Why do Wildebeest in the Serengeti-Mara calve over a short period? New evidence

Tightly synchronised reproduction in mammals is a phenomenon I have discussed previously in this blog. One of the prime examples is the White-bearded Wildebeest (Connochaetes taurinus) of the Serengeti-Mara of East Africa. Each year 80-90% of the nearly half million calves are born over a period of three weeks.

I photographed these Wildebeest moving across the Masai Mara in Kenya in 1991. They clearly prefer the man-made track.

Discussions over the years have considered the possible mechanisms that could be involved in achieving this synchronicity. The survival value, though, is clearly known: predators are both swamped and confused by a short period of super-abundance. Possible mechanisms range from a trigger that synchronises parturition with variation in the period of gestation to a trigger that synchronises oestrus with a standard length of gestation. Of course, there is no reason why both oestrus and parturition might be synchronised independently.

But first a word on the study and its limitations (which the authors not only acknowledge but draw attention to). Because of logistical and limitations of working on groups of animals kept in their natural habitat in Tanzania and to meet governmental requirements it was only possible to have five animals in each of two experimental groups.

With that caveat, the results that were obtained seem clear. In the words of the authors:

Our findings represent the first experimental evidence of a synchronizing mechanism in the Serengeti wildebeest, and the first documentation of male rutting calls as a mechanism for synchronizing reproduction across a large ungulate population. Females exposed to rutting calls initiated cycles at over three times the rate of Control individuals [isolated from males] and were consequently significantly more synchronous in their expected time to mating. While male presence and/or olfactory cues have been demonstrated to be an important modulator of female reproductive function in many mammal species, our results suggest that male vocalizations alone could be sufficient to account for the empirically observed reproductive synchrony in vast wildebeest herds.

Mathematical modelling suggested that the synchronisation of oestrus was sufficient to explain the relatively short period in which the majority of calves are born. In other words, it was not necessary to invoke an additional mechanism that synchronises the onset of parturition, i.e. to shorten or lengthen pregnancy.

So the present evidence indicates that it is the rutting calls of the males that accelerates the oestrous cycles of the females and therefore synchronises the time of mating of an entire female population. That leaves another question: what triggers the onset of the rut in these tropical mammals?

Calabrese JM, Clay AM, Estes RD, Thompson KV, Monfort SL. 2018. Male rutting calls synchronize reproduction in Serengeti wildebeest. Scientific Reports (2018) 8:10202 | DOI:10.1038/s41598-018-28307-y