resulting in what has been described as an
evolutionary arms race between
males and
females.
Conflict over mating
Males’ reproductive successes are often limited by access to mates,
whereas females’ reproductive successes are more often limited by access
to resources. Thus, for a given sexual encounter, it benefits the male
to mate, but benefits the female to be choosy and resist. For example, male
small tortoiseshell butterfly compete to gain the best territory to mate. Another example of this conflict can be found in the Eastern carpenter bee,
Xylocopa virginica.
Males of this species are limited in reproduction primarily by access
to mates, so they claim a territory and wait for a female to pass
through. Big males are, therefore, more successful in mating because
they claim territories near the female nesting sites that are more
sought after. Smaller males, on the other hand, monopolize less
competitive sites in foraging areas so that they may mate with reduced
conflict.
Extreme manifestations of this conflict are seen throughout nature. For example, the male
Panorpa
scorpionflies attempt to force copulation. Male scorpionflies usually
acquire mates by presenting them with edible nuptial gifts in the forms
of salivary secretions or dead insects. However, some males attempt to
force copulation by grabbing females with a specialized abdominal organ
without offering a gift.
Forced copulation
is costly to the female as she does not receive the food from the male
and has to search for food herself (costing time and energy), while it
is beneficial for the male as he does not need to find a nuptial gift.
In other cases, however, it pays for the female to gain more
matings and her social mate to prevent these so as to guard paternity.
For example, in many socially monogamous birds, males follow females
closely during their fertile periods and attempt to chase away any other
males to prevent extra-pair matings. The female may attempt to sneak
off to achieve these extra matings. In species where males are
incapable of constant guarding, the social male may frequently copulate
with the female so as to swamp rival males’ sperm.
Sexual conflict after mating has also been shown to occur in both
males and females. Males employ a diverse array of tactics to increase
their success in
sperm competition.
These can include removing other male’s sperm from females, displacing
other male’s sperm by flushing out prior inseminations with large
amounts of their own sperm, creating copulatory plugs in females’
reproductive tracts to prevent future matings with other males, spraying
females with anti-aphrodisiacs to discourage other males from mating
with the female, and producing sterile parasperm to protect fertile
eusperm in the female’s reproductive tract. For example, the male spruce bud moth (
Zeiraphera canadensis)
secretes an accessory gland protein during mating that makes them
unattractive to other males and thus prevents females from future
copulation. The
Rocky Mountain parnassian
also exhibits this type of sexual conflict when the male butterflies
deposit a waxy genital plug onto the tip of the female's abdomen that
physically prevents the female from mating again.
Males can also prevent future mating by transferring an
anti-Aphrodiasic to the female during mating. This behavior is seen in
butterfly species such as
Heliconius mepomene,
where males transfer a compound that causes the female to smell like a
male butterfly and thus deter any future potential mates.
Furthermore, males may control the strategic allocation of sperm,
producing more sperm when females are more promiscuous. All these
methods are meant to ensure that females are more likely to produce
offspring belonging to the males who uses the method.
Females also control the outcomes of matings, and there exists
the possibility that females choose sperm (cryptic female choice). A dramatic example of this is the
feral fowl Gallus gallus.
In this species, females prefer to copulate with dominant males, but
subordinate males can force matings. In these cases, the female is able
to eject the subordinate male’s sperm using cloacal contractions.
Parental care and family conflicts
Parental care
is the investment a parent puts into their offspring—which includes
protecting and feeding the young, preparing burrows or nests, and
providing eggs with yolk.
There is great variation in parental care in the animal kingdom. In
some species, the parents may not care for their offspring at all, while
in others the parents exhibit single-parental or even bi-parental care.
As with other topics in behavioral ecology, interactions within a
family involve conflicts. These conflicts can be broken down into three
general types: sexual (male-female) conflict, parent-offspring conflict,
and sibling conflict.
