Madame Berthe's Mouse Lemur

MammalMadagascar

20 May

Microcebus berthae

Madame Berthe's mouse lemur is the smallest primate in the world. On average, it measures 10 centimetres (3.9 in) plus a 13-centimetre (5-in) tail and weighs just 33 grams (1.2 oz) — lighter than a golf ball.

The Primate Scale

We humans are among the largest living primates. With an average weight of 62 kilograms (137 lb), we don't quite top the scale — gorillas and orangutans tend to weigh more ¹ — but with our straight-backed postures, we stand eye to eye with gorillas and look down upon orangutans, as well as our closest relatives, chimpanzees and bonobos. We, along with the aforementioned primates, are known as Great Apes — "great" in intelligence as well as body size — a group that includes eight living species. The vast majority of primates, over 500 species, aren't nearly so "great".

That doesn't necessarily mean they're small; some of the heftiest male mandrills and baboons achieve weights of over 30 kg (66 lb). From there, however, the "lesser" primates only shrink. There are species large enough to still be intimidating, like the Tibetan macaque, proboscis monkey, and black howler. Then there are the long and lanky species like the langurs, gibbons, and spider monkeys. Capuchin, tamarin, and squirrel monkeys could sit comfortably on your shoulder. Tarsiers and marmosets could fit in your hand.

On the island of Madagascar, live the lemurs. Consisting of over 100 species, they range in size from the indri (~8kg / 18 lb) to the diademed sifaka (~6.5 kg / 14 lb), the black-and-white ruffed lemur (~3.5 kg / 7.7 lb), the ring-tailed lemur (~2.5 kg / 5.5 lb) to woolly lemurs (~1kg / 2.2 lb), sportive lemurs (<1kg / 2.2 lb), dwarf lemurs (<600 g / 1.3 lb) and the smallest of all, the mouse lemurs.

The Smallest of the Small

Over 20 species of mouse lemurs live in the forests around Madagascar. In an area of the Menabe region, within western Madagascar, lives the smallest of the small. Crouched on the palm of your hand, Madame Berthe's mouse lemur would feel less substantial than a golf ball, weighing only 33 grams (1.2 oz). And it could comfortably fit in your hand — from head to rump, it measures only 10 centimetres (3.9 in) long, plus a 13-centimetre (5 in) twirling tail. Not only is it the smallest mouse lemur, but it's also the smallest of the over 500 living primate species.

Just as its stature is tiny, so is its range. The species is restricted to small forest areas comprised of dry deciduous trees, particularly the Kirindy Forest and some surrounding regions. And its little home is no paradise. Its habitat is ever-changing; temperatures rise and drop within a single day — with fluctuations of up to 13°C (23°F) between day and night, quite drastic for a tropical climate — while a year brings five months of pouring rains and seven months of brutal drought, accompanied by extreme food scarcity. On top of that, the forest isn't lacking in predators, from boas to barn owls to the island's apex hunter, the fossa. How is this pint-sized primate expected to survive?

Honeydew & Hunters

A Madame Berthe's mouse lemur forages alone in the trees by moonlight. It scopes out the night world with giant brown eyes, each equipped with a tapetum lucidum — a structure that reflects light back through the retina to improve night vision ² — to find its particular, somewhat peculiar, favourite snack. This mouse lemur is an omnivore, but its diet is highly specialised; most of its calories come from "honeydew", a sugar-coated name for a sweet secretion exuded by a species of planthopper larvae.³ This mouse lemur also hunts, specifically for arthropods, geckos, and chameleons, although it's likely limited to smaller species or youngsters of the latter, as some chameleons within its range, such as the Malagasy giant chameleon, outweigh the mouse lemur by some fifteen times. The remainder of its diet, about 2%, is made up of gums, flowers, and fruits.

