Sexual reproduction can be quite a tricky process, involving an array of selective pressures that produce intriguing behaviors across species. Male-male competition is one common form of sexual selection and includes a variety of processes, such as direct combat. For example, look at the male-male competition that occurs within the wild ass populations of the Tibetan Plains, described by Sir David Attenborough in Episode 7 of Planet Earth’s first season. Male Equus kiang, the largest species of wild asses, engage in direct combat to establish their mating territory in hopes of attracting the interest of female asses. This process is no joking matter .
Females in the documentary are shown in groups, nomadically traveling across the plains from oasis to oasis without remaining any place too long. This roaming behavior confers an additional pressure on sexual selection, specifically, female choosiness, in which females of a species with high mating costs are more selective of the males with whom they choose to mate . Equus kiang have reported gestation periods of around a year and, being in a region with limited resources, it would be reasonable to expect a strong selection for behaviors that limit mating frequency but maximize mating efficiency . So, while Sir David Attenborough may write-off this peculiar female behavior as “unfathomable”, he is overlooking that evolutionary processes and pressures are often complex and may not be blatantly observable.
Although sexual selection by direct combat is seen in a variety of mammal species, such as within the antelope populations of greater kudus, an interesting aspect of asses and other horse-like species not discussed in the documentary is there assembly into different social classes [3,4]. Equus kiang males can fall into different social classes with different corresponding behaviors, including social bachelors, solitary territorial males, and transitory males . The social classes of female asses have not been determined. In all, this episode of Planet Earth serves as a relatively effective representation of the seemingly peculiar behavior of Equus kiang mating, with an example of sexual selection through the well-studied process of male-male direct combat and an explanation of the choosiness of the female ass as, at best, an estimated-guess.
by J.W. Allen, Kai-Shanét Blackwood, Nick Green and Andrew Jeong
Planet Earth, Season 1, Episode 7, starting at 27:21
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- H Klingel. 1977. Observations on social organization and behaviour of African and Asiatic Wild Asses (Equus africanus and Equus hemionus). Zietschrift fur Tierpsychology 44: 323-331.
- Josefa B, Carmen BG & D Laloi. 2012. Evolution of female chooiness and mating frequency: effects of mating cost, density and sex ratio. Animal Behavior 83: 131-136.
- J Berger. 1983. Predation, sex ratios, and male competition in equids (Mammalia: Perissodactyla). Journal of Zoology 201: 205-216.
- Prameek K & MH Parsons. 2017. Social class and group size as predictors of behavior in male Equus kiang. Animal Behavior and Cognition 4: 442-454.
- R Rodrigues. 2016. Burrow Ornamentation in the Fiddler Crab (Uca leptodactyla): Female mate choice and male-male competition. Marine & Freshwater Behaviour & Physiology 1023-6244.