This article is about the type of wetland. For other uses, see Bog (disambiguation).
A bog or bogland is a wetland that accumulates peat as a deposit of dead plant materials – often mosses, typically sphagnum moss. It is one of the four main types of wetlands. Other names for bogs include mire, mosses, quagmire, and muskeg; alkaline mires are called fens. A baygall is another type of bog found in the forest of the Gulf Coast states in the United States. They are often covered in heath or heather shrubs rooted in the sphagnum moss and peat. The gradual accumulation of decayed plant material in a bog functions as a carbon sink.
Bogs occur where the water at the ground surface is acidic
and low in nutrients. A bog usually is found at a freshwater soft
spongy ground that is made up of decayed plant matter which is known as
peat. They are generally found in cooler northern climates and are
formed in poorly draining lake basins. In contrast to fens, they derive most of their water from precipitation rather than mineral-rich ground or surface water. Water flowing out of bogs has a characteristic brown colour, which comes from dissolved peat tannins.
In general, the low fertility and cool climate result in relatively
slow plant growth, but decay is even slower due to low oxygen levels in
saturated bog soils. Hence, peat accumulates. Large areas of the
landscape can be covered many meters deep in peat.
Bogs have distinctive assemblages of animal, fungal, and plant species, and are of high importance for biodiversity, particularly in landscapes that are otherwise settled and farmed.
Distribution and extent
Bogs are widely distributed in cold, temperateclimes, mostly in boreal ecosystems in the Northern Hemisphere. The world's largest wetland is the peat bogs of the Western Siberian Lowlands in Russia, which cover more than a million square kilometres. Large peat bogs also occur in North America, particularly the Hudson Bay Lowland and the Mackenzie River Basin. They are less common in the Southern Hemisphere, with the largest being the Magellanic moorland, comprising some 44,000 square kilometres (17,000 sq mi) in southern South America. Sphagnum bogs were widespread in northern Europe but have often been cleared and drained for agriculture. A paper led by Graeme T. Swindles
in 2019 showed that peatlands across Europe have undergone rapid drying
in recent centuries owing to human impacts including drainage, peat
cutting and burning.
A 2014 expedition leaving from Itanga village, Republic of the Congo, discovered a peat bog "as big as England" which stretches into neighboring Democratic Republic of Congo.
Definition
Like
all wetlands, it is difficult to rigidly define bogs for a number of
reasons, including variations between bogs, the in-between nature of
wetlands as an intermediate between terrestrial and aquatic ecosystems,
and varying definitions between wetland classification systems. However, there are characteristics common to all bogs that provide a broad definition:
Peat is present, usually thicker than 30 cm.
The wetland receives most of its water and nutrients from precipitation (ombrotrophic) rather than surface or groundwater (minerotrophic).
The wetland is strongly acidic (bogs near coastal areas may be less acidic due to sea spray).
Because all bogs have peat, they are a type of peatland. As a
peat-producing ecosystem, they are also classified as mires, along with
fens. Bogs differ from fens in that fens receive water and nutrients
from mineral-rich surface or groundwater, while bogs receive water and
nutrients from precipitation.
Because fens are supplied with mineral-rich water, they tend to be
slightly acidic to slightly basic, while bogs are always acidic because
precipitation is mineral-poor.
Ecology and protection
There are many highly specialized animals, fungi, and plants
associated with bog habitat. Most are capable of tolerating the
combination of low nutrient levels and waterlogging. Sphagnum is generally abundant, along with ericaceous shrubs. The shrubs are often evergreen, which may assist in conservation of nutrients.
In drier locations, evergreen trees can occur, in which case the bog
blends into the surrounding expanses of boreal evergreen forest. Sedges are one of the more common herbaceous species. Carnivorous plants such as sundews (Drosera) and pitcher plants (for example Sarracenia purpurea) have adapted to the low-nutrient conditions by using invertebrates as a nutrient source. Orchids have adapted to these conditions through the use of mycorrhizal fungi to extract nutrients. Some shrubs such as Myrica gale (bog myrtle) have root nodules in which nitrogen fixation occurs, thereby providing another supplemental source of nitrogen.
Bogs are recognized as a significant/specific habitat type by a
number of governmental and conservation agencies. They can provide
habitat for mammals, such as caribou, moose, and beavers, as well as for species of nesting shorebirds, such as Siberian cranes and yellowlegs. Bogs contain species of vulnerable reptilians such as the bog turtle. Bogs even have distinctive insects; English bogs give a home to a yellow fly called the hairy canary fly (Phaonia jaroschewskii), and bogs in North America are habitat for a butterfly called the bog copper (Lycaena epixanthe). In Ireland, the viviparous lizard, the only known reptile in the country, dwells in bogland.
Bogs are fragile ecosystems, and have been deteriorating quickly,
as archaeologists and scientists have been recently finding. Bone
material found in bogs has had accelerated deterioration from first
analyses in the 1940s. This has been found to be from fluctuations in ground water and increase in acidity
in lower areas of bogs that is affecting the rich organic material.
Many of these areas have been permeated to the lowest levels with
oxygen, which dries and cracks layers. There have been some temporary
solutions to try and fix these issues, such as adding soil to the tops
of threatened areas, yet they do not work in the long-term.
Extreme weather like dry summers are likely the cause, as they lower
precipitation and the groundwater table. It is speculated that these
issues will only increase with a rise in global temperature and climate
change. Since bogs take thousands of years to form and create the rich
peat that is used as a resource, once they are gone they are extremely
hard to recover. Arctic and sub-Arctic circles where many bogs are
warming at 0.6 °C per decade, an amount twice as large as the global
average. Because bogs and other peatlands are carbon sinks, they are
releasing large amounts of greenhouse gases as they warm up.