This mouse lemur clambers through the canopy, barely bending each branch with its insignificant weight, as it forages some 10 meters above the ground. It grabs onto handholds with long, spindly, naked fingers, each tipped with bulbous pads that improve grip and provide it with tactile information. Its large, rounded ears are alert to the soundscape around it; perhaps listening for the scuttle of a tasty bug or, more urgently, the sound of a stalking predator.

A white-browed owl (*Athene superciliaris*).

A Malagasy tree boa (*Sanzinia madagascariensis*).

With a short coat of reddish-brown, orange-tinged fur and a creamy underbelly, Madame Berthe's mouse lemur is fairly inconspicuous against the similarly coloured tree bark. A person on a night safari in the Kirindy Forest would have to be very observant or very lucky to spot one; a speck in the branches far above. Predators are better equipped. Owls — such as the white-browed and Madagascar owl — search for prey from their perches, their eyes more keen in the darkness than even the mouse lemur's. Other predators don't rely on their eyes. The Malagasy tree boa has thermoreceptive pits near its mouth that can detect the heat radiating from a warm-blooded mouse lemur. Civets and mongooses follow trails of scent through the trees. The fossa, a unique cat-monkey-weasel hybrid, is also a danger — although, to this apex predator and Madagascar's largest carnivore, a mouse lemur is scarcely more than a bite-sized snack.

Sleeping as a Social Activity

Madame Berthe's mouse lemurs spend most of their active hours on their own, but after a long and lonely night of foraging and avoiding predators, many will seek comfort in one another. As the rising sun signals bedtime, several mouse lemurs converge on a sleeping site — an open nest of leaves, vines, bark, and branches — huddle together and doze off. These mouse lemurs are pretty laissez-faire about their sleeping arrangements and partners; sleeping groups can include members of both sexes, with some groups retaining the same members night after night, even when changing to a different nest, while others shuffle around every night.

Each mouse lemur seems to differ in its level of sociality. Just as people fall on a scale between extroversion and introversion, some individual mouse lemurs are more likely to participate in cuddle sessions while others prefer to snooze on their own. The benefits of group napping aren't exactly clear. Perhaps the lemurs huddle for warmth or gather to protect themselves from predators. Maybe there's simply a shortage of good sleeping spots, or maybe it's just nice to have some company.

Smelly Sign Posts

Humans are pretty oblivious to the smells wafting through the air around us. In a way, we are "blind" to an entire sensory world. We can smell, to be sure, but just as a mole, which sees in vague outlines of light and dark, may miss out on the intricacies of the visual world around it, we miss most of the nuances of the scents we inhale. To us, urine is a waste product with a smell we tend not to, or try not to, notice (unless we've recently had asparagus), and we don't typically think of saliva as "scented" unless someone has bad breath. To a mouse lemur, however, the urine and saliva of other individuals are chemical codes, akin to smelly Post-it notes.

A mouse lemur has no scent glands, but it'll use any available bodily secretion to create its messages, from saliva to faeces — scent-marking most often when leaving its nest for the night. One common method is called urine washing, wherein a lemur rubs urine on its hands and feet, and then, like an unhygienic finger painter, uses them to create scent marks on selected branches. Alternatively, a lemur may employ a mouth wiping technique, where it wipes the corner of its mouth and face along a branch to leave behind its scent.

Relative to its large eyes and ears, the mouse lemur's nose looks tiny — a little dot at the end of its short snout — but if you look closer, you'll find a well-developed rhinarium (the furless skin surrounding the nostrils of many mammals), and if you dig deeper, all the way to the mouse lemur's genome, you'll find the largest repertoire of functional vomeronasal (VNO) receptor genes among all primates.⁴ Scent-marking among mouse lemurs probably communicates more information than we know. The scent marks can be used to identify individuals, and, given that mouse lemurs have complex urinary chemistry that is distinct between species, the marks could potentially help mouse lemurs discriminate between close species (to avoid unproductive encounters). Scent-marking is used to establish territory and dominance between males and to leave behind warnings of danger. And, for mature mouse lemurs during breeding season, scent marks become advertisements of virility and availability.

