Western Woolly Lemur, Avahi occidentalis
WESTERN WOOLLY LEMUR
Geographic Distribution and Habitat
Like all lemurs, the western woolly lemur—also called the Lorenz Von Liburnau’s woolly lemur or the western avahi— is endemic to Madagascar, the island country off of East Africa. As their name suggests, these lemurs are found mainly in the western and northwestern parts of the country. The Betsiboka River, Madagascar’s largest river, limits their distribution in the south.
The western part of Madagascar is on a slope and receives only a little rain. As a result, there are fewer trees than in a tropical forest. The main tree species in the region are deciduous varieties that lose their leaves seasonally. Western woolly lemurs thrive in these deciduous forests where they cling and leap onto trees as they forage for leaves.
The dry months usually last for eight months and, in this season, about half of the trees lose their leaves to conserve water. In North America, you may be familiar with autumn when the deciduous trees lose all their leaves. But in Madagascar, trees do not lose all their leaves at once. Rather, leaves fall out as they dry and new leaves replace them. And in some species, the old leaves only fall once new leaves push them out. This way there is a supply of leaves that can support leaf-eating animals, like the western woolly lemur, throughout the year.
Island ecology is a fascinating laboratory for evolution because the species on an island have to share limited plants and other landscape resources. In Madagascar, lemurs have evolved into at least 111 different species that share resources by feeding on different types of food, or by feeding at different times of the day. This phenomenon of resource sharing is called “niche partitioning” and it is a strategy for more species to coexist in a habitat without competing with each other.
One result of niche partitioning is that these species tend to be specialized to exist on specific resources. The western woolly lemur only lives in a specific habitat of Madagascar and specializes in eating specific deciduous trees in order to survive. Niche partitioning of resources allows western woolly lemurs to live in the same habitat as other tree-dwelling primates. It also means that we see some particular adaptations in the western woolly lemur that we do not see in other primates.
The genus name Avahi likely refers to the high-pitched “wo-he,” “va-hii,” and “vou-hii” sounds emitted by these nocturnal lemurs, and occidentalis refers to something originating in the west. The taxonomical journey of the western woolly lemur has been bumpy. Until the 1980s, taxonomists classified all woolly lemurs as a subspecies of the eastern woolly lemur (Avahi laniger). Later in the 1990s, the western woolly lemur was elevated to a separate species, Avahi occidentalis, because it was smaller and geographically separated from the eastern woolly lemur (Avahi laniger). Currently, there are nine Avahi species. DNA studies have confirmed they correctly classified the western woolly as a distinct species.
Size, Weight, and Lifespan
Western woolly lemurs are one of the smaller species in the genus, weighing between 1.5 and 2.3 pounds (700–1005 g). From head to the base of their tail, they are 9.8–11.6 inches (25–29.5 cm) long. That’s about as tall as a standard notebook! Their tails are longer than their bodies, measuring about 12.2–14.3 inches (31–36.5 cm). They use these long tails to balance as they leap from tree to tree, where they spend most of their time. There are no reliable data on their lifespans because there are no long-term studies on the wild populations.
Western woolly lemurs have large bright yellow eyes that are forward-facing. Their round-shaped face gives them a cuteness factor that is typical of many lemur species. Aptly named, woolly lemurs have a thick, slightly curled, woolly coat on their backs. Their faces are covered in much shorter hair. The base of their tail is orange-red, which is easily distinguished from the dull brown of their bodies. Their throat and underside have thin fur that is gray or beige with reddish highlights, much like the color at the base of the tail. All woolly lemurs have concealed ears and whitish inner thighs. The white part of their thighs is often what is spotted first when looking for this species because they hold their legs up against their bellies, which shows off the white coloring, especially in moonlight or in a spotlight. Locals in Madagascar often call woolly lemurs “fotsife,” which means “white leg.”
Western woolly lemurs are smaller and paler than the eastern species, but what sets the western woolly lemurs apart from others is the distinctive cream-colored triangular patch of fur near their rump or hind quarters, called the pygal patch. The western woolly lemur’s white face mask, contrasting dark eye-rings, and rounder face are distinctive features that also set them apart from other Avahi species. There is no sexual dimorphism, so males and females look similar. Young woolly lemurs are darker so that they can camouflage easily in their mother’s fur.
Western woolly lemurs are folivores, which means they get most of their nutrition from leaves. They prefer young leaves, buds, and shoots because these new growths are more tender and higher in sugars. But leaves, in general, are “all crunch” and have little nutrition. The lignin or fiber gives the leaf structure, but it is low in sugar and other nutrients. So why would primates depend on leaves? The advantage is that leaves are abundant and easy to find, which should mean that folivores spend less energy foraging.
Western woolly lemurs tend to be specialized folivores, feeding from about 20 relatively rare tree species. Most folivores use the most common tree species, which are easier to find. But if there is a lot of competition for leaves, then specializing in rare species that other animals do not eat is a way to avoid intense competition. Ecologists think that western woolly lemurs use this method of niche partitioning so that they can coexist with other folivore lemurs in their habitat, such as Milne-Edwards’ sportive lemur (Lepilemur edwardsi).