Types of parental care
There are many different patterns of
parental care
in the animal kingdom. The patterns can be explained by physiological
constraints or ecological conditions, such as mating opportunities. In
invertebrates, there is no parental care in most species because it is
more favorable for parents to produce a large number of eggs whose fate
is left to chance than to protect a few individual young. For example,
female
L. figueresi die after stocking their larvae's cells with pollen and nectar and before their larvae hatch.
In birds, biparental care is the most common, because reproductive
success directly depends on the parents' ability to feed their chicks.
Two parents can feed twice as many young, so it is more favorable for
birds to have both parents delivering food. In mammals, female-only care
is the most common. This is most likely because females are internally
fertilized and so are holding the young inside for a prolonged period of
gestation, which provides males with the opportunity to desert. Females also feed the young through
lactation
after birth, so males are not required for feeding. Male parental care
is only observed in species where they contribute to feeding or carrying
of the young, such as in
marmosets. In fish there is no parental care in 79% of
bony fish. In fish with parental care, it usually limited to selecting, preparing, and defending a nest, as seen in
sockeye salmon, for example. Also, parental care in fish, if any, is primarily done by males, as seen in
gobies and
redlip blennies. The cichlid fish
V. moorii exhibits biparental care.
In species with internal fertilization, the female is usually the one
to take care of the young. In cases where fertilization is external the
male becomes the main caretaker.
Familial conflict
Familial conflict is a result of trade-offs as a function of lifetime
parental investment. Parental investment was defined by
Robert Trivers
in 1972 as “any investment by the parent in an individual offspring
that increases the offspring's chance of surviving at the cost of the
parent’s ability to invest in other offspring”.
Parental investment includes behaviors like guarding and feeding. Each
parent has a limited amount of parental investment over the course of
their lifetime. Investment trade-offs in offspring quality and quantity
within a brood and trade offs between current and future broods leads to
conflict over how much parental investment to provide and to whom
parents should invest in. There are three major types of familial
conflict: sexual, parent-offspring, and sibling-sibling conflict.
Sexual conflict
There is conflict among parents as to who should provide the care as
well as how much care to provide. Each parent must decide whether or not
to stay and care for their offspring, or to desert their offspring.
This decision is best modeled by
game theoretic approaches to
evolutionarily stable strategies
(ESS) where the best strategy for one parent depends on the strategy
adopted by the other parent. Recent research has found response matching
in parents who determine how much care to invest in their offspring.
Studies found that parent
great tits match their partner’s increased care-giving efforts with increased provisioning rates of their own. This cued parental response is a type of behavioral negotiation between parents that leads to stabilized compensation.
Parent-offspring conflict
Blackbird chicks in a nest
According to
Robert Trivers’s theory on relatedness,
each offspring is related to itself by 1, but is only 0.5 related to
their parents and siblings. Genetically, offspring are predisposed to
behave in their own self-interest while parents are predisposed to
behave equally to all their offspring, including both current and future
ones. Offspring selfishly try to take more than their fair shares of
parental investment,
while parents try to spread out their parental investment equally
amongst their present young and future young.
There are many examples of parent-offspring conflict in nature. One
manifestation of this is asynchronous hatching in birds. A behavioral
ecology hypothesis is known as Lack's brood reduction hypothesis.
Lack's hypothesis posits an evolutionary and ecological explanation as
to why birds lay a series of eggs with an asynchronous delay leading to
nestlings of mixed age and weights. According to Lack, this brood
behavior is an ecological insurance that allows the larger birds to
survive in poor years and all birds to survive when food is plentiful. We also see sex-ratio conflict between the queen and her workers in social
hymenoptera. Because of
haplodiploidy,
the workers (offspring) prefer a 3:1 female to male sex allocation
while the queen prefers a 1:1 sex ratio. Both the queen and the workers
try to bias the sex ratio in their favor. In some species, the workers gain control of the sex ratio, while in other species, like
B. terrestris, the queen has a considerable amount of control over the colony sex ratio. Lastly, there has been recent evidence regarding
genomic imprinting
that is a result of parent-offspring conflict. Paternal genes in
offspring demand more maternal resources than maternal genes in the same
offspring and vice versa. This has been show in imprinted genes like
insulin-like growth factor-II.
