Gray Mouse Lemur, Microcebus murinus
GRAY MOUSE LEMUR
Microcebus murinus
Geographic Distribution and Habitat
Gray mouse lemurs are tiny primates native to Madagascar. They are widely distributed on the island and are particularly prevalent along the western coast. These nocturnal primates are highly adaptable and thrive in various habitats. They can be found in dry deciduous forests, sub-arid thorn scrublands, secondary forests, and even man-made environments such as plantations. Their ability to inhabit fragmented landscapes and diverse ecological zones highlights their resilience and adaptability in the face of habitat changes.
Mouse lemurs are prosimians that are considered to be ancestral primates with smaller brains and more primitive features than monkeys and apes. Thanks to their moist noses, they have a more highly developed sense of smell than simian primates (monkeys and apes). (The noses of monkeys and apes are dry, like ours.) The Greek origin of the genus name Microcebus refers to the fact that mouse lemurs are small (“micro”) and long-tailed primates (“cebus”). The Latin root of the species name, murinus, refers to their mouse-like appearance.
Mouse lemurs are largely understudied because they are difficult to observe due to their small size and nocturnal nature. The larger gray mouse lemur is the best-studied species in the group and, for many years, they were considered the primary mouse lemur species with a variety of subspecies. Over time, further zoological studies on skull size, coat coloration, and genetics have revealed twenty-four mouse lemur species! Scientists think these different mouse lemur species evolved to suit Madagascar’s many and diverse habitats. This means that each species has a specific niche, or living requirement, that is closely linked to these habitats.
Size, Weight, and Lifespan
Mouse lemurs are the smallest primates in the world; the gray mouse lemur is one of the larger species in the genus, weighing between 1.5-3 ounces (58-67 g). Their weight fluctuates throughout the year as they fatten up during the rainy season and expend these fat reserves during the dry season when food is scarce.
They are so small, they could fit in your palm. Their body length (measured from the head to the base of their tail) is 4.7-5.5 inches (12-14 cm), and their longer tails are 5.1-5.9 inches (13-15 cm). Their tails can store extra calories as fat so that they can use it when food sources are low, similar to how camel use their humps to store water.
The oldest living captive gray mouse lemur was 18.2 years old. In the wild, lifespans are much shorter as these tiny primates are preyed upon. In captivity, caretakers have noted that, as gray mouse lemurs age, their mental faculties and ability to retain muscle decline, similar to humans. Researchers are using genetic studies to understand similarities between mouse lemur aging and human age-related diseases like Alzheimer’s.
Appearance
Gray mouse lemurs have rounded ears, soft gray-red fur, and a cute, pointed, wet nose that makes them look mouse-like. They also have large, round, forward-facing eyes and a long, bushy tail, characteristic of lemurs. Their hind legs are long, and their forearms are muscular with strong grasping digits, which allow them to cling to thin branches and pull up their body weight. They can balance on their hind feet and grasp insects with their hands.
Their hind feet have a modified claw that they use for grooming. Additionally, their canines and lower incisors are finely spaced, resembling a comb that they use to groom themselves and keep their fur clear of parasites and dirt.
Their small size, soft fur, and long fluffy tails help conserve body heat, especially during cold, dry seasons when they cannot get enough food to maintain daily activities.
If you were to use a flashlight to spot them in the dark, the first thing you would likely see is the red glow from their eyes. This “eye-shine” comes from a reflective structure called tapetum lucidum behind their eyes, which amplifies their night vision.
It can be difficult to distinguish between males and females in the wild, but most female gray mouse lemurs are heavier and have broader heads than males. So, sexual dimorphism (visual differences between males and females) does exist; however, it takes some expertise to tell the sexes apart.
Diet
Gray mouse lemurs are omnivores that feed mainly on insects (with a preference for beetles), fruits, flowers, and gum. Since they must travel to find enough food, insects are an excellent protein source that can help maintain their energy requirements. When resources are low, they may feed on lower-energy foods. like leaves. or more energy-intensive meals that require active hunting, such as reptiles and amphibians. They spend most of their time in the tree canopy, but they can also hunt at ground level.