These changes have resulted in a severe decline of biodiversity and
species populations of peatlands throughout Northern Europe.
Types
Bog habitats may develop in various situations, depending on the climate and topography (see also hydroseresuccession).
By location and water source
Bogs may be classified on their topography, proximity to water, method of recharge, and nutrient accumulation .
Valley bog
These develop in gently sloping valleys or hollows. A layer of peat
fills the deepest part of the valley, and a stream may run through the
surface of the bog. Valley bogs may develop in relatively dry and warm
climates, but because they rely on ground or surface water, they only
occur on acidic substrates.
These develop from a lake or flat marshy area, over either non-acidic or acidic substrates. Over centuries there is a progression from open lake, to a marsh, to a fen (or, on acidic substrates, valley bog), to a carr, as silt
or peat accumulates within the lake. Eventually, peat builds up to a
level where the land surface is too flat for ground or surface water to
reach the center of the wetland. This part, therefore, becomes wholly
rain-fed (ombrotrophic),
and the resulting acidic conditions allow the development of bog (even
if the substrate is non-acidic). The bog continues to form peat, and
over time a shallow dome of bog peat develops into a raised bog. The
dome is typically a few meters high in the center and is often
surrounded by strips of fen or other wetland vegetation at the edges or
along streamsides where groundwater can percolate into the wetland.
The various types of raised bog may be divided into:
In cool climates with consistently high rainfall (on more than c. 235
days a year), the ground surface may remain waterlogged for much of the
time, providing conditions for the development of bog vegetation. In these circumstances, bog develops as a layer "blanketing" much of the land, including hilltops and slopes. Although a blanket bog is more common on acidic substrates, under some conditions it may also develop on neutral or even alkaline
ones, if abundant acidic rainwater predominates over the groundwater. A
blanket bog can occur in drier or warmer climates, because under those
conditions hilltops and sloping ground dry out too often for peat to
form – in intermediate climates a blanket bog may be limited to areas
which are shaded from direct sunshine. In periglacial climates a patterned form of blanket bog may occur, known as a string bog.
In Europe, these mostly very thin peat layers without significant
surface structures are distributed over the hills and valleys of
Ireland, Scotland, England, and Norway. In North America, blanket bogs
occur predominantly in Canada east of Hudson Bay. These bogs are often still under the influence of mineral soil water (groundwater). Blanket bogs do not occur north of the 65th latitude in the northern hemisphere.
Quaking bog
A quaking bog, schwingmoor, or swingmoor
is a form of floating bog occurring in wetter parts of valley bogs and
raised bogs and sometimes around the edges of acidic lakes. The bog
vegetation, mostly sphagnum moss anchored by sedges (such as Carex lasiocarpa), forms a floating mat approximately half a meter thick on the surface of water or above very wet peat. White spruce (Picea pungens)
may grow in this bog regime. Walking on the surface causes it to move –
larger movements may cause visible ripples on the surface, or they may
even make trees sway. The bog mat may eventually spread across the water
surface to cover bays or even entire small lakes. Bogs at the edges of
lakes may become detached and form floating islands.
Cataract bog
A cataract bog
is a rare ecological community formed where a permanent stream flows
over a granite outcropping. The sheeting of water keeps the edges of the
rock wet without eroding the soil, but in this precarious location, no
tree or large shrub can maintain a roothold. The result is a narrow,
permanently wet habitat.
Uses
Industrial uses
After drying, peat is used as a fuel,
and it has been used that way for centuries. More than 20% of home heat
in Ireland comes from peat, and it is also used for fuel in Finland,
Scotland, Germany, and Russia. Russia is the leading exporter of peat
for fuel, at more than 90 million metric tons per year. Ireland's Bord na Móna ("peat board") was one of the first companies to mechanically harvest peat, which is being phased out.
The other major use of dried peat is as a soil amendment (sold as moss peat or sphagnum peat) to increase the soil's capacity to retain moisture and enrich the soil. It is also used as a mulch. Some distilleries, notably in the Islay whisky-producing region, use the smoke from peat fires to dry the barley used in making Scotch whisky.
Once the peat has been extracted it can be difficult to restore the wetland, since peat accumulation is a slow process. More than 90% of the bogs in England have been damaged or destroyed.In 2011 plans for the elimination of peat in gardening products were announced by the UK government.
Other uses
The
peat in bogs is an important place for the storage of carbon. If the
peat decays, carbon dioxide would be released to the atmosphere,
contributing to global warming. Undisturbed, bogs function as a carbon sink.
As one example, the peatlands of the former Soviet Union were
calculated to be removing 52 Tg of carbon per year from the atmosphere. Therefore, the rewetting of drained peatlands may be one of the most cost-effective ways to mitigate climate change.
Peat bogs are also important in storing fresh water, particularly in the headwaters of large rivers. Even the enormous Yangtze River arises in the Ruoergai peatland near its headwaters in Tibet.
Sphagnum bogs are also used for outdoor recreation, with activities including ecotourism
and hunting. For example, many popular canoe routes in northern Canada
include areas of peatland. Some other activities, such as all-terrain vehicle use, are especially damaging to bogs.
