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Sexual Size Dimorphism in Raptors

Differentiating males from females

In many birds, males and females can be differentiated by their plumage and size – this expression of differences in plumage and size between sexes is termed sexual dimorphism. The degree of dimorphism can vary between species, some being very dimorphic such as the Montagu’s Harrier and others less so like the African White-backed Vulture. So why the difference?


Sexual dimorphism begins in the reproductive cells of each sex where females produce few large immotile egg cells and males produce many small motile sperm cells. This system is called Anisogamous sex and is the basis for sexual selection. The female contributes a greater investment as the egg cells produced are larger and fewer compared to the sperm cells from the male and therefore, in most cases, the males compete amongst themselves for this limited resource. And since females are making the larger investment they can be choosy as to who they want to mate with – female choice - which, in many species, leads males to evolve elegant plumages and colours and perform elaborate courtship rituals. Females are usually less elegantly coloured simply because they don’t need to be, it would attract a lot of unwanted attention in the form of predators. The males on the other hand are trading their safety for a chance to pass on their genes – survival for reproductive success. So one way that sexual dimorphism may arise is due to females repeatedly choosing a certain trait in males and so more males with this trait are able to pass on their genes and eventually a population of males with the specific trait is what will remain. There are other ways in which sexual dimorphism is thought to have evolved that include competition between males as well as well as competition for resources.


Dimorphism in raptors

Raptors generally don’t show much colour dimorphism but are more dimorphic in size. In almost all raptors females are larger than males, many people may know this as reverse size dimorphism – but for this blog I will refer to it as sexual size dimorphism. Although sexual size dimorphism in raptors varies from species to species, females are either similar in size or larger than the males. This was considered unusual as in most mammals and birds it is males that are larger and so it was subject to a fair bit of scientific scrutiny. Lots of theories and debates have gone into why this reverse dimorphism in size occurs and how it may have been brought about – they’ve been debating for centuries and it is still going on!

Very evident example of sexual size dimorphism in a pair of Cooper's Hawks, found in North America. Photo by Hal Trachtenberg.
Very evident example of sexual size dimorphism in a pair of Cooper's Hawks. Photo by Hal Trachtenberg.

One hypothesis, put forward as early as 1241 by Frederick II of Hohenstaufen, a Roman Emperor who was very passionate about falconry, was that females are larger because they have more moisture. The reasoning behind this was that when substances that hold more moisture were heated they would expand more than substances that held less moisture. The active lifestyle of birds of prey, i.e. hunting, generated heat and because females hold more moisture this stimulated them to grow more than males. We’ve come a long way.


Tom Cade, Dean Amadon and Leslie Brown drew attention to the fact that the greatest size difference in birds of prey is shown in those species that kill the largest prey and especially those that kill birds. Females of large falcons and Accipiters are nearly twice the weight of their males. The theory here being that size dimorphism allows the sexes to hunt different prey and so reduces competition by allowing each sex to occupy a different feeding niche. This theory appears to hold true for some strongly dimorphic species, but on its own it doesn’t necessarily explain why the female ended up being the larger sex. Or maybe it does for the very act of mating, if reversed you would have a tremendously powerful male mounting a tiny female...which could have disastrous results!


This hypothesis fits in quite well with the type of prey males hunt and their role as the main food providers for chicks. While speaking to Simon on the subject he stated that in most, if not all, terrestrial habitats the amount of small prey will always outnumber the amount of large prey (due to the way energy and biomass are distributed in food chains) and males being the smaller sex would generally go for smaller prey. Small food is also nutritionally better for growing young and therefore it would make sense that the smaller male brings in the food.

An interesting comparison of a female Martial eagle’s foot (left) and a male’s (right). The female’s foot is markedly larger and indicates that these raptors are able to take different sized prey. Photos by Shiv Kapila.


