Review of Female Mate Competition in Birds and Mammals
John Kobrossi
April 2, 2023
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Overview
It is a cool autumn day in the Great Plains of North America, and two male bison stand opposite
each other. They size each other up before they charge, slamming each other’s heads together.
Eventually, one of the bulls falls, and the other gets access to the entire group of females the
other has been guarding. How often have you seen this scenario? And what if research told you
that this is not the only version of the story? In fact, in some species of birds and mammals, it is
actually the females who fight with each other for access to males. In general, this is a form of
intrasexual selection called mate competition, where individuals of the same sex compete for
access to mate with the opposite sex (Andersson and Iwasa 1996). Mate competition is so rare in
females (hence why you are probably so familiar with the example of male bison fighting)
because of sex differences in investment. Females invest so much in each egg and raising
offspring that to maximize fitness, they are picky and choose quality in males over quantity. On
the other hand, males invest relatively so little each time they mate that they go for quantity over
quality in terms of reproduction. The reason why female mate competition is such an unusual
and underdeveloped field of research is because it occurs so rarely in birds and mammals, giving
little opportunities to study the phenomenon in these taxa. Although there is still much unknown,
there are unique examples of female mate competition across specific conditions. With these few
examples, we can begin to understand the conditions under which female mate competition
evolves. Here, I will review these instances of female mate competition and the evolutionary
theories behind them. I will first go over this in a variety of studies in bird species and in
mammal species, and lastly I will conclude with an overview of the current state of this research.
My goal is to give a clear synopsis of the research of female mate competition in birds and
mammals.
Background Theory
Sexual selection is a specific type of natural selection that selects for traits that help an individual
compete with its rivals over mates, called intrasexual selection, and that attract the opposite sex,
called intersexual selection (Andersson and Iwasa 1996). Intrasexual selection, often called mate
competition, is the theoretical explanation behind why males develop large body sizes and
weapons for fighting other males in order for access to females. Examples can be found in the
large antlers of elk, the over-sized claws of many crabs, and the general pattern of larger body
sizes in males of many species. Intersexual selection, which is often called mate choice, is the
selection of traits that help an individual, usually the male, attract potential mates of the opposite
sex. This is the explanation behind the costly and conspicuous traits found in males like the tail
feathers of the peacock and the extravagant, colorful feathers of male birds in general.
The pattern of males being the typical sex to compete and try to win over the females can
be explained by a theory called Bateman’s Principle. It states that the reason sexual selection is
more pronounced in males is because they usually have high mating success (measured in
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number of matings) relative to the costs of offspring production (Andersson and Iwasa 1996).
This ratio is what determines the strength of sexual selection in the sexes of a species. This ratio
is usually high in males, and low in females (the costs of producing offspring are high relative to
each successful mating). This makes sense mechanistically, as males produce large quantities of
sperm that are relatively low in energy cost and can be easily replenished for multiple matings.
On the other hand, females produce costly eggs in a limited quantity, and once they mate, they
have to wait until they have started ovulating or have finished caring for their present offspring
before they can mate again.
Besides this inherent difference in the costs of producing eggs versus sperm, there are
two main theories researchers use to predict female and male offspring investment and thus
sexual selection (male or female mate choice versus competition). One is by differences in
parental investment, described by Trivers in 1972 as cited in Andersson and Iwasa in 1996,
which details how the sex that displays greater parental care toward the offspring already born
may have less opportunities to mate and be more picky of the quality of mates rather than the
quantity to maximize fitness. The other way is explained by operational sex ratio theory, by
Emlen and Oring in 1977 as cited by Andersson and Iwasa in 1996. This theory states that sexual
selection direction can be predicted by the ratio of individuals of each sex that are ready to mate.
In this case sexual selection acts more strongly on males than females because they are more
numerous in terms of individuals ready to mate (because males have a faster reproduction rate,
because they do not carry offspring or develop eggs). Therefore, the limiting factor would be the
other sex, and so individuals of the more available sex will compete with each other, exhibiting
intrasexual selection, in order to mate.
There is also a similar term, adult sex ratio, which just refers to the proportion of adult
males to females in a population (Liker 2013). This broader term is also measured and used to
determine sexual selection conditions because it is easier to measure in the wild. A population
with a biased adult sex ratio will exhibit sexual selection (mate competition) on the more
abundant sex for the more rare sex (the rare sex will then show mate choice and be picky). All of
these theories apply well to birds and mammals because they are not just influenced by sex ratio,
but also by parental investment (unlike a lot of fish and amphibians). There is a lot of parental
care necessary after birth for younglings to survive, e.g. nest care in birds and general nursing in
mammals. Overall, it is hard to determine why females exhibit mate competition because these
patterns of sex ratio and parental investment rarely reverse (where males invest more in offspring
and are less available to mate). Thus, in species where females do compete with each other for
mates, there is usually a unique situation in terms of the distribution of resources or mates.