Archaeology
The anaerobic environment and presence of tannic acids within bogs can result in the remarkable preservation of organic material. Finds of such material have been made in Slovenia, Denmark, Germany, Ireland, Russia, and the United Kingdom. Some bogs have preserved bog-wood such as ancient oak logs useful in dendrochronology, and they have yielded extremely well-preserved bog bodies, with hair, organs, and skin intact, buried there thousands of years ago after apparent Germanic and Celtic human sacrifice. Excellent examples of such human specimens include the Haraldskær Woman and Tollund Man in Denmark, and Lindow man found at Lindow Common
in England. The Tollund Man was so well preserved that when the body
was discovered in 1950, the discoverers thought it was a recent murder
victim and researchers were even able to tell the last meal that the Tollund Man ate before he died: porridge and fish.
This process happens because of the low oxygen levels of bogs in
combination with the high acidity. These anaerobic conditions lead to
some of the best preserved mummies and offer a lot of archeological
insight on society as far as 8,000 years back. Céide Fields in County Mayo in Ireland, a 5,000-year-old neolithic farming landscape has been found preserved under a blanket bog, complete with field walls and hut sites. One ancient artifact found in various bogs is bog butter, large masses of fat, usually in wooden containers. These are thought to have been food stores, of both butter and tallow.
Sexual dimorphism is the condition where sexes of the same species exhibit different morphological characteristics, particularly characteristics not directly involved in reproduction. The condition occurs in most animals and some plants. Differences may include secondary sex characteristics,
size, weight, color, markings, or behavioral or cognitive traits.
Male–male reproductive competition has evolved a diverse array of
sexually dimorphic traits. Aggressive utility traits such as "battle"
teeth and blunt heads reinforced as battering rams are used as weapons
in aggressive interactions between rivals. Passive displays such as
ornamental feathering or song-calling have also evolved mainly through
sexual selection. These differences may be subtle or exaggerated and may be subjected to sexual selection and natural selection. The opposite of dimorphism is monomorphism, when both biological sexes are phenotypically indistinguishable from each other.
Overview
Ornamentation and coloration
Common and easily identified types of dimorphism consist of ornamentation
and coloration, though not always apparent. A difference in the
coloration of sexes within a given species is called sexual
dichromatism, commonly seen in many species of birds and reptiles. Sexual selection
leads to the exaggerated dimorphic traits that are used predominantly
in competition over mates. The increased fitness resulting from
ornamentation offsets its cost to produce or maintain suggesting complex
evolutionary implications, but the costs and evolutionary implications
vary from species to species. The costs and implications differ depending on the nature of the ornamentation (such as the color mechanism involved).
The peafowl constitute conspicuous illustrations of the principle. The ornate plumage of peacocks, as used in the courting display, attracts peahens.
At first sight, one might mistake peacocks and peahens for completely
different species because of the vibrant colours and the sheer size of
the male's plumage; the peahen is of a subdued brown coloration.
The plumage of the peacock increases its vulnerability to predators
because it is a hindrance in flight, and it renders the bird conspicuous
in general. Similar examples are manifold, such as in birds of paradise and argus pheasants.
Another example of sexual dichromatism is that of the nestling blue tits. Males are chromatically more yellow than females. It is believed that this is obtained by the ingestion of green Lepidopteran larvae, which contain large amounts of the carotenoidslutein and zeaxanthin. This diet also affects the sexually dimorphic colours in the human-invisible ultraviolet spectrum.
Hence, the male birds, although appearing yellow to humans actually
have a violet-tinted plumage that is seen by females. This plumage is
thought to be an indicator of male parental abilities.
Perhaps this is a good indicator for females because it shows that they
are good at obtaining a food supply from which the carotenoid is
obtained. There is a positive correlation between the chromas of the
tail and breast feathers and body condition. Carotenoids play an important role in immune function for many animals, so carotenoid dependent signals might indicate health.
Frogs constitute another conspicuous illustration of the
principle. There are two types of dichromatism for frog species:
ontogenetic and dynamic. Ontogenetic frogs are more common and have
permanent color changes in males or females. Ranoidea lesueuri is an example of a dynamic frog with temporary color changes in males during the breeding season. Hyperolius ocellatus
is an ontogenetic frog with dramatic differences in both color and
pattern between the sexes. At sexual maturity, the males display a
bright green with white dorsolateral lines.
In contrast, the females are rusty red to silver with small spots. The
bright coloration in the male population attracts females and is an aposematic sign to potential predators.
Females often show a preference for exaggerated male secondary sexual characteristics in mate selection. The sexy son hypothesis
explains that females prefer more elaborate males and select against
males that are dull in color, independent of the species' vision.
Similar sexual dimorphism and mating choice are also observed in many fish species. For example, male guppies
have colorful spots and ornamentations, while females are generally
grey. Female guppies prefer brightly colored males to duller males.
In redlip blennies,
only the male fish develops an organ at the anal-urogenital region that
produces antimicrobial substances. During parental care, males rub
their anal-urogenital regions over their nests' internal surfaces,
thereby protecting their eggs from microbial infections, one of the most
common causes for mortality in young fish.
Plants
Most flowering plants are hermaphroditic but approximately 6% of species have separate males and females (dioecy). Sexual dimorphism is common in dioecious plants and dioicous species.
Males and females in insect-pollinated species generally look similar to one another because plants provide rewards (e.g. nectar) that encourage pollinators to visit another similar flower, completing pollination. Catasetum orchids are one interesting exception to this rule. Male Catasetumorchids violently attach pollinia to euglossine bee pollinators. The bees will then avoid other male flowers but may visit the female, which looks different from the males.
Various other dioecious exceptions, such as Loxostylis alata
have visibly different sexes, with the effect of eliciting the most
efficient behavior from pollinators, who then use the most efficient
strategy in visiting each gender of flower instead of searching, say,
for pollen in a nectar-bearing female flower.
