Howler monkey study examines mechanisms of new species formation

December 22, 2018, University of Michigan

A new University of Michigan study of interbreeding between two species of howler monkeys in Mexico is yielding insights into the forces that drive the evolution of new species.

How do new species emerge in nature? One common but overly simplified version of the story goes like this: A population of animals or plants becomes geographically isolated—by a river that changes course or a mountain range that rises up, for example—and the two separated groups accumulate genetic differences over time as they adapt to their environments in isolation.

Eventually, the DNA of the two groups is so different that the two populations are considered distinct species. Voilà, speciation has occurred.

In reality, the process is much more complex than that. While geographic isolation can start the speciation process, evolutionary biologists believe that other forces—including various forms of natural selection—can help to complete it.

The new U-M study provides rare empirical evidence that multiple forms of natural selection, including a contentious one called reinforcement, are helping to complete the speciation process in a natural howler monkey "hybrid zone," a place where the two species coexist and occasionally interbreed in a process called hybridization.

The study is scheduled for online publication Dec. 22 in the journal Molecular Ecology. In the paper, the researchers use the primate hybrid zone to identify parts of the genome that are likely to contain genes underlying speciation and to test for signals of the selection forces that shaped them.

"We observed patterns in the genetic data suggesting that hybridization is playing a direct role in completing the speciation process by enhancing genetic differences between species," said U-M doctoral candidate Marcella Baiz, the study's first author. The other authors are Liliana Cortés-Ortiz and Priscilla Tucker of the U-M Department of Ecology and Evolutionary Biology.

"We found a signal for multiple forms of natural selection driving species differences, including reinforcement, a process that has been highly debated," Baiz said. "This result is particularly notable because empirical evidence for reinforcement is extremely rare, especially genetic evidence."

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Big monkey voice 'means less sperm'

By Victoria Gill

Science reporter, BBC News

23 October 2015 

The deep, growling roar of the howler monkey may hide reproductive shortcomings, according to biologists.

A study by an international team of scientists has revealed that the primates either develop big voices, or big testes - but not both.

Scientists made the discovery while trying to understand the "evolution of the animals' incredible roars".

The findings suggest such evolutionary trade-offs may be more common that previously thought.

They are published in the journal Current Biology.

Howler monkeys are named for their impressive howling roars - sounds they make to intimidate rivals and impress potential mates.

And their anatomical musical instrument is a bone in their throat called the hyoid bone, which acts as a resonator

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Boa Constrictor Seen Eating Howler Monkey in a First

If a snake eats a monkey in the forest and no one sees it, does it make a difference? New evidence suggests that it does.

For the first time, scientists have witnessed a boa constrictor attacking and eating a howler monkey. The finding is noteworthy since reports of primates being eaten by predators are relatively rare, according to the study, published this month in the journal Primates.

"This may cause us to rethink how vulnerable [these] primates are to predation," said Paul Garber, a primatologist at the University of Illinois, who wasn't involved in the study.

Vulnerable to predators

Predation does happen to primates and monkeys, particularly by snakes, large raptors and big cats — but it has not been witnessed very often, Garber told LiveScience. That's due in part to the fact that primates live in groups, wherein each member looks out for threats, providing "coordinated predator detection," he said. Primates also generally have good vision that enables them to spot would-be attackers. It's also possible that the presence of scientists watching primates helps drive predators away, he added.

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