Sexual politics in the jungle

A couple of days ago I wrote about how gender shapes chicken and their ancestral relatives, the red jungle fowl, Gallus gallus. Males fight each other to try to monopolize breeding with groups of females. This leads to larger sizes and extreme combativeness among males. The female is solely responsible for rearing chicks. Not only is she smaller, she is also drab coloured, so as to be less noticeable. The pattern of males being larger and more aggressive than females is also seen in other animals with similar social organization, for example, spotted deer (Axis axis), lions (Panthera leo), monkeys such as Northern Plains Langurs (Semnopithecus entellus). There are conjectures that sex-linked size and aggression in Homo sapiens is also due to social organization of this kind when our ancestors roamed the grasslands of Africa.

Thurber's cartoon

One of the more commonly visible birds in Pench national park was the Oriental honey buzzard (Pernis ptilorhynchus). The first one we saw was sitting on a nest, probably incubating a clutch of eggs (featured photo). The Family saw it instantly since she was looking through her new binoculars. I was peering through my camera, and I saw the nest but not the bird, until I zoomed the image. We kept seeing these birds through the next couple of days: flying low over us, sitting on a branch, diving into the grass. Once I photographed it on a branch (photo below), just before it dived into the dry grass below the tree. I could see it wrestle and worry something in the grass, so I waited for it to emerge with something in its beak. Unfortunately it appeared without anything and flew away.

Male oriental honey buzzard in Pench National Park

This was the breeding season, and clearly the intense activity was related to that. The females sat in nests, either brooding over eggs, or looking after hatchlings. I guessed that probably the males were doing most of the flying. Later reading told me an interesting fact: apparently in this, and many other species of hunting birds, the male is smaller than the female. The reason again seems to do with social organization.

In honey buzzards and other raptors, a male and a female bond as a breeding pair. The pair cooperate in raising the young. The mother spends more of her time in the nest, while the father spends longer periods foraging for the family. The wing span of male and female honey buzzards is about the same. So the smaller body of the male makes it more manoeuvrable, and a slightly better hunter. The same holds true for other hunting birds. Interestingly, those which hunt faster prey have relatively smaller males. Vultures show almost no size difference between the sexes.

Traumatic beginning of adulthood

Interestingly, all birds abandon the young once they are grown. This is again a theme that recurs throughout the animal kingdom, even among H. sapiens. To get back to our main theme, the full story of sexual dimorphism among birds may be more complicated than this. Studies show that strongly coloured males and drab females arise in bird species where the male tries to dominate a breeding group of females, but a significant fraction of the offspring are not the dominant males. This is extreme sexual politics. However, the mutually supportive roles of the two parents in the cooperative rearing of offspring prevents sexual politics from arising amongst honey buzzards.