If you watched the video from the previous blog post, you may have some questions about a certain part of the male chimpanzee anatomy. Think of this post as Everything You Wanted to Know About Chimp Testicles, But Were Afraid to Ask.
Burrito is not unique – male chimpanzees have rather large testicles compared to other apes. And there’s a very interesting explanation for this. Most people are familiar with the concept of males competing for access to females. For example, male gorillas will compete for control over a harem of females – whoever controls the harem has primary mating access. But female chimpanzees will regularly mate with multiple males during estrus (see this recent article for an interesting take on chimp mating behavior). Therefore, much of the competition between males takes place between their sperm within the female’s reproductive tract.
If you think of mating as a lottery, with the prize being a fertilized egg, then whoever can produce the most sperm is essentially buying the most lottery tickets. And whoever has more tickets has a better chance of winning. There are also other forms of sperm competition besides this sperm “arms race”. Chimpanzees also produce sperm plugs (also called copulatory plugs). After mating, some of the semen will coagulate to plug up the cervix, so that sperm from future mating (with other males) will be less likely to reach the egg.
So for chimpanzees, it pays to produce a lot of semen, and that requires large testicles. Now you know!
Julie Harding says
OMG….the things that make you go “hmmmm”!! Thanks, J.B. for that insight! I laughed a bit to myself (ok…out loud) because the last time I was volunteering in the chimp house I got a good view of Burrito’s “stuff” and commented to my daughter, Sarah, about it. We both had a little chuckle over it! I so appreciate the eduction you give…..and knowing if half the battle!!
Candy (Tyler, Texas) says
J.B., since you’ve opened this forum, here are my questions:
1) Is there a difference in testicle size between captive chimpanzees and wild chimpanzees?
2) In the Arnhem Zoo fighting incident, long ago, the male victim had lost his testicles in the fight. In the attack on St. James Davis he lost his, is this a common or uncommon result in chimpanzee fights?
I find it interesting that the two Arnhem Zoo male chimpanzees removed them, it’s as if they knew that’s what made the other chimp, a male.
I also like the male chimpanzee version of birth control, copulary plugs.
As far as I know, there is no difference in testicle size between captive and free-living chimps. My understanding is that the equipment itself has evolved through natural selection to provide the male chimpanzee with the capability of producing large amounts of ejaculate, whether or not it is needed. But there is evidence, at least in humans, that the volume produced is variable. For example, male humans have been shown to produce more ejaculate during sex with their partners after they have been physically separated for some time, with the theory being that his sperm may need to compete if his partner copulated with someone else during his absence.
It is very common for chimpanzees to attack a male’s scrotum during a fight. Are they doing this due to the reproductive nature of the testicles (i.e., to keep the victim from mating)? I don’t know. It is possible, and there probably is some research out there on the subject. But when chimps fight, they tend to go after extremities like fingers, toes, and ears, so it may simply be that the scrotum falls into the category of “things that can be bitten off”.
Wow, how fascinating! I love learning about chimps. When I was at Chimp Eden, mating occurred during estrus of course but it also occurred manytimes to calm the males down. Jinga, the alpha female would present to the upset male, most often Nikky, and he would mate with her. He would calm down after every episode. It seems that Jinga understood that allowing the males to mate would calm them down quickly and thus prevent fights in the group. Pretty smart girl if you ask me!The first time I saw the males in person I was amazed at the size of their scrotums. I find it extremely interesting that the sperm will coagulate in order to prevent conception by another male. Although Amadeus, at Chimp Eden, is the alpha and technically the only one who has the right to mate, he has never minded Nikky mating with the females. I was told it was likely because they have grown up together and the rules are more lax with captive chimps. It’s my understanding that in the wild this rule is enforced by the alpha. Thanks for the lesson, J.B. Very interesting!!
leslie sodaro says
that was very interesting, good to know that they are more than just advertising … and why did i get the idea that burrito was letting jb know exactly what he was packing. re: fighting and items that can be bitten: i would suspect that to have your bit-ness bitten would be very painful and give the biter an edge. as for chimps having sex outside of estrus, bonobos (the ‘other chimp’ … not the common chimp) always engage in sex as a tension reliever, greeting etc. it is said that the bonobo is the ‘better’ part of their cousins (us) personality … much less aggressive than the common chimp or the common man …
Amy M. says
JB — I want to compliment you not only on the content of your post but also for the variety of metaphors you wove in. 😉
Julie Harding says
I’ve learned so much from reading these blogs!! Thank you, Theresa, for your comments about your experience at Chimp Eden….and thank you, J.B., for furthering our my education.
Oh I can’t wait to bring this topic up tomorrow in the office!!!
I learned while squirming and crossing my legs! 🙂
So the question is, have the CSNW females let Burrito “use” his “parts” yet, or has Mr. B not yet mastered the art of Chimp politics?
MS – Early on, we witnessed Burrito attempting to mate with Jamie. It seemed to us that Jamie knew what to do, but Burrito was unsure. They never successfully completed the act, as far as we could tell. Since then, we have not seen any more attempts.
It’s not uncommon for captive chimpanzees to have difficulty or lack of interest engaging in intercourse despite showing signs of sexual arousal. Whether this is due to environment or upbringing, I can’t say. But it’s easy to imagine how being torn from your family, raised amongst members of another species, and kept in a cage for decades could have such an effect.
Obviously, most of the females in the group here at CSNW know what to do – they were used as breeders by the research industry and all but Jamie had multiple babies. Artificial insemination has been used for captive chimpanzees, but most breeding in laboratories takes place naturally (I hesitate to use the word “naturally” here, because laboratory life is anything but natural. But you know what I mean).