Golden-Brown Mouse Lemur, Microcebus ravelobensis
GOLDEN-BROWN MOUSE LEMUR
Microcebus ravelobensis
Geographic Distribution and Habitat
Like all the world’s lemur species, the golden-brown mouse lemur (Microcebus ravelobensis), also known as the (Lac) Ravelobe mouse lemur, is indigenous exclusively to the lush island country of Madagascar located off of Africa’s southeastern coast. These tiny prosimians are found in the island’s northwest region within the forest reserve of Ankarafantsika National Park and Mariarano Classified Forest (some areas that have been heavily disturbed by humans). Habitat is provided by dry, deciduous lowland forests—both primary and secondary—where these mouse lemurs reside from sea level up to 1,148 feet (350 meters). They can also be found within degraded forest patches and adjacent to human settlements, indicative of the species’ adaptable nature.
Golden-brown mouse lemurs share their isolated environment with gray mouse lemurs (M. murinus), but the two species occupy different niches. Whereas golden-brown mouse lemurs prefer humid forest habitats near water sources, gray mouse lemurs prefer drier forest habitats away from water sources and reside at higher altitudes than those occupied by golden-brown mouse lemurs. Wildlife biologists often discuss the two species in unison.
Until the early 1990s, wildlife biologists believed that mouse lemurs comprised two distinct species: the gray mouse lemur (the golden-brown mouse lemur’s close, partially sympatric cousin) and the rufous mouse lemur (M. rufus) who resides in eastern Madagascar within humid forest formations.
Taxonomic status remains in a bit of a twist, due to a dearth of comparative studies between members of multiple mouse lemur species in a given population and variation among members of the same mouse lemur species. With this caveat in mind . . . today, 24 species of mouse lemurs are recognized, including 7 species of Microcebus from western Madagascar, among them the golden-brown mouse lemur.
Discovered in 1994, the golden-brown mouse lemur was validated as a distinct species in 1997 confirmed by morphology and genetics. It belongs to the family known as Cheirogaleidae, along with other mouse and dwarf lemurs, within the suborder Strepsirrhini (so-called “wet-nosed” primates).
All lemur species, along with lorises, galagos, and tarsiers, belong to the primitive primate order known as prosimians. (They are not considered monkeys!)
Size, Weight, and Lifespan
Generally, lemurs are considered to be monomorphic (exhibiting little or no differences in appearance between the sexes, apart from genital organs). However, within the Microcebus genus, females are typically larger than males. They weigh a bit more and have larger heads.
How tiny is tiny? As full-grown adults, these little wisps weigh a mere 1.8 to 2.5 ounces (51.8 to 71.7 grams). (Their weight fluctuates depending on food availability in accordance with the season.) Average head-to-body length is 5 inches (12.7 centimeters). Tail length adds another 6 to 8 inches (15.2 to 20 centimeters) to their slender, miniature frame.
You might think that their diminutive size gives the golden-brown mouse lemur the distinction as the world’s smallest primate. (In fact, some scientists have posited such at one time or another and some sources still make this assertion.) Nope. In fact, this little prosimian is the largest of the seven western mouse lemur species. Bragging rights for the world’s smallest primate go to Madame Berthe’s mouse lemur (M. berthae), who weighs just 1.1 ounces (31 grams) with an average body length of 3.6 inches (9.2 centimeters). Tail length adds another 5.1 inches (13 centimeters) to this most miniature of frames. That’s how tiny . . . tiny can be.
The lifespan for captive mouse lemurs has been reported as 18 years or more; lifespan for those in the wild has not been reported.
Appearance
I mean, come on. Can you stand the cuteness level of this alien-looking, little primate? The dark-colored eyes are two large orbs that dominate the face. Besides their spell-binding affect, the eyes are an important morphological adaptation. An ocular attribute known as the tapetum lucidum, located at the back of each eyeball, reflects light through the retina and shines outward, providing this primate’s excellent night vision, essential to its nocturnal lifestyle. (If you were to spy a lemur in the dark of night and shine a flashlight in its face, the lemur’s eyes would appear to shimmer and glow in what we call “eye shine.”)