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Argument
Adult Sex Ratio, Male Parental Care, and Female Mate Competition
Female mate competition in birds has been associated with large biases in the adult sex ratio and
male parental care. Evidence for this comes from jacanas and greater-painted snipes, who both
have female mate competition, male-biased adult sex ratios, and male parental care (Liker et al.
2013). Furthermore in terms of parental investment, they found a positive correlation between
male biased adult sex ratio and male parental care, female polygamy, and female mating success
across many species of shorebirds. These species that were found with this correlation also had
female mate competition. This supports an argument that adult sex ratio can be a predictor of sex
roles and thus female mate competition in birds.
Operational Sex Ratio, Male Parental Care, and Female Mate Competition
In terms of operational sex ratio theory, there is evidence in all of these aforementioned
bird species with male parental care that there is also a female-biased operational sex ratio, and
that it may be contributing to female mate competition and reversed sex roles. T. H.
Glutton-Brock and A.C.J. Vincent collected data in 1991 across many species of birds in which
there was male parental care in order to compare the relationship between reproductive rates of
females and males and the direction of sexual selection. Their data showed that in species with
female mate competition and male parental care, there was always a higher potential
reproductive rate (which in turn influences and predicts operational sex ratio) in females than in
males. Potential reproductive rate was measured by the total number of independent offspring
produced in a given unit of time. What this means is that a female can produce eggs again faster
than the male partner can finish raising that female’s offspring, resulting in a female-biased
operational sex ratio.
This was again confirmed in the wattled jacana, in which species it is hypothesized that
male uniparental care together with this female biased operational sex ratio has caused sex role
reversal and female mate competition. Emlen and Wrege in 2004 researched a marked population
of wattled jacanas for six years, and obtained direct measurements of mean number of mates per
year of females and males (as a proxy measurement for operational sex ratio). Using this data, it
was determined that females have an increased opportunity for sexual selection, with a ratio of
1.61 to 1 compared to males. Additionally, males were observed providing all of the parental
care for the offspring while females defend the territories of the males and compete for and
reproduce with extra males. In the Eurasian dotterel, another bird species that shows female mate
competition and reversed sex roles, the males exhibit all of the parental care of the chicks and the
nest just like the jacana family (Owens et al. 1994).
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Female Mate Competition in Mammals
In mammals, the relationship between adult sex ratio and female mate competition has
also been observed, albeit more rarely, in social mammals in which there are complex social
groups or unusual mating systems. For example, chacma baboons of southern Africa live in very
large groups of hundreds of individuals, in which every individual mates with multiple partners,
in a polygynandrous mating system. In 2012, Cheney et al. measured rates of female intrasexual
aggression in chacma baboons over the span of 15 years and found that there was a correlation
between a high, female-biased adult sex ratio and increased rates of female to female aggression
for mate access. This was confirmed by corroborating evidence in a 2011 study by Huchard and
Cowlishaw, who studied two wild groups of chacma baboons over 18 months and found that the
more swollen females there were, the more mate competition (measured in agonistic
interactions) was observed. Swelling in female baboons signals sexual receptivity, and those who
were swollen and were guarded by a male were subject to more female aggression than if they
were unguarded by a male in the group. Another social mammal that shows female mate
competition is the ring-tailed lemur. In this rare case of female dominance in the social groups of
ring-tailed lemurs, female androgen levels and conflict rates increase by twofold during the
mating season. This was measured via hormonal immunoassay with fecal samples by von
Engelhardt N et al. in 2000, showing variance in female mate competition in social mammals
based on season and timing of the reproductive cycle.
Lekking and Female Mate Competition in Birds and Mammals
A unique evolutionary situation that has led to female mate competition is lek evolution.
A lek is an arena males use to show off and attract females to mate, and it has evolved in many
ungulate species, namely the topi antelope. It is unclear why leks have evolved in this species, as
it increases risk of predation, lowers food resources, and increases harassment by males, but Bro
Jorgenson in 2002 shows that on the lek compared to off, there is increased female competition
for mates, measured in agonistic encounters and disruption of matings. In addition, there is a
dominance hierarchy, where weaker, younger, and smaller females are interrupted in 15% of
mating bouts by stronger, more dominant females as compared to lower rates of interruptions
when dominant females mate. There is more agonism and mating disruption among females in
the center of the lek, where there is judged to be the highest quality males. Another instance of
female mate competition on leks comes from a lot of lekking bird species, like the black grouse.
