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Study reveals mating strategies of critically endangered muriquis

Since Northern muriquis are one of the most endangered species of monkey in the world, choosing good mates and rearing healthy offspring is critical for the species’ long-term survival. 

To better understand the mating and reproductive patterns of this species endemic to the Atlantic forest of Brazil, a team of scientists led by the University of Texas at Austin and the University of Wisconsin-Madison has recently combined genetic analyses of fecal matter with long-term behavioral observations.

Focus of the study

Unlike most other species of primates, muriquis live in peaceful, egalitarian societies, the core of which consists of related males and their mothers. 

Since study co-author Karen Strier, an anthropologist at UW-Madison, has spent four decades investigating the behavior and ecology of these monkeys in a small, preserved area of Brazilian forest, she and her team know how to identify each individual monkey and its relatives, as well as whose poop is whose, allowing them to ask unique genetic questions.

By collecting fecal samples and analyzing them at the Primate Molecular Ecology and Evolution Lab at UT Austin, the researchers confirmed that there were no mother-son pairings, an aspect already known from previous behavioral observations suggesting that muriquis can recognize their kin and thus avoid incestuous mating. 

“I knew from behavioral observations that there was lack of competition in mating and that mothers didn’t mate with their sons or close male relatives. But the only way to know who the fathers are is with genetics,” Strier explained.

What the researchers discovered 

The genetic analyses revealed that females tended to reproduce with males who have a more diverse set of genes – known as “the major histocompatibility complex” (MHC) – coding for molecules that play a critical role in the body’s immune responses to pathogens and other environmental stressors. 

“Our finding that male sires have higher MHC diversity than expected by chance is one of the things we would expect if that diversity contributes to male fitness or is one of the dimensions of female mate choice,” said study senior author Anthony Di Fiore, a professor of Anthropology at UT Austin.

In this case, male fitness refers to the capacity of a male mate to provide offspring with genes that provide them the best chance at survival. 

Further research is needed 

Moreover, having a higher diversity in their MHC genes may also give offspring increased protection against pathogens and environmental stressors. This should make females choose mates that not only have high MHC diversity, but that also have MHC genes different from theirs. 

However, the analysis revealed that, although females seem to choose mates with higher MHC diversity overall, they don’t generally choose those with genetic variants different from their own.

Future research is needed to clarify the reasons behind this surprising finding, and identify what other factors besides MHC diversity might influence muriqui mate choice by both sexes and provide offspring the best chances to survive and thrive.

More about Northern muriquis

The Northern muriqui (Brachyteles hypoxanthus) is one of the world’s most endangered primates, native to the Atlantic Forest of southeastern Brazil. 


Northern Muriquis are unique in their social structure and behavior, demonstrating a high level of cooperation and egalitarianism. 

They live in large groups of up to 50 individuals, with little to no dominance hierarchy. This is unusual among primates, many of whom have strict social structures. Additionally, muriquis have been observed to display peaceful behaviors, like mutual grooming and sharing food, which is considered uncommon among many primate species.


Sadly, habitat loss due to deforestation and hunting have critically threatened this species, leading to their status as critically endangered. There are significant conservation efforts ongoing to protect and preserve these unique primates.


Northern muriquis, also known as woolly spider monkeys, are among the largest primates in the Americas. They are typically about 2-3 feet long (not including their tails) and weigh between 20 and 30 pounds. They’re known for their long, thin limbs and prehensile tails, which they use for balance and support as they move through the treetops.


The diet of the Northern muriqui is predominantly folivorous, meaning that they mainly eat leaves. They also eat fruits, seeds, flowers, and bark, usually spending most of their day foraging.

The study is published in the journal Proceedings of the Royal Society B Biological Sciences. 


By Andrei Ionescu, Staff Writer

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