A new study found that while chimps sleep around a lot, they’re pretty selective about who they make little chimps with. The team found that these primates are more likely to conceive with the individuals that most differ from them genetically.

Image credits Pascal Renet / Pexels.

A couple of days ago we’ve talked about how humans tend to pick their mates after similarities in genome — a practice called assortative mating. From a biological point of view, it gives couples a higher chance of passing on desirable traits to their offspring, such as height, intelligence, and so on. It does, however, also come with potential drawbacks.

So let’s take a look at the opposite mating strategy — negative assortative mating. Postdoc associate in evolutionary anthropology Kara Walker and her team at Duke University found that chimps, our closest living relatives, are more likely to reproduce with genetically-different mates.

They took DNA samples from roughly 150 adult chimps from the Gombe National Park in Tanzania, and examine between 8 to 11 variable sites in their genome. They used this data to estimate the genetic similarity between every possible pairing of mates.

Getting some strange

Chimps get down a lot. But their adventures don’t always lead to offspring. The team compared the pairings that produced infants to those that didn’t, and found that females conceived with males that were genetically less similar to them than the average male. They were somehow able to determine genetic similarity among unfamiliar mates who were far removed from them in the family tree, the team concluded.

The female chimps of Gombe NP usually leave their family group when they reach adolescence, seeking a new group (with new males) to reproduce with. These females, even though they had few or no male relatives in the community they immigrated to, showed an even stronger preference for genetically-dissimilar mates than the native females. The researchers say that the females’ mate choices are driven by inbreeding depression — which is the drawback I was referring to earlier.

When two genetically-similar individuals have offspring, they have a higher chance of passing on beneficial genes — but they also have a higher chance of passing on harmful one. In the absence of another gene version to override it, this harmful gene will become active. Over time, the process increases whole populations’ vulnerability to certain pathogens or environmental factors. That is inbreeding depression in a nutshell — a whole group sharing one or more Achilles’s heel because everyone is related to everyone else.

Gene-dar on the ready

Instinctually, this is what makes you and chimps not cool with parent-offspring or sibling-sibling pairing. Such pairings are rare in chimps, and when it occurs it’s less likely to produce individuals that survive to maturity than their peers.

However, while we can determine the genetic makeup of our mate through DNA tests (you should probably not suggest that on your first date), chimps can’t. The researchers are now trying to find out how the chimps can recognize genetically-distant mates. They suggest that the primates do more than simply avoid potential mates they grew up with, being able to distinguish even among unfamiliar partners. It’s not sure yet exactly how they discriminate but it might be a best guess based on appearance, smell, or sound, said professor of evolutionary anthropology at Duke and senior author of the paper Anne Pusey.

Timing could also play a factor. The females might be pickier about partners during their most fertile period. The team is also considering processes that take place after mating, such as a female unconsciously choosing some males’ sperm over others or influencing the outcome of a pregnancy, Walker said.

The full paper “Chimpanzees breed with genetically dissimilar mates” has been published in the journal Royal Society Open Science.