Madagascar’s combination of unique organisms and the threat of extinction they face due to human activities makes it one of the world’sbiodiversity hotspots. All of the world’s approximately 100 surviving species of lemur are endemic to Madagascar, as were the 17 species that have gone extinct to date. These extinct lemurs were “giant,” weighing more than the biggest living lemur and ranging in size from “baboon” to “silverback gorilla.” Researchers have uncovered a wealth of information about how these lemurs lived using ample evidence from subfossil material (when bones are too young to have completely fossilized and still contain some organic matter). They were almost all diurnal and had large areas of occurrence relative to lemurs today. Palaeopropithecus ingens ate leaves and fruit, and was adapted for slowly climbing trees and extended periods of suspension, earning it the name sloth lemur. The monkey lemur, Hadropithecus stenognathus, was omnivorous, terrestrial, and lived in open grasslands.
These diverse giant lemurs were all alive when humans arrived in Madagascar 2,300 years ago. The monkey lemur persisted until as recently as 500 years ago and may even live on in oral histories of people from southeastern Madagascar. So, what happened to the giant lemurs? We know from the spores of fungus that only grows in the dung of very large animals that Madagascar’s megafaunal population crashed a suspicious 500 years after human arrival and continued to decline over the next few centuries. The most damning evidence is marks on hundreds of subfossil bones showing that humans hunted and butchered the giant lemurs. Further, which lemurs disappeared also indicates that hunting played a major role in early extinctions. Large, diurnal animals are ideal prey for humans, thus hunting probably was the catalyst and major driving force behind giant lemur extinctions.
Conversely, if lemurs needing large home ranges or those relying on patchy resources like fruit had been overrepresented among the extinct, then habitat fragmentation or disruption might have been a more likely trigger. That’s not to say that human-driven habitat fragmentation isn’t a huge problem for lemurs today or that it didn’t help the extinction process along once human predation set it in motion. Charcoal and pollen records indicate that human efforts and natural climate drying and warming caused dramatic changes in vegetation across the island during a major drought 950 years ago, coinciding with many of the first extinctions. Hunting and the deforestation process probably had synergistic consequences for the lemurs: removing so many big herbivores from the environment made forests denser and more prone to severe fires, making permanent the conversion of forests into unsuitable grasslands habitat. The giant lemurs also had relatively slow reproductive rates, which meant that populations were slow to recover from this host of stressors.
It is easy enough to say that the largest living lemur, the indri, is at high risk of extinction due to its body size based on past evidence, but what other species are at extreme risk given what we know about extinction? Sustained habitat loss has probably become the predominant force pushing living lemurs to the brink, and satellite images from the last 60 years indicate that this process may have accelerated. The aye-aye has the largest home range of any lemur, the slowest reproductive rate, and genetic diversity more like the giant lemurs’ before they went extinct than other living lemurs. None of those traits bode well for animals surviving in diminishing forests.
The most recent lemur listings on IUCN Red List don’t all reflect the current level of effective protection accorded to the protected areas in Madagascar. Political instability has increased in Madagascar since a coup in 2009, thwarting many conservation initiatives that were driven by national politics. Worse yet, early work documenting changes since the coup indicates that illegal deforestation and human incursions into Masoala National Park have spiked, and hunting of lemurs has again become common. We also know now that lemurs are living in non-preferred habitat through a process called ecological retreat, in which animals take refuge in places they are not necessarily adapted to in order to avoid humans and other pressures. Without more work to curb these pressures, the aye-aye and indri both may go the way of their giant relatives.