Parent-offspring conflict resolution
Parents
need an honest signal from their offspring that indicates their level
of hunger or need, so that the parents can distribute resources
accordingly. Offspring want more than their fair share of resources, so
they exaggerate their signals to wheedle more parental investment.
However, this conflict is countered by the cost of excessive begging.
Not only does excessive begging attract predators, but it also retards
chick growth if begging goes unrewarded. Thus, the cost of increased begging enforces offspring honesty.
Another resolution for parent-offspring conflict is that parental
provisioning and offspring demand have actually coevolved, so that
there is no obvious underlying conflict.
Cross-fostering experiments in
great tits (
Parus major) have shown that offspring beg more when their biological mothers are more generous. Therefore, it seems that the willingness to invest in offspring is co-adapted to offspring demand.
Sibling-sibling conflict
The lifetime
parental investment
is the fixed amount of parental resources available for all of a
parent's young, and an offspring wants as much of it as possible.
Siblings in a brood often compete for parental resources by trying to
gain more than their fair share of what their parents can offer. Nature
provides numerous examples in which sibling rivalry escalates to such an
extreme that one sibling tries to kill off broodmates to maximize
parental investment. In the
Galapagos fur seal,
the second pup of a female is usually born when the first pup is still
suckling. This competition for the mother’s milk is especially fierce
during periods of food shortage such as an
El Niño year, and this usually results in the older pup directly attacking and killing the younger one.
In some bird species, sibling rivalry is also abetted by the
asynchronous hatching of eggs. In the
blue-footed booby,
for example, the first egg in a nest is hatched four days before the
second one, resulting in the elder chick having a four-day head start in
growth. When the elder chick falls 20-25% below its expected weight
threshold, it attacks its younger sibling and drives it from the nest.
Sibling relatedness in a brood also influences the level of sibling-sibling conflict. In a study on
passerine birds, it was found that chicks begged more loudly in species with higher levels of
extra-pair paternity.
Brood parasitism
Some animals
deceive other species into providing all parental care. These brood parasites selfishly exploit their hosts' parents and host offspring. The
common cuckoo
is a well known example of a brood parasite. Female cuckoos lay a
single egg in the nest of the host species and when the cuckoo chick
hatches, it ejects all the host eggs and young. Other examples of brood
parasites include
honeyguides,
cowbirds, and the
large blue butterfly.
Brood parasite offspring have many strategies to induce their host
parents to invest parental care. Studies show that the common cuckoo
uses vocal mimicry to reproduce the sound of multiple hungry host young
to solicit more food.
Other cuckoos use visual deception with their wings to exaggerate the
begging display. False gapes from brood parasite offspring cause host
parents to collect more food. Another example of a brood parasite is
Phengaris butterflies such as
Phengaris rebeli and
Phengaris arion, which differ from the cuckoo in that the butterflies do not oviposit directly in the nest of the host, an ant species
Myrmica schencki.
Rather, the butterfly larvae release chemicals that deceive the ants
into believing that they are ant larvae, causing the ants to bring the
butterfly larvae back to their own nests to feed them. Other examples of brood parasites are
Polistes sulcifer, a paper wasp that has lost the ability to build its own nests so females lay their eggs in the nest of a host species,
Polistes dominula, and rely on the host workers to take care of their brood, as well as
Bombus bohemicus, a bumblebee that relies on host workers of various other
Bombus species. Similarly, in
Eulaema meriana, some Leucospidae wasps exploit the brood cells and nest for shelter and food from the bees.
Vespula austriaca is another wasp in which the females force the host workers to feed and take care of the brood. In particular,
Bombus hyperboreus, an Arctic bee species
, is also classified as a brood parasite in that it attacks and enslaves other species within their subgenus,
Alpinobombus to propagate their population.
Mating systems
Various types of mating systems include
monogamy,
polygyny,
polyandry,
promiscuity, and
polygamy.