Some plants, such as some species of Geranium have what amounts to serial sexual dimorphism. The flowers of such species might, for example, present their anthers on opening, then shed the exhausted anthers after a day or two and perhaps change their colours as well while the pistil
matures; specialist pollinators are very much inclined to concentrate
on the exact appearance of the flowers they serve, which saves their
time and effort and serves the interests of the plant accordingly. Some
such plants go even further and change their appearance once fertilized,
thereby discouraging further visits from pollinators. This is
advantageous to both parties because it avoids damaging the developing
fruit and wasting the pollinator's effort on unrewarding visits. In
effect, the strategy ensures that pollinators can expect a reward every
time they visit an appropriately advertising flower.
Females of the aquatic plant Vallisneria americana have floating flowers attached by a long flower stalk that are fertilized if they contact one of the thousands of free-floating flowers released by a male. Sexual dimorphism is most often associated with wind-pollination in plants due to selection for efficient pollen dispersal in males vs pollen capture in females, e.g. Leucadendron rubrum.
Sexual dimorphism in plants can also be dependent on reproductive development. This can be seen in Cannabis sativa,
a type of hemp, which have higher photosynthesis rates in males while
growing but higher rates in females once the plants become sexually
mature.
Every sexually reproducing extant species of the vascular plant
has an alternation of generations; the plants we see about us generally
are diploidsporophytes,
but their offspring are not the seeds that people commonly recognize as
the new generation. The seed actually is the offspring of the haploid generation of microgametophytes (pollen) and megagametophytes (the embryo sacs in the ovules).
Each pollen grain accordingly may be seen as a male plant in its own
right; it produces a sperm cell and is dramatically different from the
female plant, the megagametophyte that produces the female gamete.
Insects
Insects display a wide variety of sexual dimorphism between taxa including size, ornamentation and coloration. The female-biased sexual size dimorphism observed in many taxa evolved despite intense male–male competition for mates. In Osmia rufa, for example, the female is larger/broader than males, with males being 8–10 mm in size and females being 10–12 mm in size. In the hackberry emperor females are similarly larger than males. The reason for the sexual dimorphism is due to provision size mass, in which females consume more pollen than males.
In some species, there is evidence of male dimorphism, but it
appears to be for distinctions of roles. This is seen in the bee species
Macrotera portalis in which there is a small-headed morph, capable of flight, and large-headed morph, incapable of flight, for males. Anthidium manicatum
also displays male-biased sexual dimorphism. The selection for larger
size in males rather than females in this species may have resulted due
to their aggressive territorial behavior and subsequent differential
mating success. Another example is Lasioglossum hemichalceum, which is a species of sweat bee that shows drastic physical dimorphisms between male offspring. Not all dimorphism has to have a drastic difference between the sexes. Andrena agilissima is a mining bee where the females only have a slightly larger head than the males.
Weaponry leads to increased fitness by increasing success in male–male competition in many insect species. The beetle horns in Onthophagus taurus are enlarged growths of the head or thorax expressed only in the males. Copris ochus also has distinct sexual and male dimorphism in head horns. These structures are impressive because of the exaggerated sizes. There is a direct correlation between male horn lengths and body size and higher access to mates and fitness. In other beetle species, both males and females may have ornamentation such as horns.
Generally, insect sexual size dimorphism (SSD) within species increases with body size.
Sexual dimorphism within insects is also displayed by dichromatism. In butterfly genera Bicyclus and Junonia, dimorphic wing patterns evolved due to sex-limited expression, which mediates the intralocus sexual conflict and leads to increased fitness in males. The sexual dichromatic nature of Bicyclus anynana is reflected by female selection on the basis of dorsal UV-reflective eyespot pupils. The common brimstone also displays sexual dichromatism; males have yellow and iridescent wings, while female wings are white and non-iridescent. Naturally selected deviation in protective female coloration is displayed in mimetic butterflies.
Spiders and sexual cannibalism
Many arachnid groups exhibit sexual dimorphism, but it is most widely studied in the spiders. In the orb-weaving spider Zygiella x-notata, for example, adult females have a larger body size than adult males. Size dimorphism shows a correlation with sexual cannibalism, which is prominent in spiders (it is also found in insects such as praying mantises). In the size dimorphic wolf spiderTigrosa helluo, food-limited females cannibalize more frequently.
Therefore, there is a high risk of low fitness for males due to
pre-copulatory cannibalism, which led to male selection of larger
females for two reasons: higher fecundity and lower rates of cannibalism.
In addition, female fecundity is positively correlated with female body
size and large female body size is selected for, which is seen in the
family Araneidae. All Argiope species, including Argiope bruennichi, use this method. Some males evolved ornamentation
including binding the female with silk, having proportionally longer
legs, modifying the female's web, mating while the female is feeding, or
providing a nuptial gift in response to sexual cannibalism. Male body size is not under selection due to cannibalism in all spider species such as Nephila pilipes, but is more prominently selected for in less dimorphic species of spiders, which often selects for larger male size. In the species Maratus volans, the males are known for their characteristic colorful fan which attracts the females during mating.
Fish
Ray finned
fish are an ancient and diverse class, with the widest degree of sexual
dimorphism of any animal class. Fairbairn notes that "females are
generally larger than males but males are often larger in species with
male–male combat or male paternal care ... [sizes range] from dwarf
males to males more than 12 times heavier than females."
There are cases where males are substantially larger than females. An example is Lamprologus callipterus,
a type of cichlid fish. In this fish, the males are characterized as
being up to 60 times larger than the females. The male's increased size
is believed to be advantageous because males collect and defend empty
snail shells in each of which a female breeds.
