In zoos, keepers strive to preserve as much of the natural experience as possible for their animals. But not everything can be left up to nature behind zoo walls. While encouraging reproduction can be a zoo mission for captive endangered species, other species can’t be allowed to procreate without limits, lest the zoo run out of room for booming families. In primates, zookeepers turn to a familiar method of birth control – the same hormone-based contraception developed for humans. But does putting a gorilla on “the pill” change more than the animal’s ovulation cycle?

This unusual topic was the basis for University of Chicago graduate Anna Sarfaty’s undergraduate research project. For over a year, Sarfaty and her co-authors closely observed four female gorillas at Lincoln Park Zoo, keeping score of sexual, social, and aggressive behaviors to see if hormonal birth control disrupted their normal activity. Published in the journal Zoo Biology with co-authors Susan Margulis and Sylvia Atsalis, the results offer new information for zookeepers on the effects of contraception.

“Zoos don’t want to separate males from females,” Sarfaty said. “So hormonal birth control is a great option, and we know that it works since it’s been given for many years. But researchers like to understand how animals may be acting differently and understand how the behavior we’re seeing might be different from the natural world.”

Unlike most published studies, Sarfaty’s paper can name names – the stars of the experiment were Rollie, Tabibu, Madini, and Bulera, four of the seven females in the zoo’s gorilla population. Each female gorilla received birth control pills on the same schedule that a female human does – three weeks of estrogen and progestin, followed by one week of placebo pills. Under normal conditions in the wild, gorillas are known to increase certain types of sexual activities known as “estrous behaviors” in the second week of their cycle, near the time of ovulation. So researchers watched their four subjects for 20 minutes a day, four to five times a week, for over a year, to see whether the same behavioral patterns were preserved in the captive, contraceptive-fed females.

Behaviors were scored according to an “ethogram” – a dictionary of behaviors that is “more difficult to write than you would think,” Sarfaty said. The catalog, reproduced in the article, is extensive: listing everything from social play and grooming to biting and chasing to more risque actions such as mounting and masturbation. The researchers also monitored how much time the females spent in the vicinity of the group’s dominant male silverback gorilla, which is a provocative move in gorilla culture.

“Because the gorilla social system is so strict, just sitting close to the silverback male and doing nothing is still a big deal,” Sarfaty said.

The researchers used the null hypothesis that birth control would not change the timing of the gorillas’ behavior. But when the results came in, a significant departure from natural behavior was revealed. Instead of elevated sexual behavior in the second week of the cycle, the peak was seen in the first week, during the placebo phase of the pill schedule, when the animals are menstruating. To explain this deviation from natural behavior, the authors speculate that the placebo pills may allow the natural hormonal system to briefly rebound, producing a brief burst of estrogen that can promote estrous behavior during the “off week.” But with only four subjects – and no untreated gorillas to serve as a control – Sarfaty said that it was tough to draw any conclusion or alarm from the study without further research.

“We know that it’s changing their social system, so there are many directions to go with it,” she said. “I don’t think we should really be concerned, primarily because the pill is sort of necessary in captive situations, as we don’t have a better method of contraception. But there are definitely more ways for scientists to study this change in behavior.”

As for her own future, Sarfaty has moved on post-graduation to smaller animals, working at a veterinary hospital in Connecticut and planning to apply to vet school. But she’ll always fondly remember her extended time observing the gorillas, both for research purposes and otherwise.

“When you watch a group of gorillas and you don’t know much about them, you see them as animals. But when you spend a number of months with them, you see the dynamics of the group, the individual personalities, the playful behaviors of the young gorillas,” Sarfaty said. “It’s a very satisfying part of the project.”

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Sarfaty A, Margulis SW, & Atsalis S (2011). Effects of combination birth control on estrous behavior in captive western lowland gorillas, Gorilla gorilla gorilla. Zoo biology PMID: 21674603

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