Each is differentiated by the sexual behavior between mates, such as
which males mate with certain females. An influential paper by Stephen
Emlen and Lewis Oring (1977)
argued that two main factors of animal behavior influence the diversity
of mating systems: the relative accessibility that each sex has to
mates, and the parental desertion by either sex.
Mating systems with no male parental care
In a system that does not have male parental care,
resource Dispersion,
predation, and the effects of
social living
primarily influence female dispersion, which in turn influences male
dispersion. Since males' primary concern is female acquisition, the
males either indirectly or directly compete for the females. In direct
competition, the males are directly focused on the females.
Blue-headed wrasse demonstrate the behavior in which females follow resources—such as good
nest sites—and males follow the females.
Conversely, species with males that exemplify indirectly competitive
behavior tend towards the males’ anticipation of the resources desired
by females and their subsequent effort to control or acquire these
resources, which helps them to achieve success with females.
Grey-sided voles demonstrate indirect male competition for females. The
males were experimentally observed to home in on the sites with the
best food in anticipation of females settling in these areas. Males of
Euglossa imperialis,
a non-social bee species, also demonstrate indirect competitive
behavior by forming aggregations of territories, which can be considered
leks, to defend fragrant-rich primary territories. The purpose of these
aggregations is largely only facultative, since the more suitable
fragrant-rich sites there are, the more habitable territories there are
to inhabit, giving females of this species a large selection of males
with whom to potentially mate.
Leks
and choruses have also been deemed another behavior among the phenomena
of male competition for females. Due to the resource-poor nature of the
territories that lekking males often defend, it is difficult to
categorize them as indirect competitors. For example, the
Ghost moth
males display in leks to attract a female mate. Additionally, it is
difficult to classify them as direct competitors seeing as they put a
great deal of effort into their defense of their territories before
females arrive, and upon female arrival they put for the great mating
displays to attract the females to their individual sites. These
observations make it difficult to determine whether female or resource
dispersion primarily influences male aggregation, especially in lieu of
the apparent difficulty that males may have defending resources and
females in such densely populated areas.
Because the reason for male aggregation into leks is unclear, five
hypothesis have been proposed. These postulates propose the following as
reasons for male lekking: hotspot,
predation reduction, increased female attraction, hotshot males, facilitation of female choice. With all of the mating behaviors discussed, the primary factors influencing differences within and between species are
ecology, social conflicts, and life history differences.
In some other instances, neither direct nor indirect competition is seen. Instead, in species like the
Edith's checkerspot
butterfly, males' efforts are directed at acquisition of females and
they exhibit indiscriminate mate location behavior, where, given the low
cost of mistakes, they blindly attempt to mate both correctly with
females and incorrectly with other objects.
Mating systems with male parental care
Monogamy
Monogamy
is the mating system in 90% of birds, possibly because each male and
female has a greater number of offspring if they share in raising a
brood.
In obligate monogamy, males feed females on the nest, or share in
incubation and chick-feeding. In some species, males and females form
lifelong pair bonds. Monogamy may also arise from limited opportunities
for polygamy, due to strong competition among males for mates, females
suffering from loss of male help, and female-female aggression.
Polygyny
In
birds, polygyny occurs when males indirectly monopolize females by
controlling resources. In species where males normally do not contribute
much to parental care, females suffer relatively little or not at all.
In other species, however, females suffer through the loss of male
contribution, and the cost of having to share resources that the male
controls, such as nest sites or food. In some cases, a polygynous male
may control a high-quality territory so for the female, the benefits of
polygyny may outweigh the costs.
Polyandry threshold
There
also seems to be a “polyandry threshold” where males may do better by
agreeing to share a female instead of maintaining a monogamous mating
system.
Situations that may lead to cooperation among males include when food
is scarce, and when there is intense competition for territories or
females. For example, male
lions sometimes form coalitions to gain control of a pride of females. In some populations of
Galapagos hawks,
groups of males would cooperate to defend one breeding territory. The
males would share matings with the female and share paternity with the
offspring.