In a 2002 study by Karvonen et al., not only was female-female aggression observed on more
female crowded leks, but more counts of aggressive behavior were observed when the male
being fought over was of a higher rank. One last thing to note is that it was observed that the
more central the male in the lek was, the higher its lifetime performance was. Lekking and
female mate competition is also seen in the Eurasian dotterel. The species exhibits plumage
variation in both sexes, and Owens et al. in 1994 observed that it is the females who compete
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with each other for males on the arena and who also choose which male they will mate with,
exhibiting both mate competition and mate choice. Female dotterels exhibit a secondary sexual
trait that is plumage brightness to signal to other females their “strength”, and it was actually
found that brighter females win more of their fights than duller females and mate with brighter
males.
Discussion
It is pretty clear early on in the research of female mate competition that intrasexual selection is
very rare in females of birds and mammals. Just a few families in birds exhibit female mate
competition, and even fewer species in mammals. To confound the problem of studying this
phenomenon, the conditions in which female mate competition occurs do not exclusively explain
why it has evolved. Many explanations may be happening in one species, with no clear telling
which one is the main cause of why female competition has evolved. In birds, female mate
competition occurs in a variety of social and environmental conditions. Those observed include
sex role reversed birds, lekking birds, and shorebirds.
Biased Adult Sex Ratio and Direction of Sexual Selection
A measurement that can be used to predict sexual selection direction and thus female
mate competition is adult sex ratio. Adult sex ratio is just a proxy measurement for operational
sex ratio that is used because it is much easier to measure in nature, as counting males and
females is easier than telling which individuals are ready to mate and which ones are not. In the
2013 Liker et al. paper where male biased adult sex ratio is positively correlated with male
parental care, female polygamy and mating success in shorebirds including the spotted
sandpiper, it is demonstrated that adult sex ratio could be a way to predict and explain the variety
of mating systems and parental care in birds. A possible explanation that researchers have come
up with is that at least in birds, parental care is actually very flexible. Both the male and female
can and will evolve to provide full parental care given the correct environment, namely a biased
adult sex ratio. For example, as seen with shorebirds, given a large male-biased adult sex ratio,
there are so many more males than females that males will start to raise their own chicks and
provide parental care because it is more advantageous for them to ensure the survival of their
chicks than to try and mate again with so little chances of finding a female that is nearby and
receptive. Hence, the females will be the ones not investing in parental care, and given more time
to mate again, they will compete for males to mate with. Therefore, it is the adult sex ratio that
can predict the investment of each sex in parental care and thus the direction of sexual selection
(thereby predicting female mate competition). This is just one explanation of how a biased adult
sex ratio may influence parental care investments between the sexes, and there is more research
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needed to be done about exactly what happens in each species that has a biased adult sex ratio,
male parental care, and female mate competition.
Parental Care and Biased Operational Sex Ratios in Sex-Role Reversed Birds
In sex-role reversed birds like the jacana, the dotterel, and some shorebirds, it is hypothesized
that both the high reproductive rate of females relative to males and uniparental care by the
males contributed to sex role reversal and thus female mate competition. In the Wattled jacana
(Emlen and Wrege 2004) and the Eurasian dotterel (Owens et al. 1994), a female can actually
produce a second batch of eggs faster than a male can finish raising the clutch of offspring
produced by the male and that female. Because of male uniparental care and this ability to
reproduce a batch of eggs so quickly, the operational sex ratio has become biased toward
females. This is seen in Emlen and Wrege 2004 paper, as the ratio of sexual opportunities being
1.61 to 1 in favor of females. Sexual selection will then favor females to mate more often and
compete for males so that they can have as many of their own clutches raised as possible at any
given time, precisely because males are the less available sex. This will lead to secondary sexual
traits seen in the jacana female like larger body size, aggressive behavior toward other females
and territory guarding, brighter feathers, etc. A large unknown in the study of the wattle jacana
and Eurasian dotterel is that no one knows what has led to male parental care in the first place.
There are ideas that it has to do with predation pressure or the distribution of food and other
resources, but there are no ideas that have been proven.
Female Mate Competition Theories in Mammals
In mammals, female mate competition research is understandably more complex, as mammalian
social systems are more intricate in general. It is also a much more rare phenomenon in
mammals, only occurring in a couple species throughout the entire class of animals. In chacma
baboons therefore it was assumed that female competition was just over resources like food, but
researchers noticed that the inter-female aggression was positively correlated with how many
individuals of the group were females (Cheney et al. 2012) and if a female had sexual swellings
(Huchard and Cowlishaw 2011), suggesting that female competition was really about mating
access to males rather than food. Explanations of this behavior in chacma baboons are numerous
and diverse, representing the complex behavior of mammals. Cheney et al. describe the various
theories already present in the literature. One says that females compete with and stop each other
from mating so that they alone will require the extra food resources for raising offspring, giving
themselves more of the resource. This indeed would be true for any mammal that displays female
mate competition and has high competition for food. Another theory states that in species like the
chacma baboon in which offspring require a lot of parental investment and risk infanticide from
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other males, females may compete over males who will provide parental care and defend their
offspring from threats of other males. This is an ongoing area of research, as finding direct
evidence for exclusively one hypothesis is hard in complex group social interactions in
mammals. Another area research has found mammalian female mate competition is in species
with female dominated groups, like in spotted hyenas and ring-tailed lemurs. Evidence for this
comes from the observation that female aggression increases significantly (almost doubling)
during mating season over males in the group (von Engelhardt N et al. 2000). Unlike chacma
baboons, there are no concrete hypotheses that can explain why these female dominated groups,
with sex role reversals and female mate competition have evolved in the first place. This too is
an ongoing area of research in the ring-tailed lemur and spotted hyena.