Another example of the western woolly lemur’s specialization is their nocturnal habits. Without light, leaves cannot photosynthesize to make sugars. Therefore at night, leaves contain less nutrition than they do during the day. Since western woolly lemurs feed at night, they select young leaves of the plants that most primates do not feed on. Since young leaves are generally more nutritious than older leaves, the lemurs may spend more time foraging, but they consume the most energy-rich foods available to them.
To deal with their specific diet, woolly lemurs have a specialized digestive system to help process that large amount of leaves they have to eat. The leaves get fermented in a chambered large intestine (cecum) and an elongated colon. Having these chambers and long intestines that are coiled in the gut allows the food to spend a long time in the digestive system. The longer duration gives the body enough time to extract the maximum amount of nutrients and water from the leaves.
Behavior and Lifestyle
Woolly lemurs are arboreal and spend most of their time in trees foraging for leaves. As they search for young leaves, they leap from branch to branch. When they rest, they sit vertically, clinging onto the tree limbs with horizontal branches for support. They sleep on open branches and choose from multiple sleeping sites. They prefer to use higher branches for feeding and travel, but will move their sleeping sites to lower heights during the dry season.
When western woolly lemurs have to travel on the ground, they leap in a sideways manner with their arms held up. This is called saltatory movement and is common among some lemur species, especially sifakas.
Despite their small size, western woolly lemurs travel further than most saltatory primates as they leap through branches. Each night, they travel an average of 0.7 miles (1.2 km). Their tendency to select young, more nutritious leaves may explain why they travel so far.
These woolly lemurs are most active at night, usually from two hours after dusk to two hours after dawn. Traveling and searching for food takes a lot of energy. In order to meet these energy needs, woolly lemurs can spend most of their active time at night searching for and eating food. If food is abundant, then they will conserve their energy and travel very little.
Ideally, animals try to find as much food as they can without wasting too much time searching for the food. There are two strategies animals use to balance time spent foraging and selecting high-quality food: One is to either spend more time searching for high-quality food that provides the animal with lots of energy (energy-maximizing strategy). The other is to save time and settle for lower-quality food that is close by (time-maximizing strategy). Ecologists think that western woolly lemurs use the energy-maximizing strategy because they travel far distances daily to feed on young leaves of specific trees.
Western woolly lemurs are peaceful and try to avoid conflict and confrontation. Rather, members in a pair groom each other. The most aggressive behaviors seen are individuals slapping away or forcing their partners to move. If there are any contests between members in a pair, it is almost always started by females and most often won by females too. Males tend to be submissive and retreat from aggressive behaviors. In most lemurs species, females have a higher rank in the family group.
- Western woolly lemurs are monogamous, folivorous, and nocturnal, which is a rare combination of traits among primates.
- They are specialist folivores and have never been raised successfully in captivity.
- They are one of the smallest woolly lemurs but travel more distances than most similar-sized primates.
- Female western woolly lemurs are more dominant than males.
Western woolly lemurs can spend up to 70% of their active time searching for and eating food. If food is abundant, then they will conserve their energy and travel much less.
Adult western woolly lemurs form pairs in which a male and female will live together for most of their lives. This pair-living dynamic is uncommon among primates, especially folivores. Typically, primates live in small groups when there is intense competition for food. Folivorous primates usually have an abundant supply of leaves to feed from. Less competition means that more individuals can live and forage together, especially because living in larger groups is a good protection from predators. A few folivore species, including the western woolly lemur, go against these expectations which leads to a phenomenon called the “folivore paradox.” The benefits of pair-living include reliable access to a sexual partner who will pass genetic material to the next generation without fear of troop takeovers or leader displacement. Also, western woolly lemurs select rare plant species, which makes finding food for a smaller group easier and more sustainable than a large troop.
Western woolly lemur offspring stay with their parents for a few years and they move together as a small family unit. Conflicts between family members are rare. As with most lemurs, females are more dominant than males.
Primates who are active during the day (diurnal) have a range of visual communication signals they can use to maintain cohesion or close relationships with each other. For nocturnal primates like the western woolly lemur, visual communication signals are less helpful because seeing each other in the dark is difficult. Also, when you live in dense forests, visual communication is less useful because trees and leaves often block your view. However, sound can travel through the forest and signals can reach individuals even if they cannot see each other. So western woolly lemurs depend on audio signals to maintain contact with each other because they can hear each other better than can see each other in the dark.
Pair-living and small family groups are also long-distance communication challenges. Many primates in larger groups rely on other members of their family to relay important messages, such as alarm calls, when a predator is nearby. But the smaller group size of western woolly lemurs means fewer eyes to look out for predators and there are fewer individuals to relay important information. And so audio communication is, once again, really useful in such cases.
