We humans —along with apes, monkeys, and lemurs— are primates, and exploring primate evolution helps us understand where we came from. Indigenous to Madagascar, lemurs form a unique branch of the primate family tree. But there is one important obstacle to understanding where lemurs came from and how they evolved– there are no ancient fossil lemurs. This void makes it difficult to track the stages of evolution often observed in transitional fossil forms. Making matters more confusing, but fascinating, there are remains of lemurs extinct around 2,000 years old. Those lemurs were radically different from the ones alive today. Many of the extinct lemurs were giants, with some weighing as much as gorillas! Placing them in the tree with living species has been difficult because of their unique features. To understand the factors that may place species at risk of extinction, it is important to compare the traits the extinct species had to those of living species. For my Ph.D. research, the first step was to do just that. With my committee member and collaborator, Stony Brook University Professor Dr. Liliana M. Dávalos, I studied the evolutionary history of lemurs in detail with support from the National Science Foundation. The findings are out this week in the journal Systematic Biology. For the first time since the early 1990s, I combined data from living and extinct species with cutting-edge techniques to place all lemurs in a single tree, providing the most complete tree of lemur evolution yet.
Surprisingly, the new phylogeny finds some of the subfossil species were unique branches with no living close relatives. The giant, extinct koala lemur (Megaladapis) had short limbs and long finger and toe bones, which may have allowed it to cling to branches, much as living koalas do. Megaladapis also had an extremely long snout with no upper front teeth, and may have even had long dexterous lips for manipulating food. The koala lemur was thought to be closely related to either of two living groups: the sportive lemurs (Lepilemur), or ‘true’ lemurs, like the ring-tailed lemur (Lemuridae). Because of the unique features with few similarities to living species, placing them in the lemur tree based on skeletal traits has been difficult. Fragments of ancient DNA had suggested a close link between koala and true lemurs. By combining genetic and anatomical data, the new tree finds the koala lemur was a completely extinct, unique branch of the lemur tree.
Other fossil groups are more closely related to living species, but the fossil groups were giant compared to living species. While the largest living lemur is about 22 pounds, the largest and closely related extinct lemurs may have weighed over 300 pounds! Large body size is one important component of extinction risk, and so by understanding the evolution of body size, we can better understand the factors threatening lemurs today. The implications for the future of lemurs are substantial; 95% of living lemurs are threatened with extinction. With all living and extinct lemurs together in the same tree, we can begin to understand how evolution and extinction have worked in the past, and project risks into the future.
Another way the study helps us understand primate evolution is by illuminating how geography has contributed to splitting off species. Madagascar has been an island separate from all other landmasses for at least 90 million years, long before primates first appeared. As in previous analyses of molecular clocks, the new study suggest lemurs first evolved ~50-60 million years ago. This means the ancestor of lemurs likely arrived on Madagascar by dispersal – perhaps washed from Africa to Madagascar floating on vegetation, or with some small stepping-stone islands to help along the way. A surprising new twist arises from the new lemur family tree: could lemurs have dispersed from Madagascar back to Africa? In some analyses, there was evidence a fossil primate found on Africa 34 million years ago (Plesiopithecus) was most closely related to the aye-aye, the first lemur to diverge from other species. If confirmed, this evolutionary pattern means the aye-aye and the ancestor of all other lemurs dispersed from Africa to Madagascar separately, or the ancestor of all lemurs arrived on Madagascar and this unique branch dispersed back to Africa. This finding warrants an intense focus on the similarities between the aye-aye and the African fossil Plesiopithecus to determine whether they are the result of common ancestry, or if this link is an artifact of similar ecological pressures leading to similar traits.
With this new evolutionary tree, we have a much better understanding of the evolutionary dynamics of primates and how evolution shaped the primates of Madagascar over the last 50 – 60 million years. We can now compare the factors associated with the extinction of all subfossil lemurs, and predict how living lemurs may fare in the future. This information is crucial if we hope to prevent further extinctions of these distant human relatives.
Check out the Huffington Post's piece on this article
And Forbes too!