A dark ring encircles each of the golden-brown lemur’s eye for an even more arresting appearance. Extending from the forehead, between the eyes, a strip of white or light-colored fur accents the narrow nose bridge, which is punctuated by a moist button nose. Light-colored fur also surrounds the mouth. The ears are outrageous. Big and scalloped, they extend from either side of the temples and stretch upward—as if they might lift this little animal high into the stratosphere to whatever planet it came from. But seriously. The enormous ears provide keen hearing and are an adaptation, courtesy of Mother Nature, that allows these prosimians to detect subtle sounds in the forest.
As this prosimian’s common name tells us, the plush pelage that cloaks its physique is golden-brown. A thick undercoat provides insulation and protection against the elements. Fur on the back can be a mottled-red, and the underside is a yellowish white. The long tail is furred in brown, and the long, bony fingers and toes are hairless.
Characteristic of all lemur species, except for the aye-aye, the lower incisor teeth of golden-brown mouse lemurs are forward-leaning. This so-called “toothcomb” is an ancestral trait used for grooming or gouging tree bark. And like other lemurs (along with lorises, galagoes, and tarsiers), golden-brown mouse lemurs are fitted with a specialized, curved nail known as a “grooming claw” on the second toe digit of each foot. The other digits, on both hands and feet, are flattened nails.
Diet
These miniature primates are mostly frugivorous, meaning they eat primarily fruits along with flowers, buds, leaves, and nectar. Theirs is a seasonal diet. When fruits are scarce, they consume more arthropods, small vertebrates, insect secretions, and tree gums. Tree gums, in particular, but also sugary insect secretions provide important, supplemental nutrients to mouse lemur diets. Like all prosimians, lemurs are able to synthesize vitamin C within their bodies.
The diet of golden-brown mouse lemurs is similar to that of the gray mouse lemur and the brown mouse lemur. But unlike these lemur cousins whose tails and hind legs can store fat—up to a combined 35 to 40 percent of their body weight—the longer and thinner tail of golden-brown mouse lemurs is incapable of this morphological feat. We don’t know why Mother Nature chose to forgo this trait, a crucial backup energy reserve, when she created golden-brown mouse lemurs. Perhaps she needed a nap.
Behavior and Lifestyle
Golden-brown mouse lemurs are both nocturnal (active during nighttime hours) and arboreal (spending most of their time in the trees). They are agile creatures, traveling through the forest canopy by leaping from tree limb to tree limb. Sharp claws on their hands and feet enable them to grasp branches as they advance, while their long tail provides balance as they stick their landings. Their mode of locomotion differs from their close gray mouse lemur cousins, who travel quadrupedally (using all four limbs), as do most mouse lemurs. Wildlife biologists speculate the locomotor differences between the two species are likely attributed to differences in body morphology (size and shape) and preferred forest strata (level).
Sleeping accommodations also differ; when gray mouse lemurs share habitat with golden-brown mouse lemurs, the former choose tree holes to slumber away the daylight hours. Golden-brown mouse lemurs, like other mouse lemur species, are primarily leaf or nest builders. Females, particularly lactating females, assume this responsibility. Using small branches, leaves, and surrounding vegetation, they begin constructing their sleeping nest during the early evening hours (so it is ready for occupation when daylight comes), an endeavor that takes about 60 minutes. Nests are occupied by lactating or dominant females, their offspring, and mixed lineage group members of both sexes.
Although golden-brown mouse lemurs frequently switch their nesting sites (one study recorded 17 sleeping sites), the multimale/multifemale group composition remains stable. These male/female sleeping groups serve two important purposes: to help with thermoregulation (maintain body temperature) during cooler months and to decrease chances of becoming victims to a predator attack. Their multimember sleeping pattern is in contrast to their solitary nocturnal lifestyle.
