How Noisy Males Control the Gnu’s Cycle

New research shows that ovulation in Serengeti wildebeests is accelerated and synchronized by the yammering of eager males

Clay's team captured 15 female gnus for study. Following controlled exposure to male mating calls in an experimental setting, the quadrupeds (and their offspring) were released back into the wild. Richard Estes, SCBI

Trundling through Tanzania’s Grumeti Game Reserve on a course to intercept a migrating herd of wildebeests is not what most people imagine when they think of scholarly scientific research. But for the authors of a newly published paper on wildebeest reproduction cues, four-by-four escapades worthy of Indiana Jones was their field work.

It was 15 years ago that this initial excitement took place. The Smithsonian Conservation Biology Institute’s Allison Moss Clay, co-lead author on the paper, was at the time a starry-eyed doctoral student, thrust into the Serengeti out of the blue thanks to an idea from the preeminent biologist Richard Estes.

Estes, who has been dubbed the "guru of gnu" for his knowledge of wildebeest behavior, had a hunch that the animals' frenzied mating periods hinged on a female physiological response to rapid-fire vocalizations from males, and invited his colleague Steven Monfort to bring Clay on a trip to southeast Africa to scope out the situation firsthand.

With permission from the Tanzania Wildlife Research Institute and the aid of a professional Serengeti veterinarian adept with a tranquilizer rifle, Clay, Estes and other researchers tracked a group of transient wildebeests across the sweeping plains, dropping 15 females with darts containing a knockout concoction of etorphine and xylazine. Estes helped the vet identify pregnant targets—ideal specimens for the experiment, since they were guaranteed to be fertile. “We were darting these pregnant females, actually in migration, from the back of a Land Rover,” Clay recalls. “It was pretty crazy.”

This week, the fruits of the team’s ambitious gnu-napping expedition appeared in the journal Scientific Reports. The paper confirmed Estes’s suspicion that the collective drone of bull wildebeests’ mating calls had a significant effect on the ovulation of females. In fact, the link between audio from rutting males and the rapidity and synchronicity of the she-beests’ menstrual cycles was so strong that the researchers tentatively concluded it was the primary mechanism by which the gnus keep themselves on schedule in mating season.

The exhilarating off-road chase was the capstone on months of grueling manual-labor groundwork. “There was no research camp when I first went out there,” Clay says. “No cellphone, no radio. It was literally in the middle of nowhere. No electricity, nothing.” In addition to getting shelter and rudimentary infrastructure set up—a single propane generator was what the group installed to supply its power—Clay and her colleagues had to erect an enclosure encompassing dozens of grassy hectares for the captive gnus.

How Noisy Males Control the Gnu’s Cycle
Unlike the white-tailed deer of North America, African gnus cannot effectively hide their calves from the eyes of predators. Their solution? Multiply in as brief a timespan each year as possible. Richard Estes, SCBI

Once the wildebeests were secured and their babies born, the experiment began in earnest. Having divided the 15 females into three groups of five, researchers exposed Group 1 (the control) to no male stimuli whatsoever, Group 2 to male vocalizations characteristic of mating season, and Group 3 to male vocalizations as well as a flesh-and-blood eligible bull wildebeest. What they found was a striking affirmation of Estes’s hypothesis: the mating call of male wildebeests concretely impacted the menstrual cycle of females whether or not a bull was physically present, speeding up ovulation by a factor of three. In the wild, this effect would guarantee mating in 80 percent of females within a three-week timespan.

Why the rush? It has to do with survival, says Clay. Wildebeests on the Serengeti do not have the luxury of hiding their young like forest-dwelling deer—out in the sunshine and short grass, the big bovids and their offspring are constantly exposed. If wildebeests mated all across the calendar year, their sparse youngsters would be picked off by predators at every turn. Gnus’ adaptive solution is to have their young all at once, to overwhelm and disorient potential aggressors.

“They go for the opposite approach,” Clay says, “and just have so many [babies] at once that it swamps the predators.” The herd has safety in numbers this way, and it becomes physically infeasible for predators to make off with more than a few of the young gnus. “If you are a female wildebeest and you have a calf outside of that peak,” she adds, “it stands out like a sore thumb, and is at a much higher risk of predation.”

For the baby bombardment strategy to succeed, wildebeests have to keep to a tight reproductive schedule. “In order for them to calve at the same time,” Clay says, “they need to get pregnant at the same time. And in order to get pregnant at the same time, they all need to produce an egg at the same time.” This is where the acceleration and synchronization of the menstrual cycle comes in. And Clay’s just-published paper illustrates that these essential effects are brought on by the characteristic chorus of hees and haws from male gnus in mating season.

How Noisy Males Control the Gnu’s Cycle
Zebras and wildebeests mingle on the Serengeti. Future research could help to make the case for gnu conservation efforts in Tanzania. Richard Estes, SCBI

How exactly the male wildebeests know when to unleash their mating cries in the first place is an enigma unto itself, and Clay’s co-lead author Justin Calabrese, also with the Smithsonian Conservation Biology Institute, is looking forward to conducting an in-depth statistical analysis of relevant literature to shed some light on that side of the equation in the coming years.

Ranking among the most abundant critters on the Serengeti, western white-bearded wildebeests—the ones studied by Clay and company—are considered a “keystone species.” They play a vital role in maintaining their ecosystem and the food web associated with it. In short, no gnus is bad news—and gnu populations are on the decline.

Clay is hopeful that subsequent, larger-scale research will help to determine whether the effectiveness of male mating calls is likely to drop off dramatically as populations wane. “If this calving synchrony is density-dependent,” Clay says, “and the density decreases, is that going to cause a precipitous loss of the population?”

Studies built on the foundation of this one could ultimately pave the way for valuable wildebeest conservationist efforts, stemming the tide of habitat destruction and hunting by humans. “If the population is dropping because of habitat loss or poaching,” Clay says, “and then on top of that the decrease in population is going to affect the efficiency of their anti-predation strategy for their young, that could seriously affect the population.”

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