Ballsy Little Lemur

Great ape sexual organs, compared for size.

This mouse lemur holds another record among the primates — this one for something atypically big. Madame Berthe's mouse lemur has the largest testicle-to-body-size ratio of all primates.⁵ This isn't just some incidental quirk; the size of a primate's package, specifically their testes, can often tell us a lot about their mating system. Gorillas, for instance, might be the largest of all living primates, but their testicles are smaller than those of the average human. On the other hand, male bonobos and chimpanzees are smaller than your typical male human, but their testicles can be 3-10 times larger than those of a man.

The size of a primate's testicles correlates roughly with the primate's level of promiscuity, in the sense that, primate species with a low level of promiscuity, like gorillas (in which one male holds the mating rights to the females in his harem) or gibbons (who have only one mate at a time) tend to have smaller testicles, while those species in which mating is a free-for-all, as seen in chimps and bonobos, tend to have larger testicles. The reason is pretty simple: sperm competition. In promiscuous species, wherein multiple males mate with multiple females, the males that can produce the largest amounts of sperm are more likely to impregnate a female and have offspring. Whereas species that practice monogamy (gibbons) or polygyny (gorillas) don't need to invest in big balls with lots of sperm, since rival males aren't likely to mate with the same females.

The mating habits of the world's smallest primate have been hard to observe, but if its balls tell us anything, it's that, in Madame Berthe's mouse lemurs, competition among males is high, as is the promiscuity of the females. Come the start of the rainy season in November, male mouse lemurs expand their home ranges in a bid to locate as many fertile females as they can. For their part, the females in estrus will increase the amount of genital scent-marks they make so that males can sniff them out. After two mouse lemurs have mated, they go their separate ways, with the father unlikely to play any role in raising his child.

It's easy to mistake an adult mouse lemur for a baby animal, as the proportions of a mouse lemur are babyish — a large head relative to its body, bulbous eyes, and a little nose — and of course, there's the fact of its diminutive size. An actual baby mouse lemur amplifies these traits to an almost absurdly adorable degree. A mother mouse lemur likely cares for her young until they're sexually mature, whereupon they strike out to establish their own territories in patches of ever-shrinking forest.

A Permanent Torpor

As of a 2018 assessment, the range of Madame Berthe's mouse lemur covers only 348 km² (216 mi²). That may seem like a lot of land for such a little primate, but compared to the natural ranges of other mouse lemur species, it's a minuscule speck on the map of Madagascar. And that shrunken range is only shrinking further. From 2000 to 2023, Madagascar saw a 29% decrease in forest cover, with a significant amount of tree loss occurring in and around the home of Madame Berthe's mouse lemur. Currently, the species is threatened by illegal logging, deforestation for charcoal production, and slash-and-burn agriculture.

The forest in which it lives, you'll remember, was already a tumultuous place before the arrival of large-scale industry. This fact is reflected in the mouse lemur's ability to enter torpor — reducing its body temperature and metabolism — during periods of drought, cold weather, and food scarcity. But no torpor, however deep or long, can save it from the razing of its home. Madame Berthe's mouse lemur is considered a critically endangered species with a decreasing population.

For millennia, this smallest of primates has survived fluctuating daily temperatures, seven months of drought and famine, and attacks from snakes, raptors, and other peerless predators. Today, it faces our fire and metal. If the onslaught continues, we'll forever lose our tiniest simian relative.

¹ Eastern gorillas, the largest living primates, weigh in at 81 kilograms (178 lb) for a female and 163 kilograms (359 lb) for a male, with the latter able to weigh as much as 250 kg (551 lb) — heavier than four average humans.

Orangutans are trickier to rank. A female orangutan weighs just 37 kg (81 lb) on average — less than the average female human — while the average male orangutan weighs 86 kg (191 lb) — heavier than the average human male.

Chimpanzees weigh between 32–60 kg (70–130 lb), smaller than your average human (although certainly not weaker), and bonobos tend to be smaller still.