Lekking, a Unique Instance of Female Mate Competition in Birds and Mammals
Lekking in birds and mammals represents another chance for female mate competition to evolve,
as female mate competition is present in a large proportion of both birds and mammals who
display lekking. The topi antelope is one of these species, and females have been observed to
interrupt matings and show aggression to other females more often on the lek than off of it (Bro
Jorgenson 2002). In addition, the center of the lek, which has the highest quality males, has also
been observed to have the largest females mating with such individuals. This all suggests that
females are not only choosing the best males, thereby securing the best genes (and survival
chance) for their offspring, but are also competing for them. A theory that was proposed to
explain this is that there is first a high variance in male quality, as seen in the variance in body
size in relation to the distance to the center of the lek. This causes females to disproportionately
mate with the highest quality males in the center of the lek, causing these males to eventually run
out of sperm because they mate so often. Hence, females compete for the sperm resources of
these highest quality males in the center of the lek.
In birds, lekking is more common, and female mate competition in lekking birds can be
seen in multiple species of multiple different families. One strong example is in the black grouse,
which like the topi antelope, hints that female mate competition in lekking species may be over
the best quality males’ sperm. Black grouse females display more competition for males of
higher rank and when there is a female biased adult sex ratio in the lek arena (Karvonen et al.
2002). And indeed there is a hypothesis that black grouses fight over males because they have to
mate within a certain short time period to ensure reproduction at the right time in the season.
Thus, having to wait to mate with the best male could have direct costs on reproduction. And just
like the topi antelope, there is another hypothesis that female mate competition may really be
competition over food resources as interrupting mating would ensure there are less females and
offspring present on the patch of resources. And lastly, again these hypotheses have been
proposed in the Eurasian dotterel (Owens et al. 1994) because females have been observed
competing for exclusive access to the brightest and largest males in the center of the lek,
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suggesting the evolutionary advantage of competition being the access to the best genes, while
depriving other females of those genes, thereby eliminating competition for resources.
Conclusion
Female mate competition in birds and mammals is very rare, as there are large amounts of
pre-birth investments that females have to provide to their offspring that cannot be avoided
biologically. Therefore, there are only very specific and rare situations where a male can provide
more overall investment in offspring than females, namely through parental investment. Thus,
male parental care can lead to sex role reversals, where the females compete for male access,
when the operational sex ratio is biased toward females, as seen in the Wattled jacana and
Eurasian dotterel, or the adult sex ratio is male-biased, as seen in the shorebirds like the spotted
sandpiper. Female mate competition in mammals is even more complex, often having to do with
the mating and social system present. In addition, sex ratio does not predict sex roles as well as
in birds because reproductive rates in mammals are even more strict in mammals. It is just seen
in the ring-tailed lemur and spotted hyena, where dominant females compete for subordinate
male access during the mating season, the evolutionary origins of why dominant females exist
and display mate competition being still unknown. Lastly, female mate competition can evolve in
lekking species, where more often than not the two conditions of female intrasexual selection
and lekking are observed to be present together in the same species, being more present in birds.
In the topi antelope, black grouse, and Eurasian dotterel, females compete for the best males in
the center of the lek for various hypothesized evolutionary reasons, including that females may
be reducing competition or competing for limited sperm within a small mating time window.
As for the overall question of why female mate competition occurs, researchers still do
not know. Although the examples of female mate competition in this review give direct evidence
that the phenomenon exists in birds and mammals, none of the papers have hard evidence for the
evolutionary reasons why it is present. Further research is needed in all of these species to
determine the exact roles parental investment and sex ratio patterns play in determining sex roles
and female mate competition. Additionally, all of this evidence has been correlational, so finding
causal relationships between these variables is the next step in understanding not only female
mate competition, but the evolution of sex roles in general. For example, male parental care,
operational sex ratio, and adult sex ratio all play a role in female mate competition in the
Eurasian dotterel. Finding out which of these three is the causal factor may answer questions
about the nature of female competition.
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