Males must be larger and more powerful in order to collect the largest
shells. The female's body size must remain small because in order for
her to breed, she must lay her eggs inside the empty shells. If she
grows too large, she will not fit in the shells and will be unable to
breed. The female's small body size is also likely beneficial to her
chances of finding an unoccupied shell. Larger shells, although
preferred by females, are often limited in availability.
Hence, the female is limited to the growth of the size of the shell and
may actually change her growth rate according to shell size
availability.
In other words, the male's ability to collect large shells depends on
his size. The larger the male, the larger the shells he is able to
collect. This then allows for females to be larger in his brooding nest
which makes the difference between the sizes of the sexes less
substantial. Male–male competition in this fish species also selects for
large size in males. There is aggressive competition by males over
territory and access to larger shells. Large males win fights and steal
shells from competitors. Another example is the dragonet, in which males are considerably larger than females and possess longer fins.
Sexual dimorphism also occurs in hermaphroditic fish. These species are known as sequential hermaphrodites. In fish, reproductive histories
often include the sex-change from female to male where there is a
strong connection between growth, the sex of an individual, and the
mating system it operates within.
In protogynous mating systems where males dominate mating with many
females, size plays a significant role in male reproductive success.
Males have a propensity to be larger than females of a comparable age
but it is unclear whether the size increase is due to a growth spurt at
the time of the sexual transition or due to the history of faster growth
in sex changing individuals. Larger males are able to stifle the growth of females and control environmental resources.
Social organization plays a large role in the changing of sex by
the fish. It is often seen that a fish will change its sex when there is
a lack of dominant male within the social hierarchy. The females that
change sex are often those who attain and preserve an initial size
advantage early in life. In either case, females which change sex to
males are larger and often prove to be a good example of dimorphism.
In other cases with fish, males will go through noticeable
changes in body size, and females will go through morphological changes
that can only be seen inside of the body. For example, in sockeye salmon,
males develop larger body size at maturity, including an increase in
body depth, hump height, and snout length. Females experience minor
changes in snout length, but the most noticeable difference is the huge
increase in gonad size, which accounts for about 25% of body mass.
Sexual selection was observed for female ornamentation in Gobiusculus flavescens, known as two-spotted gobies.
Traditional hypotheses suggest that male–male competition drives
selection. However, selection for ornamentation within this species
suggests that showy female traits can be selected through either
female–female competition or male mate choice.
Since carotenoid-based ornamentation suggests mate quality, female
two-spotted guppies that develop colorful orange bellies during the
breeding season are considered favorable to males.
The males invest heavily in offspring during the incubation, which
leads to the sexual preference in colorful females due to higher egg
quality.
Amphibians and non-avian reptiles
In amphibians and reptiles, the degree of sexual dimorphism varies widely among taxonomic groups.
The sexual dimorphism in amphibians and reptiles may be reflected in
any of the following: anatomy; relative length of tail; relative size of
head; overall size as in many species of vipers and lizards; coloration as in many amphibians, snakes, and lizards, as well as in some turtles; an ornament as in many newts
and lizards; the presence of specific sex-related behaviour is common
to many lizards; and vocal qualities which are frequently observed in frogs.
Anole
lizards show prominent size dimorphism with males typically being
significantly larger than females. For instance, the average male Anolis sagrei was 53.4 mm vs. 40 mm in females. Different sizes of the heads in anoles have been explained by differences in the estrogen pathway.
The sexual dimorphism in lizards is generally attributed to the effects
of sexual selection, but other mechanisms including ecological
divergence and fecundity selection provide alternative explanations. The development of color dimorphism in lizards is induced by hormonal changes at the onset of sexual maturity, as seen in Psamodromus algirus, Sceloporus gadoviae, and S. undulates erythrocheilus. Sexual dimorphism in size is also seen in frog species like P. bibronii.
Male painted dragon lizards, Ctenophorus pictus. are brightly conspicuous in their breeding coloration, but male colour declines with aging. Male coloration appears to reflect innate anti-oxidation capacity that protects against oxidative DNA damage.
Male breeding coloration is likely an indicator to females of the
underlying level of oxidative DNA damage (a significant component of
aging) in potential mates.
Birds
Sexual dimorphism in birds can be manifested in size or plumage
differences between the sexes. Sexual size dimorphism varies among taxa
with males typically being larger, though this is not always the case,
e.g. birds of prey, hummingbirds, and some species of flightless birds.
Plumage dimorphism, in the form of ornamentation or coloration, also
varies, though males are typically the more ornamented or brightly
colored sex. Such differences have been attributed to the unequal reproductive contributions of the sexes.
This difference produces a stronger female choice since they have more
risk in producing offspring. In some species, the male's contribution to
reproduction ends at copulation, while in other species the male
becomes the main (or only) caregiver. Plumage polymorphisms have evolved
to reflect these differences and other measures of reproductive
fitness, such as body condition or survival. The male phenotype sends signals to females who then choose the 'fittest' available male.
Sexual dimorphism is a product of both genetics and environmental factors. An example of sexual polymorphism determined by environmental conditions exists in the red-backed fairywren. Red-backed fairywren males can be classified into three categories during breeding season: black breeders, brown breeders, and brown auxiliaries.
These differences arise in response to the bird's body condition: if
they are healthy they will produce more androgens thus becoming black
breeders, while less healthy birds produce less androgens and become
brown auxiliaries. The reproductive success
of the male is thus determined by his success during each year's
non-breeding season, causing reproductive success to vary with each
year's environmental conditions.
Migratory patterns and behaviors also influence sexual
dimorphisms. This aspect also stems back to the size dimorphism in
species. It has been shown that the larger males are better at coping
with the difficulties of migration and thus are more successful in
reproducing when reaching the breeding destination.