Female desertion and sex role reversal
In
birds, desertion often happens when food is abundant, so the remaining
partner is better able to raise the young unaided. Desertion also occurs
if there is a great chance of a parent to gain another mate, which
depends on environmental and population factors.
Some birds, such as the phalaropes, have reversed sex roles where the
female is larger and more brightly colored, and compete for males to
incubate their clutches. In jacanas, the female is larger than the male and her territory could overlap the multiple territories of up to four males.
Social behaviors
Animals
cooperate with each other to increase their own
fitness. These altruistic, and sometimes spiteful behaviors can be explained by
Hamilton's rule, which states that
rB-C > 0 where
r= relatedness,
B= benefits, and
C= costs.
Kin selection
Kin selection predicts that individuals will harbor personal
costs in favor of one or multiple individuals because this can maximize
their genetic contribution to future generations. For example, an
organism may be inclined to expend great time and energy in
parental investment to
rear offspring
since this future generation may be better suited for propagating genes
that are highly shared between the parent and offspring. Ultimately, the initial actor performs apparent
altruistic actions for kin to enhance its own reproductive
fitness. In particular, organisms are hypothesized to act in favor of kin depending on their genetic
relatedness.
So, individuals are inclined to act altruistically for siblings,
grandparents, cousins, and other relatives, but to differing degrees.
Inclusive fitness
Inclusive fitness describes the component of reproductive success in both a focal individual and their relatives.
Importantly, the measure embodies the sum of direct and indirect
fitness and the change in their reproductive success based on the
actor's behavior.
That is, the effect an individual's behaviors have on: being personally
better-suited to reproduce offspring, and aiding descendent and
non-descendent relatives in their reproductive efforts.
Natural selection
is predicted to push individuals to behave in ways that maximize their
inclusive fitness. Studying inclusive fitness is often done using
predictions from Hamilton's rule.
Kin recognition
Genetic cues
One possible method of kin selection is based on genetic cues that can be recognized phenotypically. Genetic recognition has been exemplified in a species that is usually not thought of as a social creature:
amoebae.
Social amoebae form fruiting bodies when starved for food. These
amoebae preferentially formed slugs and fruiting bodies with members of
their own lineage, which is clonially related. The genetic cue comes from variable lag genes, which are involved in signaling and adhesion between cells.
Kin can also be recognized a genetically determined odor, as studied in the primitively social sweat bee,
Lasioglossum zephyrum. These bees can even recognize relatives they have never met and roughly determine relatedness. The Brazilian stingless bee
Schwarziana quadripunctata uses a distinct combination of chemical
hydrocarbons to recognize and locate kin. Each chemical odor, emitted from the organism's
epicuticles, is unique and varies according to age, sex, location, and hierarchical position. Similarly, individuals of the stingless bee species
Trigona fulviventris
can distinguish kin from non-kin through recognition of a number of
compounds, including hydrocarbons and fatty acids that are present in
their wax and floral oils from plants used to construct their nests. In the species,
Osmia rufa,
kin selection has also been associated with mating selection. Females,
specifically, select males for mating with whom they are genetically
more related to.
Environmental cues
There
are two simple rules that animals follow to determine who is kin.
These rules can be exploited, but exist because they are generally
successful.
The first rule is ‘treat anyone in my home as kin.’ This rule is readily seen in the
reed warbler,
a bird species that only focuses on chicks in their own nest. If its
own kin is placed outside of the nest, a parent bird ignores that chick.
This rule can sometimes lead to odd results, especially if there is a
parasitic bird that lays eggs in the reed warbler nest. For example, an
adult
cuckoo
may sneak its egg into the nest. Once the cuckoo hatches, the reed
warbler parent feeds the invading bird like its own child. Even with
the risk for exploitation, the rule generally proves successful.
The second rule, named by
Konrad Lorenz
as ‘imprinting,’ states that those who you grow up with are kin.
Several species exhibit this behavior, including, but not limited to the
Belding's ground squirrel.