Coat coloring provides golden-brown mouse lemurs with excellent camouflage, allowing these little creatures to blend in with their treed environment so that their predators cannot spot them. Predators include mongooses, viverrids (wild, cat-like carnivores), raptors (including the Madagascar long-eared owl and the Madagascar harrier-hawk), domestic dogs, and snakes. When a Malagasy tree boa was observed attacking an adult gray mouse lemur, several of its group members came to the rescue by mobbing and biting the snake—thereby forcing the reptile to release its death grip. Might golden-brown mouse lemurs exhibit the same “altruistic” behavior?
A physiological mechanism known as “torpor” is common among mouse lemur species, to varying degrees. A simple definition of the term is “the controlled depression of virtually all bodily function during scarce periods.” Think of temporary hibernation. Unlike full hibernation, which is voluntary, torpor is an involuntary stage of sleep that lasts only a few hours during the daytime. Torpor allows these tiny prosimians to cope with environmental and seasonal changes in their habitats.
Activity levels, body temperature, metabolism, and body mass are managed by two types of torpor: daily and seasonal. Daily torpor allows mouse lemurs to conserve energy as they enter this inactive state, while seasonal torpor helps them to survive yearly periods of scarce food resources.
Although all mouse lemurs become torpid during winter months, the degree of torpor varies. In studies of golden-brown mouse lemurs, wildlife biologists have found that the species remains active during cooler weather, irrespective of environmental changes; theirs is daily, rather than seasonal, torpor. Scientific speculation posits that the species’ small body size limits its ability to consume and store large amounts of energy sources, thereby making seasonal torpor (or hibernation) impractical.
This species takes its alternate common name, “(Lac) Ravelobe mouse lemur,” and its scientific name, ravelobensis, from the area of Lac Ravelobe, a sacred, legendary lake within Ankarafantsika National Park where the tiny primate was discovered in 1994.
The legend of Lac Ravelobe tells the story of an early 1800s tribal king who, despondent over losing his kingdom to a rival, threw himself and his family into Lac Ravelobe; all were subsequently eaten by crocodiles. But the king’s royal gatekeeper had refused to join this suicidal (and murderous) crocodile plunge. When this gatekeeper eventually died (we assume of natural causes), a crocodile on Lac Ravelobe appeared and spoke in the gatekeeper’s voice, sighing in sorrow for the dead king. Today, Lake Ravelobe’s crocodiles are revered as sacred beings, referred to as mampisento, or, “that is sighing.” Each July, locals gather and sacrifice zebu (large African cattle) to the crocodiles, so that in return, they are granted safe fishing on the lake.
Scientists theorize that lemur ancestors arrived in Madagascar by floating on mats of vegetation across the Mozambique Channel from mainland Africa. The prosimians’ small size and their adaptability between endothermic (body temperature) and ectothermic (environmental temperature) conditions allowed them to survive the journey across the Indian Ocean to colonize Madagascar—and still, today, they are found nowhere else in the world.
In comparison to their sympatric gray mouse lemur cousins, far less is known about the social organization of golden-brown mouse lemurs. That said, from the many studies conducted on the species’ nest sharing/switching patterns—which take into account spatial distribution (physical location) and temporal distribution (change in patterns/behavior over time), we know that golden-brown mouse lemurs live in overlapping multimale/multifemale societies. Ascertaining the number of individuals who comprise a family group is a bit more challenging, however. But if we decipher the available super science-y/academic articles (which go into great detail about the number of individuals trapped for study, with sidebar discussions on population densities), we can reasonably conclude that a group includes 7 to 10 individuals, but membership might be as high as 20.
Observed nesting behavior of captured individuals, both males and females, has led researchers to conclude, much to their surprise, that, unlike gray mouse lemurs, female dominance is not prominent with golden-brown mouse lemurs.
While both young males and females typically remain with their natal (birth) group into their first mating (reproductive) cycle, both sexes eventually leave home—anywhere between 36 weeks and 1 year of age. Studies show that males are more likely to postpone their dispersal, a delay that increases the risk of inbreeding.
