Gorillas have sex, too!

When Science Met Nature…

Studying gorillas is nothing if not glamorous, as you can clearly see; PC: Kathryn Jeffery

Jessica’s Science Blog- 11.01.17

Gorillas have sex, too!

Kingo, a silverback male western lowland gorilla (Gorilla gorilla) at Mondika, copulates with Mama ti Seysa, one of six adult females in his group; PC: Jessica Lodwick

Naturally, gorillas have sex from time to time. They, like us, are sexually reproducing primates in the great ape family, Hominidae. Gorillas are non-seasonal breeders, which means that females conceive throughout the year. Within gorilla groups, mating occurs during the period in which a sexually mature female enters into the fertile phase of her reproductive cycle. This physiological phase is referred to as estrous. During the ovulatory window of estrous (usually 2-3 days/month when she’s cycling), she signals her interest in copulation via a choreographed sequence of displays, postures, and gestures. Solicitation behaviors include staring, branch-waving, and lip-tightening, and approaching. The male may also solicit copulations with receptive females using a similar sequence of behaviors. I must admit, a silverback male’s tight-lip stare is fairly formidable.

I found it somewhat exciting and entertaining to watch a sexual solicitation among gorillas unfold. The act of copulating, on the other hand, was usually pretty mechanical and perfunctory, lasting a maximum of 1-2 minutes, with the female laying belly face-down on the ground and the male mounting her from behind. Copulations may occur anywhere between 1-12 times/day when females are receptive.

Kingo, a silverback male western lowland gorilla copulates with Eboka, another adult female in his group; PC: Jessica Lodwick

Interestingly, pregnant females also copulate with the group silverback, as has been shown in a 2009 paper by Doran-Sheehy and colleagues. It has been hypothesized that they do this to spite fertile, lower-ranking females, who also solicit copulations with the silverback. This spiteful behavior may at first seem like a pointless exercise. Isn’t there enough sperm to go around, as in chimpanzee society? The answer is a bit unclear and hard to test. However, considering the small size of gorilla testicles and the number of copulations/day that the male engages in with more than one female. sperm may actually be in limited supply for female gorillas during certain periods.

It’s completely unclear to me (and I would assume everyone else) how female gorillas view sex. A silverback male is typically more than twice the body weight of an adult female. In light of this extreme sexual dimorphism in body size, I couldn’t help but wonder if female gorillas feel like they’re being crushed under the weight of the silverback during sex? I will never know the answer to this question because I can’t interview wild gorillas.

Female-female mounting behavior: Eboka (top) mounts Mekome (bottom), another adult female in Kingo’s group; PC: Jessica Lodwick

 

Another question I’ve pondered is why female gorillas sometimes engage in homosexual behaviors. Over the course of three years, in which I observed habituated wild western lowland gorillas at the Mondika Research Centre, I recorded instances of female-female mounting, face-to-face presentation with genital contact, loud vocalizing, and one brief observation of what could best be described as cunnilingus from one adult female to another. Were these types of inter-female sexual behaviors performed to attract male attention and increase chance of copulation? If so, who was initiating these behaviors? Were they typically initiated by pregnant females (e.g. in an effort to outcompete their lower-ranking opponents for access to sperm)? The answers to these questions, at present time, remain a mystery.

Anecdotally, I remember that the male quickly descended a feeding tree after two of his females (who were on the ground) were observed to be engaged in female-female mounting while loudly vocalizing. He solicited and completed a copulation with the higher-ranking female on that occasion. The function of female-female sexual behavior as an advertisement to the male would be interesting to explore further, but sample size would be a problem since I only observed intrasexual behavior when two females were receptive to mating at the same time.

Adult female western lowland gorilla uses walking stick to gauge water depth at Mbeli Bai, Republic of Congo; PC: Thomas Breuer

Finally, we come to the issue of whether there is variability in sexual behavior between gorilla groups or populations. Gorillas are intelligent animals, who have been observed to make and use tools in the swamp. (I recommend you check out Breuer et al. 2005 in PLoS Biology for more details). It would be difficult to imagine a sort of stasis in sexual practice across wide geographic distances, if not for any other reason than apes are innovators. Cultural behaviors have been documented in chimpanzees and orangutans, and potential cultural traits have been described in gorillas. Though this has not yet been shown, I would hypothesize that sexual behaviors and/or positions might in fact be good candidates for cultural traits in gorillas.

The reason I say this rests on an observation made at Mondika by one of my colleagues. A young adult female gorilla had recently transferred into the Kingo study group. This meant that she had previously been living in another gorilla group in the study area. One day, in the course of soliciting a copulation with the male she presented herself to the male lying on her back rather than on her belly. Mind you, I had NEVER seen a female in Kingo’s group present herself for copulation in this manner. She was a new member of the group. The silverback then mated with her in a face-to-face position.

Face-to-face mating in wild western lowland gorillas at Mbeli Bai, Republic of Congo; PC: Thomas Breuer

Mekome (left) and Eboka (right), two adult female western lowland gorillas, stare, approach, and perform a tight-lip display at one another. These behaviors immediately preceded female-female sexual behavior; PC: Jessica Lodwick

Note: Face-to-face mating in gorillas also been observed at Mbeli Bai, another lowland gorillaresearch site in Congo. Was this a case of cultural transmission from one group to another? Another mystery that lacks sufficient data to test, but an intriguing possibility- that females may innovate and spread their behavioral variants to other groups when they transfer. Female gorillas may secondarily transfer between multiple breeding groups in their lifetime, so there is plausible mechanism for transmission of cultural behaviors across gorilla groups.

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Animal Behavior is Fascinating!

Adult female western lowland gorilla uses tool to fish for herbs at Mbeli Bai; PC: Thomas Breuer

I hope you’ve learned some new things about animal behavior in this latest installment of my blog! I seek to inform, engage, and entertain you with my true-life stories, observations, and experiences as a behavioral ecologist. Behavioral traits, just like morphological traits, can evolve over time to suit the environment in which animals live. If the evolution of a certain behavior affords a distinct advantage to individuals within a population in terms of survival and reproduction, it should be expected to persist, should it not?

Furthermore, cultural behaviors in animal societies can be transmitted from one generation to the next through social learning. Like humans, ape mothers model all sorts of behaviors for their offspring- from foraging techniques, to tool use, to grooming and gestural communication. Sexual practices in gorillas may be learned, but just how this learning is achieved remains a matter of interest.

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If you liked this post, be sure to check out my previous blog entries on When Science Met Nature….

OCT: Frogs and spiders, oh my! Surprising food web dynamics in neotropical swamps

SEP: The Long Trail

JUN, JUL, AUG: Tales from a far-flung forest: parts 1, 2, and 3,

MAY: We can be heroes! Finding your community on iNaturalist

FROM 2016: The health benefits of yoga: A scientific perspective.

Bonus image: Did I ever mention that central African forests are home to a great diversity of colorful butterflies? PC: Kathryn Jeffery

 

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