World Lemur Day and the Primates Running Out of Forest

An aye-aye finds its food by knocking. It walks along a branch tapping the wood rapidly with a thin, flexible third finger, listens with oversized ears for the hollow note of a beetle larva's tunnel, gnaws an opening with incisors that never stop growing, and hooks the grub out with that same finger. It is the largest nocturnal primate in the world, it weighs under 3 kilograms, and it is a lemur.

Lemurs get a day of their own at the end of October. The date is the simplest thing about them.

A Festival First, a Friday After

World Lemur Day falls on the last Friday of October, which makes it October 30th in 2026, the day before Halloween, and October 29th in 2027. It sits inside a longer World Lemur Festival that runs through the month, with events listed each year by the Lemur Conservation Network.

The festival has a documented start. The first one was held in 2014 in Antananarivo, Madagascar's capital, organized by Professor Jonah Ratsimbazafy of GERP Madagascar. From 2015 it spread to zoos, schools and online events around the world. Who first tied the day itself to the last Friday, and why that Friday, is not on record.

The 2018 Count

In 2018 more than 50 primate specialists met in Antananarivo for an IUCN Species Survival Commission workshop led by the Primate Specialist Group. They reassessed all 111 lemur species and subspecies, every one of them endemic to Madagascar, and provisionally rated 105 as threatened: 38 Critically Endangered, 44 Endangered and 23 Vulnerable. That is about 95%. Russ Mittermeier, chair of the group, called it "without a doubt, the highest percentage of threat for any large group of mammals" and for any large group of vertebrates.

Those were workshop ratings, published ahead of the formal review every Red List assessment goes through. The official update came in July 2020 and counted species only, leaving subspecies out. It put 103 of the 107 lemur species in a threatened category and 33 of them, almost a third, in Critically Endangered. Thirteen species moved up a threat category, among them Verreaux's sifaka and Madame Berthe's mouse lemur, the smallest primate in the world, both moved from Endangered to Critically Endangered.

The cause is forest. Slash-and-burn farming and logging for charcoal and fuelwood keep clearing what is left. Hunting adds to it, illegally, for species such as Verreaux's sifaka. The northern sportive lemur is down to so few animals that the 2020 update said it might already be extinct without the conservation money spent on it.

The Poster Species Is Endangered Too

The ring-tailed lemur, the face on most World Lemur Day posters and the one most people picture, is Endangered, and many of the survivors live at low density in isolated forest fragments. In the US, that same endangered status decides whether a private owner can legally keep one in some states before any state primate law comes into it.

The Lemur Nobody Recognizes

The aye-aye is the one that surprises people, because it looks assembled from other animals: bat ears, rodent teeth, coarse black fur and a skeletal middle finger. It is listed as Endangered. Habitat loss is the main pressure, and in some communities it carries an older one. Many villagers regard the aye-aye as an evil omen, fady, to be killed on sight.

Its hand turned out to be stranger than anyone had described. In a 2019 study, Adam Hartstone-Rose and colleagues at North Carolina State University dissected the wrists and hands of seven aye-ayes and found a pseudothumb in every one: an enlarged sesamoid bone with a dense cartilage extension, worked by three muscles, with its own pad on the palm complete with its own fingerprint ridges. No primate had been documented with one before. The closest comparison is the giant panda's famous false thumb.

The authors' explanation is the tapping. The aye-aye's fingers are so specialized for knocking and probing that they are poor at gripping, and the extra digit makes up some of the hold they lose. The rest of the kit, teeth, ears and finger working as one foraging system, lets it fill the niche woodpeckers hold elsewhere, on an island that has no woodpeckers.

Lemurs belong to the strepsirrhines, the branch of primates that also holds the lorises of Asia, and the branch has a habit of unlikely equipment. The slow loris is the only primate known to be venomous, mixing a secretion from a gland on its arm into its saliva. The aye-aye settled for an extra thumb. Every piece of that equipment is built for trees, and trees are what Madagascar keeps losing, to slash-and-burn farming and to the charcoal trade. A lemur's specializations are a bet on the forest staying.

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