Raptors are a large grouping made up of different orders. Accipitriformes include most diurnal birds of prey like eagles, kites, hawks but exclude falcons as they are in their own order (Falconiformes) as are owls (Strigiformes) and New World Vulture (Cathartiformes). Studying the genomes of birds of prey has shown that all four orders evolved independently of one another from different ancestral branches. The fact that most of the birds found in these orders display size dimorphism may indicate that size dimorphism developed as a result of the same selective pressure(s) acting on all birds of prey, although not necessarily at the same time and with the same intensity.


But If we go back far enough in evolutionary time, about 65 million years ago which is when the two major clades of birds Afroaves and Australaves separated, and find their common ancestor which may have happened to exhibit size dimorphism, then it follows that size dimorphism could have sustained itself in the raptor species we see today. There is evidence to suggest that the basal clades for Afroaves and Australaves both had predatory members and so it is quite likely that their common ancestor lived a predatory lifestyle. So we can’t exclude the idea that size dimorphism did evolve in a common ancestor and then sustained itself in birds of prey.


Cladogram showing phylogeny of Afroaves and Australaves. Image from Kaiser Science (modified from Mindell et al. 2018).
Cladogram showing phylogeny of Afroaves and Australaves. Image from Kaiser Science (modified from Mindell et al. 2018).

So far we have assumed that the female evolved from an ancestral state to become larger, how do we know that it wasn’t the males that shrunk in size while the females remained the same size or even both – the males shrunk and the females grew? A study conducted looking at the available data on Falconiformes in North America and the Western Palearctic proved, through regression analysis (some complicated sounding statistics where they looked at relationships between egg size and body size and their correlation to a dimorphism index) that it was in fact females that evolved to become larger while males remained the same size. Although this was only consistent for the order of Falconiformes.


So looking at the information we have available regarding the phylogeny of raptors (which is quite extensive and this blog hasn’t even managed to scratch the surface of), sexual size dimorphism in raptors would have had to:

  • evolve due to a selective pressure(s) acting on a specific gene or set of genes;

  • influence the reproductive success of the individuals it was acting on; and

  • act on all individuals of a population frequently during the reproductive or pre-reproductive stages of each individual.

The only hypothesis that we know of (so far) that has met these criteria and can explain size dimorphism in all raptors is the “Nest Defence Hypothesis”.


In an extensive study done in North America nest predation has been shown to be the most prevalent cause of nest failures (apart from human interference), out of 161 raptor nests observed 54 nests failed and 28% of these cases failed due to nest predation. This represents a significant loss to nest predators. The females being the sex that spends the most time on the nest would have an advantage in being more powerful to defend the nest. So to tick off the criteria above, the Nest Defence Hypothesis proposes that due to the selective pressure of nest predation directly affecting reproductive success, the females have evolved to become bigger to better defend their offspring. Being larger would mean they are more powerful, delivering more fatal injuries to nest predators. This means that individuals that could successfully defend their nests against attackers would pass on their genes and so, over time, a larger body size in females is selected for as it is an adaptive trait.


To link back to the theory of reducing competition for food between sexes – the Nest Defence Hypothesis may have been the ultimate cause of the size difference but over time the degree of dimorphism could have increased (or could have been propagated) due to competition for food between the sexes which could result in the strongly dimorphic species we see today. The Nest Defence Hypothesis is still a hypothesis and may have its flaws, but so far it’s the only hypothesis that can explain sexual size dimorphism in all raptors.

greenhumour.com, by Rohan Chakravarty, 2021

About Pranav Peshavaria

Pranav was brought up in Kenya and his fascination with birds and wildlife pushed him to pursue a

degree in Wildlife Conservation. In 2021 he volunteered with the Kenya Bird of Prey Trust to conduct

a survey of African fish eagles at Lake Naivasha as part of his undergraduate thesis. Pranav is also a

member of the Nairobi Ringing Group and takes a keen interest in raptors.

 

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Kenya Bird of Prey Trust

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