The explosion of new lemur species that began when the first of those animals arrived on Madagascar more than 50 million years ago has not died down, according to scientists who've identified three different groups of lemurs with high speciation rates.
The research led by Katie Everson of Oregon State University provides evidence that lemurs continue to defy the evolutionary principle that says rapid species expansion is followed by a slowdown. Published in Nature Communications and funded by the National Science Foundation, the study sheds new light on a primate under extreme extinction pressure.
"At the same time that more and more lemur species are still evolving, we are also driving them rapidly to extinction," said Everson, assistant professor of integrative biology in the OSU College of Science. "Ninety-five percent of lemur species are considered threatened."
There are more than 100 extant lemur species, plus at least 16 that have gone extinct over the last 2,000 years in the wake of human settlement on Madagascar. Lemurs are endemic to Madagascar, and because of that the island east of Africa, roughly the size of Oregon and California combined, is home to 15% of all primate species.
"When people think of lemurs they often think of the famous ring-tailed lemur, but lemurs take many different forms," Everson said. "They include the smallest primate in the world, Madame Berthe's mouse lemur, which is only 3.6 inches long and weighs 1 ounce, and an extinct lemur that was as large as a gorilla."
Lemurs are strepsirrhine primates, a suborder that branched off earlier in primate evolution than the haplorhines - monkeys, apes and humans. They have large eyes adapted for low light, and moist noses that enhance their sense of smell. Strepsirrhine primates are also known for having a claw and teeth specialized for grooming.
Lemurs arrived on predator-free Madagascar 53.2 million years ago according to the phylogenetic analysis led by Everson, whose team sequenced DNA from 129 animals. After reaching the island, likely floating from Africa on natural rafts, lemurs followed a course of adaptive radiation. Their evolutionary tree branched into a plethora of species well suited to the island's diverse and isolated habitats, which include rain forests, dry deciduous forests, spiny forests, montane forests, coastal forests and mangroves.
"But our research finds that this is not the whole story - lemur species diversity is still exploding today after two millennia of human presence," she said. "There are three specific groups of lemurs - mouse lemurs, sportive lemur and brown lemurs - where speciation rates are currently very high."
Speciation rate refers to the speed at which new species form and is often expressed as new species per 1 million years. Understanding the speciation rate helps scientists calculate how fast biodiversity is generated, can show patterns of evolutionary stasis, and can be used to compare different animal groups - such as lemurs and their primate sisters: the lorises, bushbabies and galagos that live on continental Africa and Asia.
The scientists found that diversification among those animals happens much more slowly, "suggesting that something special is happening on Madagascar," Everson said.
"The lemur clades with high diversification rates also have high rates of genetic material from one species becoming part of the gene pool of another, a phenomenon known as genomic introgression," she said. "That suggests that hybridization in these primates is not an evolutionary dead end, as it often can be, but potential fuel for diversification."
Collaborating with Everson were scientists from the University of Kentucky, the University of Texas San Antonio, Duke University and SUNY Binghamton, as well as several international researchers.
