Promiscuous females cause male zebras to have bigger testes and act all weird!

This article (from 1990) examined the differences in male zebra mating behavior and testicular size as influenced by polyandrous and monandrous zebra mares. The study focused on comparing the plains zebra (Equus burchelli) with Grevy’s zebra (E. grevyi) in northern Kenya. Plains zebras reside in herds consisting of several females and a single stallion. Females are considered monandrous as they will stay and mate with the same male for 9 – 26 months. This is the same type of mating behavior seen in wild horse populations. Grevy’s zebra females are generally polyandrous and typically travel in small groups without a male. These groups of females may travel through the territories of up to four different stallions each day. This polyandrous behavior is also seen in female asses (E. asinus) in wild populations. When mating, the polyandrous mares will stay with the stallion for only 1 – 48 hours. After giving birth, a normally polyandrous mare may become monandrous and stay with a male for 3 – 59 days. This longer association with the male provides that mare and her newborn with protection and allows the stallion a chance to mate with the mare.

The promiscuous mating behaviors of female Grevy’s zebras have caused the males to evolve in ways that would help to improve their reproductive success. Grevy’s zebra stallions that are mating with polyandrous females will exhibit more mating behaviors such as calling and mounting and will mate with the female more frequently than stallions with monandrous females. Polyandrous mares also exhibit a shorter estrous than monandrous mares, thus decreasing the amount of time that she will be receptive to the stallion and requiring him to mate often to increase chances of conception. The frequent mating requires greater stores of sperm and has led to the evolution of larger testes in the Grevy’s zebra. Larger testicular size enables the stallion to breed more frequently and also allows him to produce greater volumes of sperm to help improve his reproductive success.

--contributed by Abigail Hall