When viewing this in an evolutionary standpoint many theories and
explanations come to consideration. If these are the result for every
migration and breeding season the expected results should be a shift
towards a larger male population through sexual selection. Sexual
selection is strong when the factor of environmental selection is also
introduced. The environmental selection may support a smaller chick size
if those chicks were born in an area that allowed them to grow to a
larger size, even though under normal conditions they would not be able
to reach this optimal size for migration. When the environment gives
advantages and disadvantages of this sort, the strength of selection is
weakened and the environmental forces are given greater morphological
weight. The sexual dimorphism could also produce a change in timing of
migration leading to differences in mating success within the bird
population.
When the dimorphism produces that large of a variation between the
sexes and between the members of the sexes multiple evolutionary effects
can take place. This timing could even lead to a speciation phenomenon
if the variation becomes strongly drastic and favorable towards two
different outcomes. Sexual dimorphism is maintained by the counteracting
pressures of natural selection and sexual selection. For example,
sexual dimorphism in coloration increases the vulnerability of bird
species to predation by European sparrowhawks in Denmark. Presumably, increased sexual dimorphism means males are brighter and more conspicuous, leading to increased predation. Moreover, the production of more exaggerated ornaments in males may come at the cost of suppressed immune function.
So long as the reproductive benefits of the trait due to sexual
selection are greater than the costs imposed by natural selection, then
the trait will propagate throughout the population. Reproductive
benefits arise in the form of a larger number of offspring, while
natural selection imposes costs in the form of reduced survival. This
means that even if the trait causes males to die earlier, the trait is
still beneficial so long as males with the trait produce more offspring
than males lacking the trait. This balance keeps the dimorphism alive in
these species and ensures that the next generation of successful males
will also display these traits that are attractive to the females.
Such differences in form and reproductive roles often cause
differences in behavior. As previously stated, males and females often
have different roles in reproduction. The courtship and mating behavior
of males and females are regulated largely by hormones throughout a
bird's lifetime.
Activational hormones occur during puberty and adulthood and serve to
'activate' certain behaviors when appropriate, such as territoriality
during breeding season.
Organizational hormones occur only during a critical period early in
development, either just before or just after hatching in most birds,
and determine patterns of behavior for the rest of the bird's life. Such behavioral differences can cause disproportionate sensitivities to anthropogenic pressures. Females of the whinchat in Switzerland breed in intensely managed grasslands. Earlier harvesting of the grasses during the breeding season lead to more female deaths.
Populations of many birds are often male-skewed and when sexual
differences in behavior increase this ratio, populations decline at a
more rapid rate.
Also not all male dimorphic traits are due to hormones like
testosterone, instead they are a naturally occurring part of
development, for example plumage.
In addition, the strong hormonal influence on phenotypic differences
suggest that the genetic mechanism and genetic basis of these sexually
dimorphic traits may involve transcription factors or cofactors rather
than regulatory sequences.
Sexual dimorphism may also influence differences in parental investment during times of food scarcity. For example, in the blue-footed booby,
the female chicks grow faster than the males, resulting in booby
parents producing the smaller sex, the males, during times of food
shortage. This then results in the maximization of parental lifetime
reproductive success. In Black-tailed GodwitsLimosa limosa limosa
females are also the larger sex, and the growth rates of female chicks
are more susceptible to limited environmental conditions.
Sexual dimorphism may also only appear during mating season, some
species of birds only show dimorphic traits in seasonal variation. The
males of these species will molt into a less bright or less exaggerated
color during the off breeding season. This occurs because the species is more focused on survival than reproduction, causing a shift into a less ornate state.
Consequently, sexual dimorphism has important ramifications for
conservation. However, sexual dimorphism is not only found in birds and
is thus important to the conservation of many animals. Such differences
in form and behavior can lead to sexual segregation, defined as sex differences in space and resource use. Most sexual segregation research has been done on ungulates, but such research extends to bats, kangaroos, and birds. Sex-specific conservation plans have even been suggested for species with pronounced sexual segregation.
The term sesquimorphism (the Latin numeral prefixsesqui- means one-and-one-half, so halfway between mono- (one) and di-
(two)) has been proposed for bird species in which "both sexes have
basically the same plumage pattern, though the female is clearly
distinguishable by reason of her paler or washed-out colour". Examples include Cape sparrow (Passer melanurus),rufous sparrow (subspecies P. motinensis motinensis), and saxaul sparrow (P. ammodendri).
Mammals
In a large proportion of mammal species, males are larger than females. Both genes and hormones affect the formation of many animal brains before "birth" (or hatching),
and also behaviour of adult individuals. Hormones significantly affect
human brain formation, and also brain development at puberty. A 2004
review in Nature Reviews Neuroscience
observed that "because it is easier to manipulate hormone levels than
the expression of sex chromosome genes, the effects of hormones have
been studied much more extensively, and are much better understood, than
the direct actions in the brain of sex chromosome genes." It concluded
that while "the differentiating effects of gonadal secretions seem to be
dominant," the existing body of research "support the idea that sex
differences in neural expression of X and Y genes significantly
contribute to sex differences in brain functions and disease."
Pinnipeds
Marine mammals
show some of the greatest sexual size differences of mammals, because
of sexual selection and environmental factors like breeding location. The mating system of pinnipeds varies from polygamy to serial monogamy.
Pinnipeds are known for early differential growth and maternal
investment since the only nutrients for newborn pups is the milk
provided by the mother. For example, the males are significantly larger (about 10% heavier and 2% longer) than the females at birth in sea lion pups. The pattern of differential investment can be varied principally prenatally and post-natally. Mirounga leonina, the southern elephant seal, is one of the most dimorphic mammals.
