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 

What Triggers Birth in Mammals?

I was reminded of my post from 31 August 2013 on the length of pregnancy and on animals which synchronise the onset of parturition when I read the Biographical Memoir written by Peter Gluckman and Tatjana Buklijas on ‘Mont’ Liggins*. Liggins set off a whole train of research from the 1960s onwards on the control of the onset of parturition by finding that, in sheep, the fetus provides the trigger for parturition through activation of the hypophysio-adrenocortical axis and the production of glucocorticoid. Gluckman and Buklijas describe Liggins’s discoveries very well in the context of all that was happening in research on the fetus and to who was doing what, where and why.

I kept a close eye on research on the control of the onset of parturition throughout the 1970s and 80s, in view of the link between parturition and the onset of copious milk secretion which we termed ‘lactogenesis stageII’†. It was soon obvious that, and quoting from Liggins and Buklijas, …there was no shared mechanism controlling the onset of labour across the entire eutherian clade. In some species, such as mice, goats and rabbits, the maintenance of pregnancy depended on the persistence of corpus luteum to the end, and a decrease in progesterone levels signalled the onset of labour. In other species—sheep, cows and primates including humans—the placenta took over the role of the corpus luteum, so these species were termed ‘placentally dependent’, yet even this group was far from homogeneous, with sheep and cows showing much clearer evidence of a marked hormonal change preceding parturition than did primates.

A number of large teams throughout the world, including a number of personal friends, have been working on the control of parturition in the decades following Liggins discoveries, both in collaboration and competition. However, I was surprised to find that there is a stark truth. Returning to the Biographical Memoir:

Writing in the 1990s, Liggins summed up the results of the decades of research into the physiology of human parturition: first, rather than either the fetus or the mother occupying the leading role, the mechanism involved an interaction between the two participants, fetus (chorion) and mother (decidua); and second, the synthesis and release of prostaglandin F2α from decidua was a key event in the onset of parturition. Little has changed since that time, although we now know much more about the complex paracrine interactions between the amnion, decidua and chorion in driving this process and of the possible role of placental corticotrophin-releasing factor.

In short, we still do not know the trigger for parturition in any species or indeed whether there is a single trigger in a particular species. Those animals that synchronise parturition, like the wildebeest and banded mongoose, provide an intriguing glimpse into control mechanisms that, as far as I am aware, has not yet been exploited.

I end on a large ‘phew’. The link between parturition and lactogenesis stage II is much simpler.

Wildebeest (Connochaetes taurinus). Masai Mara, Kenya, 1991

*Gluckman, P., Buklijas, T. 2013. Sir Graham Collingwood (Mont) Liggins. 24 June 1926 — 24 August 2010. Biographical Memoirs of Fellows of the Royal Society 59, 195-214.

†Fleet, I.R., Goode, J.A., Hamon, M.H., Laurie, M.S., Linzell, J.L., Peaker, M. 1975. Secretory activity of goat mammary glands during pregnancy and the onset of lactation. Journal of Physiology 252, 763-773.