In an effort to describe the social organization of the species, wildlife biologists have decided upon three descriptors: gregarious, dispersed, and solitary.
A gregarious family group refers to multiple individuals, both male and female, living as a cohesive family unit. Offspring from the previous reproductive season are included.
Dispersed refers to individuals belonging to a social group but who are solitary foragers.
Solitary is as it sounds: individuals who live a completely solitary existence.
Home ranges for the species overlap with that of gray mouse and dwarf lemurs. While we don’t know the golden-brown mouse lemur’s range limit, we can make a like inference from the daily home range of gray mouse lemurs: reported as only 0.01 square mile (0.032 square kilometer) for males and just 0.007 square mile (0.018 square kilometers) for females.
Aggression has been observed between golden-brown and dwarf lemurs, but it appears that aggressive encounters (spats) are more common between gray mouse and dwarf lemurs.
Besides the gray mouse lemur, dwarf lemur, other lemur species—including the Critically Endangered Coquerel’s sifaka (Propithecus coquereli) and the rare Critically Endangered mongoose lemur (Eulemur mongoz)—other wildlife who share habitat with golden-brown mouse lemurs include crocodiles, the Critically Endangered ploughshare tortoise (Astrochelys yniphora), a variety of snakes, leaf-tailed geckos, and rare bird species including Madagascar’s national bird, the Madagascar fish eagle (Haliaeetus vocifer); Madagascan jacana (Actophilornis albinucha); and Madagascar paradise-flycatcher (Terpsiphone mutata).
Communication
Both olfactory signals and vocalizations serve equally important purposes in golden-brown mouse lemur communication. Secretions released from scent glands on these prosimians’ wrists and chests serve to mark territorial boundaries among the various groups sharing spatial distribution. (Marking behavior may occur exclusively during dispersal, according to an observation reported in one field study.)
Males emit various calls, in conjunction with scent marking, to send messages to other lemurs and to assert their dominance and reproductive rights. Acoustic vocalizations help to regulate group cohesion and coordination among group members. And each group has its own signature-specific “trill” call, used by all individuals during reunions.
It has been reported that other mouse lemur species often groom one another while relaxing after an evening meal. While this social bonding activity has not been reported in wild golden-brown mouse lemurs, we can assume—from their dental toothcomb and grooming claw that Mother Nature has fitted them with— that they likewise engage in allogrooming.
The multimale/multifemale nest-sharing/nest-switching or swapping behavior of golden-brown mouse lemurs (considered exceptional among mouse lemur species) has led researchers to conclude that the species practices promiscuity. Most wildlife biologists now concur, however, that the greater impetus, or benefit, of this sleeping arrangement is thermoregulation, rather than randy males exploiting the situation for coitus. Contrast the sleeping patterns of gray mouse lemurs, who live within the same ecological conditions but do not form mixed-sex sleeping groups. Researchers speculate that their higher quality nesting sites like tree holes provide these mouse lemur cousins with adequate insulation and safety; hence, no need to comingle.
Golden-brown mouse lemurs are mostly solitary creatures, except during mating season which occurs during the months of October and November. While females are known to mate with cohabitating males, it appears they more often mate with males from neighboring groups.
To attract a female, a male “serenades” her with vocalizations and leaves his scent secretions in her path. If he’s successful and copulation leads to pregnancy, a female gives birth to a single infant after just 60 to 62 days—one of the shortest gestation periods among primates. But infant mortality is extremely high, so Nature has seen to it that females give birth twice a year.
