In a new article available online at the Royal Society Open Science, I tested several predictions of adaptive radiation theory for lemurs of Madagascar. This charismatic group has long been suggested to be the result of adaptive radiation, but my results suggest the long evolutionary history of lemurs may not fit all the predictions.
Adaptive radiation is a key concept in evolutionary biology, with a fantastic diversity of species arising to fill every conceivable niche. Famous examples include Darwin’s finches of the Galapagos, lizards of the Caribbean, and cichlid fishes in Africa. Madagascar, an island nation off the coast of Africa, is a biodiversity hotspot, with thousands of species found nowhere else on earth. The primates – lemurs – have long been suggested to be an example of adaptive radiation, with around 100 species alive today that have unique and diverse niches. The theory of adaptive radiation makes several explicit predictions, which have not yet been tested for lemurs. For example, evolution of morphological adaptations to fill unique niches is an important part of adaptive radiation theory. Do lemurs fit these predictions?
In a new article accepted in the Royal Society Open Science, I tested multiple predictions about the rate that species diversified, and how their body size adapted to different niches of diet and activity pattern. I also tested the possibility that a mass extinction event in the past, 34 million years ago, may have changed the rate of speciation and extinction. While theory predicts that speciation rates should be highest early in the evolution of an adaptive radiation and slow down towards the present, lemur speciation rates were either constant through time, or may have even increased slightly over time. The rate of body size evolution fit predictions better than speciation rates; the major groups of lemurs seem to have differentiated into unique niches with different optimal body sizes. Lemurs that eat leaves and are active during the day have larger body size than day-active fruit eating lemurs. There are three unique groups of small-bodied lemurs that have independently evolved adaptations for eating leaves. Additionally, there are small nocturnal species adapted for eating fruits, nectar, and insects. This diversity of niches and associated body size evolution suggests that lemurs may have evolved via adaptive radiation, despite the contrary results from speciation rates.
While there was no evidence that a mass extinction event disrupted speciation dynamics in the deep past, 17 species of giant lemurs have gone extinct within the last 2,000 years, not long after the arrival of humans around 4,000 years ago. The giant lemurs were unlike most alive today, with one species as large as a silverback gorilla. The loss of these unique species represents a significant change in Madagascar ecology and evolutionary history. This study provided explicit tests of the adaptive radiation theory for lemurs, with mixed results suggesting that body size and niche evolution fit expectations, but speciation rates may tell a different tale.
Lemur phenotypes - or their morphology, ecology, and behavior - are so diverse, they fit the prediction of adaptive radiation. While some species are small, nocturnal omnivores, including the smallest primates, the mouse lemurs, other lemurs are larger, diurnal and eat leaves. Some species are even specialized to eat bamboo with enough cyanide to kill people! The largest living lemur, the 6-10kg (12-20lbs) Indri, is tiny compared to the giant lemurs that went extinct just 2,000 years ago, which were as big as gorillas - up to 150kg, or 300lbs!
Phylogeny of lemurs and other primates, illustrating living and extinct species. Figure from Herrera and Davalos 2016, Syst. Biol. Illustrations courtesy of S. Nash, used with permission.
Strepsirrhine phylogeny illustrating the distribution of body mass (black bars next to tips, scale at the bottom, natural log transformed) and adaptive niches of diet and activity pattern (colored circles next to tips). The evolution of niches was estimated at internal nodes (colored pie charts on nodes representing the proportional probability of each state) and the best estimate at each node was assigned to the descendant lineages (colored branches). This tree was used to test how body mass evolved at different rates in different niches.