Experimentation with these squirrels showed that regardless of true
genetic relatedness, those that were reared together rarely fought.
Further research suggests that there is partially some genetic
recognition going on as well, as siblings that were raised apart were
less aggressive toward one another compared to non-relatives reared
apart.
Cooperation
Cooperation
is broadly defined as behavior that provides a benefit to another
individual that specifically evolved for that benefit. This excludes
behavior that has not been expressly selected for to provide a benefit
for another individual, because there are many commensal and parasitic
relationships where the behavior one individual (which has evolved to
benefit that individual and no others) is taken advantage of by other
organisms. Stable cooperative behavior requires that it provide a
benefit to both the actor and recipient, though the benefit to the actor
can take many different forms.
Within species
Within
species cooperation occurs among members of the same species. Examples
of intraspecific cooperation include cooperative breeding (such as in
weeper capuchins) and cooperative foraging (such as in wolves). There
are also forms of cooperative defense mechanisms, such as the "fighting
swarm" behavior used by the stingless bee
Tetragonula carbonaria.
Much of this behavior occurs due to kin selection. Kin selection
allows cooperative behavior to evolve where the actor receives no direct
benefits from the cooperation.
Cooperation (without kin selection) must evolve to provide
benefits to both the actor and recipient of the behavior. This includes
reciprocity, where the recipient of the cooperative behavior repays the
actor at a later time. This may occur in vampire bats but it is
uncommon in non-human animals.
Cooperation can occur willingly between individuals when both benefit
directly as well. Cooperative breeding, where one individual cares for
the offspring of another, occurs in several species, including
wedge-capped capuchin monkeys.
Cooperative behavior may also be enforced, where their failure to
cooperate results in negative consequences. One of the best examples
of this is
worker policing, which occurs in social insect colonies.
The
cooperative pulling paradigm
is a popular experimental design used to assess if and under which
conditions animals cooperate. It involves two or more animals pulling
rewards towards themselves via an apparatus they can not successfully
operate alone.
Between species
Cooperation
can occur between members of different species. For interspecific
cooperation to be evolutionarily stable, it must benefit individuals in
both species. Examples include pistol shrimp and goby fish, nitrogen
fixing microbes and legumes, ants and aphids.
In ants and aphids, aphids secrete a sugary liquid called honeydew,
which ants eat. The ants provide protection to the aphids against
predators, and, in some instances, raise the aphid eggs and larvae
inside the ant colony. This behavior is analogous to human
domestication. The genus of goby fish,
Elacatinus also demonstrate cooperation by removing and feeding on
ectoparasites of their clients. The species of wasp
Polybia rejecta and ants
Azteca chartifex show a cooperative behavior protecting one another's nests from predators.
Market
economics often govern the details of the cooperation: e.g. the amount
exchanged between individual animals follow the rules of
supply and demand.
Spite
Hamilton's rule can also predict spiteful behaviors between non-relatives.
A spiteful behavior is one that is harmful to both the actor and to the
recipient. Spiteful behavior is favored if the actor is less related to
the recipient than to the average member of the population making r
negative and if rB-C is still greater than zero.
Spite
can also be thought of as a type of altruism because harming a
non-relative, by taking his resources for example, could also benefit a
relative, by allowing him access to those resources. Furthermore,
certain spiteful behaviors may provide harmful short term consequences
to the actor but also give long term reproductive benefits.
Many behaviors that are commonly thought of as spiteful are actually
better explained as being selfish, that is benefiting the actor and
harming the recipient, and true spiteful behaviors are rare in the
animal kingdom.
An example of spite is the sterile soldiers of the
polyembryonic
parasitoid wasp. A female wasp lays a male and a female egg in a
caterpillar. The eggs divide asexually, creating many genetically
identical male and female larvae. Sterile soldier wasps also develop and
attack the relatively unrelated brother larvae so that the genetically
identical sisters have more access to food.
Another example is bacteria that release
bacteriocins.