Behavior and Lifestyle
As arboreal primates, they are tree-dwelling and spend most of their time feeding on the narrow branches at the edge of trees. They are highly skilled at leaping and grasping onto branches as they land. These skills develop from being novice tree climbers while infants to being experts by the time they are one month old.
They forage alone, but females gather together at dawn in communal sleeping sites, usually in tree holes.
Female dominance is typical in lemurs. Female gray mouse lemurs enjoy unconditional dominance, where females win almost every competition against males, including aggressively rejecting unwanted males and having food priority over males. This dominance may be a way for females to secure enough food resources and sleeping sites to raise young gray mouse lemurs to adulthood, especially as the male does not play a role in taking care of his young.
During the dry season, temperatures are low (especially at night), fruits are scarce, and insects are less active. To survive these harsh conditions, gray mouse lemurs enter torpor, a hibernation-like state of inactivity during which their metabolic and breathing rates reduce drastically. This unique behavior allows them to survive the low food condition by reducing their caloric needs. This way, they do not waste energy searching for food they will likely not catch. Instead, they rely on their stored fat to provide enough energy to survive.
Before the dry season, gray mouse lemurs spend considerable time eating and building fat reserves they can use during torpor. When temperatures are warm enough (above 82°F or 28°C), gray mouse lemurs have an internal body temperature of 97 to 100 °F (36.5 to 37.6°C). During torpor, gray mouse lemurs’ body temperatures decrease to 63.14 °F (17.3±°C), and their metabolism reduces by 38%.
Lowering their body temperature and metabolism reduces the mouse lemurs’ food needs, but maintaining a higher body temperature is essential for many bodily functions, including sleep. Sleep is a vital part of how the body (muscles, brain, etc.) recovers, and researchers have found that mouse lemurs cannot enter this recovery sleep when they are in torpor. So, mouse lemurs must emerge from torpor and experience normal sleep for some portion of the day. Gray mouse lemurs can go into torpor for an average of 9 hours daily. They can go into torpor multiple times a night, and usually, they come out of it around noon when the day is the hottest. This way, they can regulate their sleep during the day so that they can hunt for some time during the night.
Females tend to go into torpor more than males, which probably helps maintain fat reserves for when they may get pregnant soon after the dry season.
They rely on their cryptic (secretive) nocturnal behavior and choose to freeze in the safety of the dark if they detect danger.
Gray mouse lemurs seasonally enter torpor, a state of inactivity where they regulate their metabolism to survive cold, low-food conditions.
They forage alone, but females form communal sleeping groups.
Females enjoy unconditional dominance over males and win almost all competitions for space and food.
They age similarly to humans, where their muscle development and mental faculties decline. This similarity is being studied to better understand diseases like Alzheimer’s.
They may have small brains, but they are capable of learning complex calls and navigating through dense forests using a mental map.
Gray mouse lemurs start to wake up close to sunset and emerge from their nest in tree holes or clumps of leaves to forage for food. They are very active and capable of leaping 10 feet (3 m) to catch insects. Their activity level is dependent on the surrounding temperature. When the ambient temperature falls below 82°F(28°C), gray mouse lemurs lower their internal body temperature by reducing activity and entering torpor, which preserves energy and body heat. Before the sun rises, a gray mouse lemur, usually a female, will round up the communal sleeping group with a specific vocalization that triggers members to congregate at the designated sleeping site.
Communal sleeping groups can vary depending on the season, being small (2-4 individuals) during the dry season and larger groups (up to 15 individuals) in the rainy season.
Grooming occurs primarily between mother and child and sometimes between a female and a sleeping mate’s young. Males are occasionally seen grooming females around the mating period. They use their grooming claw or dental comb to groom each other. This mutual grooming is called allogrooming.
Gray mouse lemurs engage in play fighting where one individual may jump on another and try to groom them. These playful bouts do not end with any one of them getting hurt and are a way to strengthen social bonds between individuals and also help develop strength and coordination in young mouse lemurs. Most of the studies in play fighting behavior have been done on captive individuals since they are easy to observe, and captive gray mouse lemurs do not have to expend energy foraging for food or escaping predators.
