The forests of Madagascar have a puzzling and important history. To explain their spread, scientists have turned to possibly the island's cutest inhabitants – mouse lemurs.
Madagascar is renowned as one of the world's richest sites of animal and plant biodiversity. “Current estimates hold that close to 100 percent of the island’s land mammals and native amphibians, 92 percent of its reptiles, and 90 percent of its plants are found nowhere else on earth,” observes a paper in the Proceedings of the National Academy of Sciences.
It is also notorious for being particularly under threat, adding urgency to attempts to understand their evolution.
The rainforest on the eastern side of the island and the western dry forests are thought to have once been linked. Ecologists have been keen to understand when and why these highlands-spanning connections disappeared.
"For a long time, scientists weren't sure how or why Madagascar's biogeography changed in very recent geological time, specifically at the key period around when humans arrived on the island a few thousand years ago. It has been proposed they heavily impacted the Central Highland forests," said study co-author Dr Steve Goodman of the Field Museum, Chicago in a statement. This view held that humans rapidly replaced 90 percent of Madagascar's forests with grasslands.
However, Goodman said: "This study shows the landscape was changing thousands of years before humans arrived."
Brown mouse lemur (Microcebus rufus) Ryan M Bolton/Shutterstock
To test how the forests had evolved, Goodman and his fellow authors needed a guide. They chose the world's smallest primates, the genus Microcebus or mouse lemurs, including one named after Goodman himself. Goodman claimed mouse lemurs were chosen for their short lifespan. "They reach reproductive maturity within a year, and that means that a lot of generations are produced very quickly. That enables us to see evolution at work faster than we would in an animal that took, say, five years to first reproduce."
The mouse lemurs live wherever there is forest, adapting to different forest types. Their DNA reveals that different mouse lemurs diverged more than 50,000 years ago, marking a time when two populations were physically separated, as the break-up of Madagascar's forests forced inhabitants to adapt or die.
"When we analyzed the mouse lemurs' DNA, we were able to see genetic similarities between lemur species that are closely related but today live far apart from each other. That suggests that their ancestors were able to disperse across forested habitat that no longer exists – portions of the Central Highlands that formed the bridge between the eastern and western parts of the island today," said Goodman.
Madame Berthe's mouse lemur (Microcebus berthae) is restricted to western Madagascar's dry forests, but is closely related to M. rufus, which occupies humid habitat. Dennis van de Water/Shutterstock
M. berthae and M. rufus, which now inhabit very different environments, split around 55,000 years ago when the bridges between them were cut.
Humanity's astonishing arrival on Madagascar accelerated changes to forest cover, but climatic shifts were the original cause. The forests on opposite sides of the continent are also more connected than previously thought, with Goodman describing them as “different extremes on the continuum” rather than isolated ecosystems.
Not all scientists engaging in this sort of phylogeographic analysis are as lucky as Goodman's team in their study subjects, though; similar work on northern Australia's ancient forests used dung beetles.