Why is it that with beasts, males are always larger than females, while in raptors the opposite is true?

The difference between the sexes has always been an interesting topic for biologists to discuss. Whether it is the magnificent plumage of a peacock or the majestic horns of a stag, the phenotype of the sexes is often a complex product of environmental pressures and sexual selection.

Predators can be considered the rulers of the land, while birds of prey such as eagles, falcons and owls are the rulers of the sky. However, the sexual dimorphism of raptors and carnivores is completely opposite. Most carnivores have larger males than females, while most raptors are larger females. What caused this phenomenon?

Rules

Different raptors and wild animals will have different physical differences between males and females, but not without rules. First, let's look at the raptors. In a 2005 study, Kruger summarized the sexual dimorphism of eagles, falcons, and owls. In short, the harder the raptor's prey is to catch, the smaller the male size.

Among them, the bird of prey Accipiter ventralis, distributed in the tropical forests of South America, may be the most striking bird of prey in terms of sexual dimorphism. The average wingspan of males is only 53% that of females.

Other birds of prey that feed on lively and agile prey such as birds, bats, monkeys and squirrels. the wingspan of males usually does not exceed 80% of that of females.

The manta falcon (Bat Falco) lives in the lowland rainforests of America. It eats a variety of birds and bats. Males typically have a wingspan of 61% of females. Image of a pair of manta falcons standing together. It is not difficult to see that the female on the right is not only larger in length, the body is also much more muscular.

However, as the difficulty of hunting decreased, the difference in size by sex of raptors also decreased. Taking the shape of the eagle for example, many raptors such as eagles, buzzards and snake eagles feed on the ground their prey is easier to catch than bats and other birds. The wingspan of males is usually as large as 80% or even 90% when compared to females. The same applies to all types of falcons and owls that feed on small mammals, reptiles, and large insects.

However, in scavengers, the males are usually roughly the same size as the female, and in vultures there are males even larger than the female. The wingspan of male Vultur gryphus (Andean Falcon) is usually 126% of that of females, which also makes them unique - the largest male of all bird-eaters meat.

A pair of Andean falcons, you can see the male on the right is slightly larger than the female on the left.

For carnivores, the difference in bodybuilding between the sexes is also related to the habits of the species. Law's research in 2019 found that the extent to which the sex differences in terrestrial carnivores were related to eating habits and social characteristics.

Accordingly, the higher the level of solitary life in carnivores, the greater the difference in body composition between males and females. This is most evident in carnivores of the big cat family as well as those of the weasel family.

In tigers, leopards, mountain lions, ferrets, etc., the males are usually considerably larger than the females. Among the terrestrial carnivores, the species with the largest physical difference between males and females is the Mustela itatsi, whose average weight is only 30% of that of the male.

Carnivorous groups such as jackals and wolves generally have relatively low levels of sexual dimorphism. Of course, the above rule only represents a general trend, and there are exceptions.

With the yellow mink, the male is usually significantly larger than the female.

Reason

Birds of prey and carnivores are both carnivores, so what led to the physical contrast between the sexes? In fact, like all animals, the sexual dimorphism of birds of prey and wild animals is the result of natural selection and sexual selection, the differences between which reflect differences in warfare. reproductive strategy.

Let's first consider birds of prey. To explain the sexual dimorphism of birds of prey, scientists have come up with several hypotheses.

The first hypothesis is that raptors have a rather aggressive temperament, so that if the male is larger than the female, they may misunderstand and consider the female as potential prey during tracking. chasing, flirting and this can cause injury to their mates.

Therefore, the size of the larger female is to protect herself. However, when observing the breeding process of birds of prey, scientists found that pairs of birds of prey often get along very well. Furthermore, if raptors do indeed consider their mates as prey, then larger females will obviously threaten the male's safety as well. So this hypothesis is clearly wrong.

 The size of the larger female is to protect herself?

The second theory is that males are smaller than females to attract mates. This view holds that male raptors rely on aerial displays to attract females. Because females prefer faster-flying males, sexual selection makes smaller males more successful in reproductive competition.

This hypothesis seems reasonable, and moreover, for raptors they often raise their young in pairs, and the male is important, since they usually take on most of the hunting duties.

Many people may think that the larger the size, the stronger the ability to hunt, but this is not true of raptors. Through research and observation, it can be seen that falcons are much smaller in size when compared to eagles, but they are better hunters and have higher hunting efficiency than eagles. a lot, the reason being that they are smaller and more agile.

Furthermore, smaller males will consume less food, so after each hunt they can leave more food for the hens and chicks. Therefore, for birds of prey, the smaller male will have more advantages.

 For birds of prey, the smaller male has more of an advantage.

Meanwhile, for carnivores, the difference in bodybuilding between the sexes comes mainly from the territorial competition of males. Unlike raptors, which are often monogamous, the reproductive structure of predators is predominantly polygamous. Males often need to defend a large territory that includes many territories of the female. The pressure of this sexual selection forces them to grow to a larger size to be able to defend their territory and to mate more.

In predators, females are often responsible for raising their cubs, and therefore they need to perform hunting duties to feed themselves and their cubs. Like male raptors, female raptors may be slightly inferior to males in catching large prey, but their agility and speed make it easy to catch small and medium-sized prey. . The smaller size of the female can also save more food for the cubs.

The smaller size of the female can also save more food for the cubs.

In summary, the important reason why females are larger in raptors and males are larger in carnivores is because of their different division of labor and social structure. Whether it's a lone beast or a bird of prey, smaller birds are often better at hunting and raising their families, while larger ones are more able to fight and defend territory. better.