Gray mouse lemurs are nocturnal, solitary, and live in dense forests. This makes visual communication with each other difficult, so they rely primarily on vocalizations and olfactory communication (using their refined sense of smell). Despite this, gray mouse lemurs have a visual anatomy and nervous system similar to that of more developed primates that live in more open areas. This indicates that the advanced optic (or visual) system does not require large brain sizes. In gray mouse lemurs, sight is mainly used to navigate and capture prey.
Gray mouse lemurs have an expansive vocal range that would inspire any opera singer. Most of their calls cannot be detected by the human ear because they are ultrasonic (high-frequency sound waves). One of the most common calls is a trill, which these primates make when they meet mates or other family members. Trills are complex and comprise 30 syllables strung together with different pitch modulations. Young gray mouse lemurs are not born with the ability to make high-pitched trills like adults, but researchers have observed infant lemurs emitting calls that sound like the start of a trill call. These quieter sounds, or “prototrills,” are likely to be the precursor to the young developing their vocal muscles, leading to them making full-blown trills. Researchers are studying this vocal development to understand how human babies babble to learn language eventually.
Grunt and purr calls are emitted when two gray mouse lemurs are close to each other. Grunt calls can be loud when a lemur is defending themselves, and soft purrs are used when they are allogrooming and content.
Mothers and infants have special reunion calls whenever they leave and return to the nest. This type of communication is almost ritualistic in that when the mother makes her call, the infant utters a predictable response. It truly is fantastic to see how deep-rooted this type of communication is and how it has been preserved from a tiny primitive primate like the gray mouse lemur to a more developed primate like humans.
Researchers have found that calls, especially those used to gather members of a sleeping group, are specialized or individualized for each group, similar to dialects, which are specialized languages for particular groups.
Even with their vocal abilities, gray mouse lemurs hardly use alarm calls to warn others of predators. This is likely a survival tactic because of their solitary foraging behavior. It can be risky to attract attention by making noise, and it is unlikely that another mouse lemur will be nearby to hear the alarm call reliably.
Mouse lemurs have a well-developed olfactory system and use scent marks to communicate with other members of their species. They use a combination of urine and gland secretions to communicate reproductive health, sex, identity, and other characteristics. They often deposit urine on branches along their usual routes or near sleeping sites so that others know who uses that area. Females urine mark during mating season. In response to smelling this, males will also urine mark more in the area to establish dominance over other males.
Gray mouse lemurs have a multi-male/multi-female system where both sexes can access multiple mates. Males and females become sexually mature when they are around six to seven months old. Sexual maturity in gray mouse lemurs develops sooner than social maturity. This means that, although they can mate and conceive, young mouse lemurs have not yet developed all the social skills to understand when to mate. While this delayed social development is common among primates, including humans, it is interesting to see this in a primitive primate such as the gray mouse lemur. Researchers are examining how ancestral genes preserved through different primate species are linked to similar social and reproductive development.
There is an evident mating season (around mid-September), which is timed so that females give birth when the rainy season starts and food is abundant.
Male reproductive organs (testes) remain small most of the year, and they enlarge in preparation for mating about one to two months before the mating season. Males become more active now and travel far to find a female ready to mate. He is also more aggressive to ensure he deters competition so that he has the best chance of fathering the next generation of gray mouse lemurs.
Female reproductive hormones are also dormant until the mating season starts when they enter estrus and can conceive. This hormone increase is associated with changes in female behaviors that allow the males to know she is ready to mate. In estrus, she runs around more actively and scent-marks with glands in her cheek and anogenital region (near her reproductive organ). She also vocalizes her reproductive condition with a vocal advertising call. When a mature male finds a receptive female, she will allow him to mount and mate her. After this, he leaves, and parental care falls on the female.
The gestation period (when females are pregnant) is about two months, and they usually give birth to twins. For the first two weeks of their lives, infant gray mouse lemurs stay in the nest in tree holes, and they are often left there during the night while their mother forages. This practice is referred to as the mother “parking” her infants while she forages. The infants are dependent on their mother and sleeping group for protection and care.