Of course, individual humans, just like individual chimps or gorillas, come in many different sizes, far to both sides of the average. As a species, we've seen an increase in average size — that is, height — over the past century, but we've also seen a much more drastic increase in weight thanks to the dramatic change in food availability in many parts of the world. In the past century, we've seen humans who've weighed more than twice as much as the heaviest eastern gorillas. The heaviest man ever recorded, for instance, was an American named Jon Brower Minnoch, who weighed 635 kilograms (1,400 lb) at his peak.

The planthopper *Flatida coccinea*, from which Madame Berthe’s mouse lemur gets its “honeydew”.

² The tapetum lucidum is what causes animals' eyes to glow in torchlight or a camera flash. You also see this structure in deer, horses, cattle, squirrels, ferrets, dogs, cats, as well as some frogs, reptiles, and fish. Humans do not have tapetum lucidum (the red-eye effect in photos is caused by light reflected off the blood-rich retina at the back of our eyes), indicative of our adapted commitment to a diurnal lifestyle.

³ This honeydew comes from a species of planthopper (Flatida coccinea).

⁴ Vomeronasal (VNO) receptor genes are key components of the olfactory system and are linked to the vomeronasal organ, which is especially good at detecting pheromones and chemical cues that affect an animal's behaviour.

⁵ Some sources give this award — biggest balls relative to body size — to the northern giant mouse lemur (Mirza zaza). This species is much larger in general, weighing nearly ten times as much as Madame Berthe’s at 300 grams (10.6 oz), but was only discovered in 2005 and is considered ‘endangered’.

**Where Does It Live?**

⛰️ Dry deciduous forest.
📍 The Kirindy Forest in western Madagascar.

‘Critically Endangered’ as of 22 Feb, 2022.

IUCN

Size // Small
Length // 10 cm (3.9 in) + 13 cm (5 in) tail
Weight // 33 grams (1.2 oz)
Activity: Nocturnal 🌙
Lifestyle: Mostly Solitary (Social at night) 👤/👥
Lifespan: 6–8 years in the wild
Diet: Omnivore
Favourite Food: “Honeydew” from beetle 🐞
Class: Mammalia
Order: Primates
Family: Cheirogaleidae
Genus: Microcebus
Species: M. berthae

Not only is Madame Berthe's mouse lemur the smallest of the 500+ primate species, but it also has the largest testicle-to-body-size ratio of them all (some sources give this award to the northern giant mouse lemur).
Its range is as tiny as its stature — restricted to dry forest patches in western Madagascar, totalling just 348 km² (216 mi²).
This lemur is an omnivore, but its diet is highly specialised; most of its calories come from "honeydew", a sugar-coated name for a sweet secretion exuded by a species of planthopper larvae.
It also hunts prey, including chameleons — small species or juveniles, surely, as some of its chameleon neighbours, such as the Malagasy giant chameleon, outweigh the mouse lemur by as much as fifteen times.
After a night of solitary foraging, some of these mouse lemurs will converge on a sleeping site — an open nest of leaves, vines, bark, and branches — to huddle together and doze off. Some groups retain the same members night after night, even when changing to a different nest, while others shuffle around every night.
This mouse lemur has no scent glands, but it'll create scent markings using anything from saliva (wiping the corner of its mouth along a branch), to urine (coating its hands and feet in pee and using them to smear marks), to faeces.
Mouse lemurs have the largest repertoire of functional vomeronasal (VNO) receptor genes among all primates — genes associated with the vomeronasal organ, specialised for detecting pheromones and chemical cues that influence behaviour.
Madame Berthe's mouse lemur can enter torpor — reducing its body temperature and metabolism — during periods of drought, cold weather, and food scarcity. But no torpor, however deep or long, can save it from illegal logging, deforestation for charcoal production, and slash-and-burn agriculture. It is considered a critically endangered species with a decreasing population.