Mothers nurse their infants for 5 to 6 weeks, at which time the young are considered weaned. (Some female mouse lemur species are known to have three pairs of nipples, but confirmation of this physical adaptation is lacking for golden-brown mouse lemurs.) For their first few weeks of life, youngsters cling to their mother’s belly, then transition to riding on her back as she travels. By 2 to 3 months of age, youngsters begin exploring their treed environment—but they keep mom in sight in case they need rescuing. During this period, young golden-brown mouse lemurs learn necessary survival skills, including foraging for food, navigating the forest canopy, and avoiding predators. By 6 months old, they are considered fully independent. Full sexual maturity (for males, able to sire offspring; for females, able to conceive and bear offspring) occurs between 1 to 2 years of age. Females are physiologically capable, however, of reproducing at just 9 months of age (though earlier childbirth is more prevalent with female gray mouse lemurs, who have a greater body mass).
According to wildlife biologists who conducted an exhaustive study comparing the reproductive dynamics of golden-brown mouse lemurs to gray mouse lemurs over a 24-year period, golden-brown mouse lemurs exhibit a high degree of environmentally driven reproductive plasticity. Say what? Research found that golden-brown mouse lemurs are more flexible (than their gray mouse lemur cousins) to changes in their environment (environmental plasticity), which thereby allows them to adjust their reproductive state in response to resource availability. To point, during years with more rainfall, females became reproductively active later. Or, environmental conditions may be such that they reproduce three times during one year.
Overall, the species exhibits an impressive ability to adapt and thrive in its forested environment, despite its tiny size and the large challenges it routinely faces.
Thanks to their largely frugivorous diet, golden-brown mouse lemurs are natural habitat regenerators. The seeds of the many fruits they eat pass through their digestive tract and are excreted, via their feces, throughout their environment, thereby encouraging new plant growth.
Golden-brown mouse lemurs are classified as Vulnerable by the International Union for Conservation of Nature (IUCN, May 2018), appearing on the IUCN Red List of Threatened Species. The most recently documented extent of occurrence (ECO)—that is, the area believed to encompass the entire population, is 3,663 square miles (9,486 square kilometers).
Though the species’ vulnerable classification is a slight improvement from its 2014 classification of Endangered, the population continues to decline as this mouse lemur’s range increasingly becomes more severely fragmented. Fragmented habitats prevent or limit natal dispersal, leading to inbreeding and an abnormal gene pool.
Continuing anthropogenic (human-caused) negative activities have already destroyed nearly 50 percent of Madagascar’s forests and pose a grave threat to the island’s extraordinary endemic biodiversity. Ninety-eight percent of all lemur species are currently under threat of extinction; 36 percent are Critically Endangered.
Habitat loss due to slash-and-burn agriculture (pristine forests are razed and burned, then transformed into agricultural tracts of land, pastures for cattle grazing, or construction of human settlements) is the premier threat to the golden-brown mouse lemur’s survival. Seasonal bushfires also threaten the species, and the tiny primates are also poached (illegally hunted). And like everywhere else on our planet, perilous climate change looms over them.
The golden-brown mouse lemur is listed in Appendix I of the Convention on International Trade in Endangered Species (CITES), an international agreement between governments whose goal is to ensure that international trade in specimens of wild animals and plants does not threaten their survival.
Although Ankarafantsika National Park is supposed to be a protected area and Mariarano Classified Forest offers limited protections—destructive anthropogenic activities occur in both these localities where golden-brown mouse lemurs occur. Thus, conservationists have called for the species’ protection status to be elevated.
One conservation group dedicated to saving all of Madagascar’s lemurs is the Lemur Conservation Network (LCN). Founded in 2014 as a project of the IUCN Species Survival Commission Primate Specialist Group, it became an independent not-for-profit organization registered in the USA in January 2021. The organization organizes an annual World Lemur Festival, anchored by World Lemur Day celebrated on the last Friday of October. This fun, educational activity helps to raise awareness and garner support for lemur species not only among locals but worldwide.
Planet Madagascar is a local organization working hard (and partnering with LCN) to protect golden-brown mouse lemurs and all lemur species. Focusing on outreach, educational programs, fire management, and community livelihood programs, the organization strives to cultivate an appreciation and pride for Madagascar’s treasured lemurs to save them from extinction.
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Written by Kathleen Downey, July 2024