When infants are three weeks old, they may explore the area outside their nest. At this time, the nervous and muscular systems of the young lemurs are not developed enough to grasp firmly or balance on narrow branches. It takes about a month for young mouse lemurs to become strong and coordinated as adults. They are completely weaned off of their mother’s milk by the age of 3 months. Gray mouse lemurs are considered adults at the age of six months.
When juvenile males become sexually mature, they leave the natal site, which is the area where they were born. Males travel about 196-207 yards (180-960 m) from their birthplace. The distance ensures that they are far enough away from closely related females so that they can find unrelated female mates.” Young females stay in the area and can form strong bonds with their mothers and other related females in the communal sleeping site.
Sleeping groups are the equivalent of a family because they are stable units that stay together even if they move on to a new sleeping site. The social structure of having related females stay together while males disperse to different areas reduces the chances of closely related males mating with females to whom he is related. Female sleeping groups also have the advantage of thermoregulation, that is, the sleeping sites are kept warm from the group’s body heat. Multiple females living together also help share the child-care duties.
Female home ranges overlap more than males’, especially if they share sleeping sites. Males have larger home ranges and sleep alone. These overlapping home ranges of males and females are called individualized neighborhoods.
Mouse lemurs are the only primates in Madagascar that use the thin, delicate outer branches of forest trees and vines. This specialized niche, sometimes referred to as the “fine-branch niche,” allows gray mouse lemurs to take advantage of spaces and food sources that are not accessible by other omnivore species.
Mouse lemurs are a part of the ecosystem, providing valuable nutrition to predators. Raptors (predatory birds like owls), snakes, and fossas are their main predators.
Gray mouse lemurs have a broad diet spectrum, feeding on insects and nectar, which makes them adaptable to different environments in their range. As insectivores, they control the abundant insect population in the area. Some have observed mouse lemurs feeding on moths that were, in turn, feeding on the nectar from baobab flowers. The mouse lemur was covered in pollen and scurried to the higher branches to feed on the moth. In this way, mouse lemurs act as pollinators for trees. Baobabs are large, thick-trunked trees that can grow to be over 1,000 years old. Their presence and growth are mysterious as no fossils of this tree species can tell us how they evolved to be successful for so long. The mutually beneficial relationship between the mouse lemur, a prosimian, and the long-lived baobab is likely a vital part of the baobab’s survival and the development of that habitat.
The International Union for Conservation of Nature lists gray mouse lemurs as Least Concern (IUCN, 2020), appearing on the IUCN Red List of Threatened Species.
Madagascar’s rapid habitat destruction is the main threat to wildlife in the area. Much of the island has been converted to plantations, deforested for human development, or taken over by mining companies. Despite the loss of habitat, gray mouse lemurs are highly adaptable and remain the most abundant species of the genus in Madagascar. Currently, their population is not threatened by extinction, though it is declining.
Habitat fragmentation reduces the number of viable habitats for animals and also results in patchy areas with corridors and open regions that provide little cover for small animals. In the case of nocturnal animals, like the gray mouse lemur, the addition of light pollution (from street lights and human developments) can affect how these lemurs travel. Studies have shown that gray mouse lemurs try to avoid illumination and explore their environment less when there is too much light and not enough dark corridors to hide in. This means that light pollution reduces the lemur’s hunting and foraging opportunities, which affects their ability to feed themselves and survive in degraded environments.
Occasionally, a captured gray mouse lemur may be kept as a pet, but it is not traded at the scale of other primates.
The gray 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.
Most of the species conservation efforts come from general habitat protections from establishing national parks. As there is no immediate conservation threat, there are no specific laws or action plans for the species.
Gray mouse lemurs have been successfully kept in captivity and used in studies that could benefit future conservation studies. For example,
The Baylor College of Medicine and the Human Genome Sequencing Center (BCM-HGSC) have decoded the gray mouse lemur genome and are trying to understand how their genetics are linked with their behaviors (like torpor), diversity, and health risks.
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Written by Acima Cherian, July 2024