The bacteria that releases the bacteriocin may have to die to do so,
but most of the harm is to unrelated individuals who are killed by the
bacteriocin. This is because the ability to produce and release the
bacteriocin is linked to an immunity to it. Therefore, close relatives
to the releasing cell are less likely to die than non-relatives.
Altruism and conflict in social insects
Many insect species of the order
Hymenoptera (bees, ants, wasps) are
eusocial.
Within the nests or hives of social insects, individuals engage in
specialized tasks to ensure the survival of the colony. Dramatic
examples of these specializations include changes in body morphology or
unique behaviors, such as the engorged bodies of the
honeypot ant Myrmecocystus mexicanus or the
waggle dance of honey bees and a wasp species,
Vespula vulgaris.
In many, but not all social insects, reproduction is monopolized by the queen of the colony. Due to the effects of a
haplodiploid mating system, in which unfertilized eggs become male
drones and fertilized eggs become worker females, average
relatedness values between sister workers can be higher than those seen in humans or other
eutherian mammals. This has led to the suggestion that
kin selection
may be a driving force in the evolution of eusociality, as individuals
could provide cooperative care that establishes a favorable benefit to
cost ratio (
rB-c greater than 0). However, not all social insects follow this rule. In the social wasp
Polistes dominula, 35% of the nest mates are unrelated.
In many other species, unrelated individuals only help the queen when
no other options are present. In this case, subordinates work for
unrelated queens even when other options may be present. No other social
insect submits to unrelated queens in this way. This seemingly
unfavorable behavior parallels some vertebrate systems. It is thought
that this unrelated assistance is evidence of altruism in
P. dominula.
Cooperation in social organisms has numerous ecological factors that
can determine the benefits and costs associated with this form of
organization. One suggested benefit is a type of "life insurance" for
individuals who participate in the care of the young. In this instance,
individuals may have a greater likelihood of transmitting genes to the
next generation when helping in a group compared to individual
reproduction. Another suggested benefit is the possibility of "fortress
defense", where soldier castes threaten or attack intruders, thus
protecting related individuals inside the territory. Such behaviors are
seen in the snapping shrimp
Synalpheus regalis and gall-forming aphid
Pemphigus spyrothecae.
A third ecological factor that is posited to promote eusociality is
the distribution of resources: when food is sparse and concentrated in
patches, eusociality is favored. Evidence supporting this third factor
comes from studies of
naked mole-rats and
Damaraland mole-rats, which have communities containing a single pair of reproductive individuals.
Conflicts in social insects
The sex-ratio conflict arises from a
relatedness asymmetry, which is caused by the
haplodiploidy nature of
Hymenoptera.
For instance, workers are most related to each other because they
share half of the genes from the queen and inherit all of the father’s
genes. Their total relatedness to each other would be 0.5+ (0.5 x 0.5) =
0.75. Thus, sisters are three-fourths related to each other. On the
other hand, males arise from unfertilized larva, meaning they only
inherit half of the queen’s genes and none from the father. As a
result, a female is related to her brother by 0.25, because 50% of her
genes that come from her father have no chance of being shared with a
brother. Her relatedness to her brother would therefore be 0.5 x
0.5=0.25.
According to Trivers and Hare’s population-level sex-investment
ratio theory, the ratio of relatedness between sexes determines the sex
investment ratios.
As a result, it has been observed that there is a tug-of-war between
the queen and the workers, where the queen would prefer a 1:1 female to
male ratio because she is equally related to her sons and daughters
(r=0.5 in each case). However, the workers would prefer a 3:1 female to
male ratio because they are 0.75 related to each other and only 0.25
related to their brothers.
Allozyme data of a colony may indicate who wins this conflict.
Conflict can also arise between workers in colonies of social
insects. In some species, worker females retain their ability to mate
and lay eggs. The colony's queen is related to her sons by half of her
genes and a quarter to the sons of her worker daughters. Workers,
however, are related to their sons by half of their genes and to their
brothers by a quarter. Thus, the queen and her worker daughters would
compete for reproduction to maximize their own reproductive fitness.