Illustrations copyright S. Nash, used with permission
The first chapter of my dissertation to be published is now out in the Journal of Animal Ecology, detailing a close connection between the diversity of primates and the abundance of their food tree resources. Lemurs are primates endemic to Madagascar – they are indigenous and found nowhere else on earth. They are also highly threatened with extinction; 95% of lemurs are considered at risk of extinction. The primary threats to lemur persistence are habitat loss due to deforestation for agriculture, cattle grazing, logging, and mining, as well as hunting for bush meat. The new study illustrates the tight links between lemur diversity and ecosystem health.
In case this is your first time on my page, my name is James Herrera, and I am a researcher at the American Museum of Natural History. I conducted the study as a Ph.D. student at Stony Brook University. I spent two years trekking across rainforests in southeast Madagascar to count lemur species and determine their abundance in relation to habitat characteristics such as human disturbance, altitude, and tree community composition. I surveyed five different habitats with teams of Malagasy field assistants and graduate students. The study was conducted in Ranomafana National Park and an adjacent forest corridor that connects Ranomafana to the Fandriana Park to the north. Some habitats were low altitude pristine rainforests like green cathedrals of massive, broad canopy trees. Other habitats were high mountain peaks of granite, festooned with forest clinging to rocks. I also surveyed heavily disturbed sites, some of which had been logged or cultivated in the past and subsequently grew back, while others were actively being logged and mined for gold around his camp site. This variety of habitat types allowed me to compare the primate communities in relation to variation in habitat features.
The study revealed the overwhelming effect of food tree abundance on lemur diversity. While some small nocturnal lemurs eat insects, many lemurs are vegetarians, and most specialize on the fruits or leaves of just a few key tree species to obtain most of their diet. Collaborating with Malagasy botanists, I enumerated the food trees in each habitat compared to non-food trees, and found the relative abundance of preferred food trees was most closely tied to lemur diversity. Where food trees were most abundant, the lemur communities were dominated by a few large-bodied species that form large groups, while small species with small group size were rare or absent. When food trees became scarce, those previously dominant large-bodied species became rare or absent and other species increased in abundance. This result illustrates how resource limitation affects the abundance and diversity of lemurs. This finding is especially pertinent for healthy ecosystem functioning. The fruit-eating lemurs are important seed dispersers; when lemurs eat the fruits, they pass the seeds whole which germinate much better than seeds not passed by lemurs. Different lemurs specialize on different trees and disperse the seeds far from the parent trees, like gardeners sowing the future forests. Without these lemurs, the trees may not be able to reproduce as efficiently, and so lemur extinctions could have cascading effects on ecosystems.
Surprisingly, human disturbance did not have a strong effect on lemur diversity, possibly because of the indirect effects on food tree abundance. For example, while some heavily degraded sites had low food tree abundance, so did high altitude pristine forests. The study illustrates that some lemur species may be resilient to habitat disturbance, especially those small-bodied species that have small group sizes and can be sustained with lower resource abundance. The result is important for understanding how entire lemur communities will respond to future habitat loss, which is especially important because of the ongoing degradation of natural habitats. While I surveyed corridor forests that connect two national parks, migrants from far-away cities were cutting forest, damming rivers and panning for gold. The recent surge is especially troubling because it attracts bandits who rob gold miners and threaten the safety of rural villages. Further, many of the miners are immigrants from cities, not the local land owners with rights to the forest resources. Protective action is needed to suppress the burgeoning deforestation, and this corridor is especially important to maintain safe passageways for lemurs to move among parks and maintain healthy stable populations.
This study was funded by the National Science Foundation, Margot Marsh Biodiversity Foundation, Conservation International, Explorer’s Club, Leakey Foundation, Primate Conservation, Inc., Rufford Small Grants Foundation, International Primatological Society, American Society of Primatologists, La Conservatoire pour la Protecion des Primates, and the Turner Fellowship while at Stony Brook University, as well as a Gerstner Scholarship and Postdoctoral Research Fellowship from the Richard Gilder Graduate School, American Museum of Natural History. I also acknowledge the help of over 30 Malagasy research assistants, especially Tongasoa Lydia, a Malagasy Ph.D. student who worked with me during this study.