Top: Stylised illustration of humans on the Pioneer plaque, showing both male (left) and female (right).
Bottom: Comparison between male (left) and female (right) pelvises.
According to Clark Spencer Larsen, modern day Homo sapiens show a range of sexual dimorphism, with average body mass between the sexes differing by roughly 15%.
Substantial discussion in academic literature considers potential
evolutionary advantages associated with sexual competition (both
intrasexual and intersexual), as well as short- and long-term sexual
strategies.
According to Daly and Wilson, "The sexes differ more in human beings
than in monogamous mammals, but much less than in extremely polygamous
mammals."
The average basal metabolic rate
is about 6 percent higher in adolescent males than females and
increases to about 10 percent higher after puberty. Females tend to
convert more food into fat, while males convert more into muscle
and expendable circulating energy reserves. Aggregated data of absolute
strength indicates that females have, on average, 40–60% the upper body
strength of males, and 70–75% the lower body strength.
The difference in strength relative to body mass is less pronounced in
trained individuals. In Olympic weightlifting, male records vary from
5.5× body mass in the lowest weight category to 4.2× in the highest
weight category, while female records vary from 4.4× to 3.8×, a weight
adjusted difference of only 10–20%, and an absolute difference of about
40% (i.e. 472 kg vs 333 kg for unlimited weight classes; see Olympic weightlifting records).
A study, carried about by analyzing annual world rankings from 1980 to
1996, found that males' running times were, on average, 11% faster than
females'.
In early adolescence, females are on average taller than males (as females tend to go through puberty earlier),
but males, on average, surpass them in height in later adolescence and
adulthood. In the United States, adult males are on average 9% taller and 16.5% heavier than adult females.
Males typically have larger tracheae and branching bronchi, with about 30 percent greater lung volume per body mass. On average, males have larger hearts, 10 percent higher red blood cell count, higher hemoglobin, hence greater oxygen-carrying capacity. They also have higher circulating clotting factors (vitamin K, prothrombin and platelets). These differences lead to faster healing of wounds and lower sensitivity to nerve pain after injury. In males, pain-causing injury to the peripheral nerve occurs through the microglia, while in females it occurs through the T cells (except in pregnant women, who follow a male pattern).
Females typically have more white blood cells (stored and circulating), as well as more granulocytes and B and T lymphocytes. Additionally, they produce more antibodies at a faster rate than males, hence they develop fewer infectious diseases and succumb for shorter periods. Ethologists argue that females, interacting with other females and multiple offspring in social groups, have experienced such traits as a selective advantage.
Females have a higher sensitivity to pain due to aforementioned nerve
differences that increase the sensation, and females thus require higher
levels of pain medication after injury.
Hormonal changes in females affect pain sensitivity, and pregnant women
have the same sensitivity as males. Acute pain tolerance is also more
consistent over a lifetime in females than males, despite these hormonal
changes. Despite differences in the physical feeling, both sexes have similar psychological tolerance to (or ability to cope with and ignore) pain.
In the human brain, a difference between sexes was observed in the transcription of the PCDH11X/Y gene pair unique to Homo sapiens.
Sexual differentiation in the human brain from the undifferentiated
state is triggered by testosterone from the fetal testis. Testosterone
is converted to estrogen in the brain through the action of the enzyme
aromatase. Testosterone acts on many brain areas, including the SDN-POA, to create the masculinized brain pattern.
Brains of pregnant females carrying male fetuses may be shielded from
the masculinizing effects of androgen through the action of sex hormone-binding globulin.
The relationship between sex differences in the brain and human
behavior is a subject of controversy in psychology and society at large. Many females tend to have a higher ratio of gray matter in the left hemisphere of the brain in comparison to males.
Males on average have larger brains than females; however, when
adjusted for total brain volume the gray matter differences between
sexes is almost nonexistent. Thus, the percentage of gray matter appears
to be more related to brain size than it is to sex. Differences in brain physiology between sexes do not necessarily relate to differences in intellect. Haier et al.
found in a 2004 study that "men and women apparently achieve similar IQ
results with different brain regions, suggesting that there is no
singular underlying neuroanatomical structure to general intelligence
and that different types of brain designs may manifest equivalent
intellectual performance". (See the sex and intelligence article for more on this subject.) Strict graph-theoretical analysis of the human brain connections revealed that in numerous graph-theoretical parameters (e.g., minimum bipartition width, edge number, the expander graph property, minimum vertex cover), the structural connectome of women are significantly "better" connected than the connectome of men. It was shown
that the graph-theoretical differences are due to the sex and not to
the differences in the cerebral volume, by analyzing the data of 36
females and 36 males, where the brain volume of each man in the group
was smaller than the brain volume of each woman in the group.
Sexual dimorphism was also described in the gene level and shown
to extend from the sex chromosomes. Overall, about 6500 genes have been
found to have sex-differential expression in at least one tissue. Many
of these genes are not directly associated with reproduction, but rather
linked to more general biological features. In addition, it has been
shown that genes with sex-specific expression undergo reduced selection
efficiency, which lead to higher population frequencies of deleterious
mutations and contributing to the prevalence of several human diseases.
Immune function
Sexual
dimorphism in immune function is a common pattern in vertebrates and
also in a number of invertebrates. Most often, females are more
'immunocompetent' than males. This trait is not consistent among all
animals, but differs depending on taxonomy, with the most female-biased
immune systems being found in insects.
In mammals this results in more frequent and severe infections in males
and higher rates of autoimmune disorders in females. One potential
cause may be differences in gene expression of immune cells between the
sexes.