Worker reproduction is limited by other workers who are more related to
the queen than their sisters, a situation occurring in many polyandrous
hymenopteran species. Workers police the egg-laying females by engaging
in
oophagy or directed acts of aggression.
The monogamy hypothesis
The monogamy hypothesis states that the presence of monogamy in insects is crucial for
eusociality to occur. This is thought to be true because of Hamilton’s rule that states that rB-C>0. By having a
monogamous
mating system, all of the offspring have high relatedness to each
other. This means that it is equally beneficial to help out a sibling,
as it is to help out an offspring. If there were many fathers the
relatedness of the colony would be lowered.
This monogamous mating system has been observed in insects such as termites, ants, bees and wasps.
In termites the queen commits to a single male when founding a nest. In
ants, bees and wasps the queens have a functional equivalent to
lifetime monogamy. The male can even die before the founding of the
colony. The queen can store and use the sperm from a single male
throughout their lifetime, sometimes up to 30 years.
In an experiment looking at the mating of 267 hymenopteran species, the results were mapped onto a
phylogeny.
It was found that monogamy was the ancestral state in all the
independent transitions to eusociality. This indicates that monogamy is
the ancestral, likely to be crucial state for the development of
eusociality. In species where queens mated with multiple mates, it was
found that these were developed from lineages where sterile castes
already evolved, so the multiple mating was secondary.
In these cases, multiple mating is likely to be advantageous for
reasons other than those important at the origin of eusociality. Most
likely reasons are that a diverse worker pool attained by multiple
mating by the queen increases disease resistance and may facilitate a
division of labor among workers.
Communication and signaling
Communication is varied at all scales of life, from interactions
between microscopic organisms to those of large groups of people.
Nevertheless, the signals used in communication abide by a fundamental
property: they must be a quality of the receiver that can transfer
information to a receiver that is capable of interpreting the signal and
modifying its behavior accordingly. Signals are distinct from cues in
that evolution has selected for signalling between both parties, whereas
cues are merely informative to the observer and may not have originally
been used for the intended purpose. The natural world is replete with
examples of signals, from the luminescent flashes of light from
fireflies, to chemical signaling in
red harvester ants to prominent mating displays of birds such as the
Guianan cock-of-the-rock, which gather in
leks, the pheromones released by the
corn earworm moth, the dancing patterns of the
blue-footed booby, or the alarm sound
Synoeca cyanea make by rubbing their mandibles against their nest. Yet other examples are the cases of the
grizzled skipper and
Spodoptera littoralis where pheromones are released as a sexual recognition mechanism that drives evolution.
The nature of communication poses evolutionary concerns, such as the potential for
deceit
or manipulation on the part of the sender. In this situation, the
receiver must be able to anticipate the interests of the sender and act
appropriately to a given signal. Should any side gain advantage in the
short term, evolution would select against the signal or the response.
The conflict of interests between the sender and the receiver results in
an evolutionarily stable state only if both sides can derive an overall
benefit.
Although the potential benefits of deceit could be great in terms
of mating success, there are several possibilities for how dishonesty
is controlled, which include indices,
handicaps,
and common interests. Indices are reliable indicators of a desirable
quality, such as overall health, fertility, or fighting ability of the
organism. Handicaps, as the term suggests, place a restrictive cost on
the organisms that own them, and thus lower quality competitors
experience a greater relative cost compared to their higher quality
counterparts. In the common interest situation, it is beneficial to both
sender and receiver to communicate honestly such that the benefit of
the interaction is maximized.
Signals are often honest, but there are exceptions. Prime examples of dishonest signals include the luminescent lure of the
anglerfish, which is used to attract prey, or the
mimicry of non-poisonous butterfly species, like the
Batesian mimic
Papilio polyxenes of the poisonous model
Battus philenor.
Although evolution should normally favor selection against the
dishonest signal, in these cases it appears that the receiver would
benefit more on average by accepting the signal.
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