Most of the communication in western woolly lemurs occurs between members of a family unit. They mainly use three types of calls to communicate with each other. The “avahee” call and the “whistle” call are the loudest, and the “growling” call is softer. As you might imagine, western woolly lemurs use loud calls when pairs become separated, and quieter sounds when pairs are just out of view. Despite woolly lemurs getting their name from the “avahee” call, western woolly lemurs use this call less than the “whistle” call. The “avahee” call is used mostly near the borders of the home range and is most likely used for territorial defense. Both sexes emit and respond to these calls to maintain contact or group distance with each other. Researchers think that “avahee” call is used to ensure adequate spacing between different pairs, while the other two calls are used to coordinate travel and activities within a pair of lemurs. Researchers have noted that western woolly lemurs are mostly peaceful and avoid aggressive or territorial behaviors. However, during the dry seasons, resources like food and sleeping sites are harder to find. Therefore, the competition between family groups increases during this time, and the avahee call is used more often.
Olfactory communication, through scent, is another effective method of communicating when animals cannot see one another. Western woolly lemurs have scent glands in their chin that are larger in males than females. Zoologists are still not sure how and when they use these glands, but it is likely that the glands indicate individual identity, health, and maybe even sexual status as the gland is more pronounced in males.
Mating usually occurs toward the end of the rainy season, between March and May, and infants are born between September and October. The gestation period is between 120 and 150 days. Usually, only one infant is born at a time. Mothers carry their infants against their bellies for the first two months. As the babies grow bigger and become more independent, they move further away from their mothers to feed on leaves. Juveniles stay with the family for about two years. A family group could consist of offspring from two consecutive years plus an infant.
Western woolly lemurs form monogamous mating pairs in which an adult male and female will spend their entire lives together. Monogamy is uncommon among primates. It probably exists in western woolly lemurs because of their dependence on rare tree species, which are distributed unevenly throughout the forest. The knowledge of the location, seasonality, and travel routes to these food sources is important to the survival of the species. An individual that knows where good food spots are will have a better chance of finding a mate and so researchers suspect that when western woolly lemurs choose their mate, they will favor a mate with this knowledge. Monogamy is a good way to keep this valuable information as a family secret where the knowledge is passed to members of the family and future generations of the same genetic line.
The western woolly lemur’s survival and success are completely intertwined with the ecology of the dry, deciduous forest they live in. Almost their entire life is arboreal and dependent on trees, from sleeping on branches to feeding on leaves.
Western woolly lemurs are a prey source for avian predators such as goshawks, as well as for Madagascar’s main mammal predator, the fossa (Cryptoprocta ferox). These predators are experts at hunting in trees and often catch lemurs during the day when the lemurs are resting. So, the woolly lemurs’ selection of sleeping sites is very important to their survival. Western woolly lemurs sleep in bare branches that are surrounded by dense leaves. The leaves not only provide cover but also keep the sleeping site warm. During the dry season, the forest canopy becomes less dense and there are fewer protected sleeping sites for the lemurs. It turns out that western woolly lemurs have multiple sleeping sites (40 or more) and they choose these sites based on the season.
The International Union for Conservation of Nature lists the western woolly lemur as Vulnerable (IUCN, 2018), appearing on the IUCN Red List of Threatened Species. They are classified as Vulnerable because their home range and habitat are severely fragmented or patchy and what little habitat is available is continuing to decline. Forest habitats on all island nations are competing with human development and economy, and so the major threat for the western woolly lemurs is habitat loss, caused mostly due to clearing trees for agriculture. Density estimates of western woolly lemurs are about 2 to 3 individuals per 2.5 acres (1 ha) and most of their activities occur within a 2.5–5 acre (1 to 2 ha) area of forest.
Habitat loss affects every aspect of the western woolly lemurs’ lives. Within their seasonal dry habitat, western woolly lemurs depend on traveling between patches of rare tree species to feed. Habitat loss can remove both their already-patchy food sources and the connecting pathways the lemurs travel to reach their food. Western woolly lemurs live in monogamous pairs and have relatively low reproductive rates. When their numbers are low, it is difficult for the population to bounce back because there are fewer individuals to form pairs and give birth to offspring, which delays the next generation of western woolly lemurs. So even a small amount of habitat disturbance can have a drastic effect on western woolly lemur populations.
The western woolly 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. This listing means that the export of western woolly lemurs or any parts of them is prohibited and can only be done in extreme cases. Some of these extreme cases may include old museum specimens that were collected before the 1970s when CITES was signed.
Even when new species were described and differentiated from the western woolly lemur, scientists carefully handled wild individuals from wild populations so as to minimize the harm to the decreasing population of woolly lemurs. Prior to conservation laws and agreements, scientists would have trapped and sometimes killed specimens for collections. Captive breeding for reintroduction is not an option for the western woolly lemur because people have yet to keep these lemurs alive in captivity. This is probably because the western woolly lemurs have a specialized niche and diet, which are difficult to mimic outside of their habitat.
The Ankarafantsika National Park is the main protected area for the western woolly lemur in Madagascar and national wildlife laws protect the western woolly lemurs and their habitat. However, this species is still illegally hunted for bush markets.
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Written by Acima Cherian, November 2022