Another explanation is that endocrinological differences between the
sexes impact the immune system – for example, testosterone acts as an
immunosuppressive agent.
Cells
Phenotypic differences between sexes are evident even in cultured cells from tissues. For example, female muscle-derived stem cells have a better muscle regeneration efficiency than male ones. There are reports of several metabolic differences between male and female cells and they also respond to stress differently.
Reproductively advantageous
In
theory, larger females are favored by competition for mates, especially
in polygamous species. Larger females offer an advantage in fertility,
since the physiological demands of reproduction are limiting in females.
Hence there is a theoretical expectation that females tend to be larger
in species that are monogamous.
Females are larger in many species of insects, many spiders, many fish, many reptiles, owls, birds of prey and certain mammals such as the spotted hyena, and baleen whales such as blue whale. As an example, in some species, females are sedentary, and so males must search for them. Fritz Vollrath and Geoff Parker
argue that this difference in behaviour leads to radically different
selection pressures on the two sexes, evidently favouring smaller males. Cases where the male is larger than the female have been studied as well, and require alternative explanations.
One example of this type of sexual size dimorphism is the bat Myotis nigricans,
(black myotis bat) where females are substantially larger than males in
terms of body weight, skull measurement, and forearm length.
The interaction between the sexes and the energy needed to produce
viable offspring make it favorable for females to be larger in this
species. Females bear the energetic cost of producing eggs, which is
much greater than the cost of making sperm by the males. The fecundity
advantage hypothesis states that a larger female is able to produce more
offspring and give them more favorable conditions to ensure their
survival; this is true for most ectotherms. A larger female can provide
parental care for a longer time while the offspring matures. The
gestation and lactation periods are fairly long in M. nigricans, the females suckling their offspring until they reach nearly adult size.
They would not be able to fly and catch prey if they did not compensate
for the additional mass of the offspring during this time. Smaller male
size may be an adaptation to increase maneuverability and agility,
allowing males to compete better with females for food and other
resources.
Some species of anglerfish
also display extreme sexual dimorphism. Females are more typical in
appearance to other fish, whereas the males are tiny rudimentary
creatures with stunted digestive systems. A male must find a female and
fuse with her: he then lives parasitically, becoming little more than a
sperm-producing body in what amounts to an effectively hermaphrodite
composite organism. A similar situation is found in the Zeus water bug Phoreticovelia disparata
where the female has a glandular area on her back that can serve to
feed a male, which clings to her (although males can survive away from
females, they generally are not free-living). This is taken to the logical extreme in the Rhizocephala crustaceans, like the Sacculina,
where the male injects itself into the female's body and becomes
nothing more than sperm producing cells, to the point that the
superorder used to be mistaken for hermaphroditic.
Some plant species also exhibit dimorphism in which the females are significantly larger than the males, such as in the moss Dicranum and the liverwort Sphaerocarpos. There is some evidence that, in these genera, the dimorphism may be tied to a sex chromosome, or to chemical signalling from females.
Another complicated example of sexual dimorphism is in Vespula squamosa,
the southern yellowjacket. In this wasp species, the female workers
are the smallest, the male workers are slightly larger, and the female
queens are significantly larger than her female worker and male
counterparts.
The first step towards sexual dimorphism is the size differentiation of sperm and eggs (anisogamy). Anisogamy
and the usually large number of small male gametes relative to the
larger female gametes usually lies in the development of strong sperm competition, because small sperm enable organisms to produce a large number of sperm, and make males (or male function of hermaphrodites) more redundant.
This intensifies male competition for mates and promotes the
evolution of other sexual dimorphism in many species, especially in vertebrates including mammals.
However, in some species females compete for mates in ways more
usually associated with males (usually species in which males invest a
lot in rearing offspring and thus are no longer considered as so
redundant).
Sexual dimorphism by size is evident in some extinct species such as the velociraptor.
In the case of velociraptors the sexual size dimorphism may have been
caused by two factors: male competition for hunting ground to attract
mates, and/or female competition for nesting locations and mates, males
being a scarce breeding resource.
Volvocine algae have been useful in understanding the evolution of sexual dimorphism and species like the beetle C. maculatus, where the females are larger than the males, are used to study its underlying genetic mechanisms.
In many non-monogamous species, the benefit to a male's
reproductive fitness of mating with multiple females is large, whereas
the benefit to a female's reproductive fitness of mating with multiple
males is small or nonexistent. In these species, there is a selection pressure for whatever traits enable a male to have more matings. The male may therefore come to have different traits from the female.
These traits could be ones that allow him to fight off other males for control of territory or a harem, such as large size or weapons; or they could be traits that females, for whatever reason, prefer in mates. Male–male competition poses no deep theoretical questions but mate choice does.
Females may choose males that appear strong and healthy, thus likely to possess "good alleles" and give rise to healthy offspring.
In some species, however, females seem to choose males with traits that
do not improve offspring survival rates, and even traits that reduce it
(potentially leading to traits like the peacock's tail). Two hypotheses for explaining this fact are the sexy son hypothesis and the handicap principle.
The sexy son hypothesis states that females may initially choose a
trait because it improves the survival of their young, but once this
preference has become widespread, females must continue to choose the
trait, even if it becomes harmful. Those that do not will have sons that
are unattractive to most females (since the preference is widespread)
and so receive few matings.
The handicap principle states that a male who survives despite
possessing some sort of handicap thus proves that the rest of his genes
are "good alleles". If males with "bad alleles" could not survive the
handicap, females may evolve to choose males with this sort of handicap;
the trait is acting as a hard-to-fake signal of fitness.