The Strange Aye-Aye Lemur of Madagascar and Its Pseudothumb
A Bizarre Animal
The aye-aye is a strange lemur that is often said to have a "bizarre" appearance. The animal has large ears, teeth that never stop growing, long and slender fingers, and coarse and untidy hair. It very long and skinny third finger is especially noticeable and is used for a hunting method known as tap foraging. The animal has the large eyes that are typical of nocturnal creatures.
The aye-aye is native to Madagascar but is found in captivity in various countries. Lemurs are primates like us and therefore have five visible fingers and five toes. Researchers have recently made an interesting discovery in relation to the fingers of the aye-aye. They say that an extra and elongated bone inside each of the animal's hands is movable and acts as a sixth digit. They call this digit a pseudothumb and say that it plays a helpful role in the aye-aye's life.
The aye-aye belongs to the class Mammalia and the order Primates, like humans, apes, and monkeys. It belongs to the suborder Strepsirrhini (which contains lemurs, galagos or bush babies, pottos, and lorises), the superfamily Lemuroidea, and the family Daubentoniidae. The derivation of the name "aye-aye" is uncertain.
Habitat and Distribution
The scientific name of the aye-aye is Daubentonia madagascariensis. It's the only living member in the family Daubentoniidae. The animal is arboreal and mainly solitary in the wild. It lives in the rainforest of eastern Madagascar and in smaller areas in the northern and northwestern part of the country. Madagascar is an island nation off the southeast coast of South Africa. It contains many organisms that are found nowhere else on Earth in the wild, including the aye-aye.
Physical Features of the Animal
An adult aye-aye is about the size of a house cat. Its average body length (excluding the tail) is around sixteen inches. The animal has a small and pointed face that's covered with short and mostly white hair. The nose is often pink. The rest of its body is dark brown or black in colour but is sprinkled with lighter hairs. The thick and bushy tail resembles that of a squirrel while the ever-growing nature of the teeth resembles that of rodents. The long and thin fingers are the most noticeable feature for many people. Each finger bears a curved claw rather than a nail.
When an aye-aye becomes scared or excited, the white hairs on its body may stand erect, making the animal look larger than its real size. This may be a helpful tactic to scare predators away in the wild. Researchers have noticed that captive animals sometimes perform the behaviour when they're moved to a new enclosure or when a mother is playing with her child.
Unfortunately, the animal's strange features have caused some people to believe that its appearance in their community is an evil omen. It's sometimes thought that in order to prevent bad luck from pervading a village, an aye-aye that has been seen must be killed.
Daily Life of an Aye-Aye
The aye-aye spends most of the night looking for food in the trees. It may spend some time on the ground. During much of the day, the animal sleeps. It makes a nest of leaves in a fork in tree branches. Nest may be used more than once.
The aye-aye is a territorial animal. Males have much larger territories than females. The territories of different animals may overlap. Meetings between the animals may or may not be peaceful. Duke University says that in general the only social interactions in the wild occur during courtship and when a female is caring for a nursing youngster.
Some researchers report that they have seen pairs of wild adults travelling through the forest together as they forage, which suggests that they are not always solitary. In captivity, the Duke University scientists have found that "a male/female pair and their single infant might coexist peacefully for years".
The lemur is omnivorous and feeds mostly on insects and fruit. It has an interesting method of finding the larvae in the wood of trees. It taps the tree with its third finger and then listens for the sound created in the hollow passages that the larvae create as they burrow. If it hears the right sound, the lemur gnaws the area with its teeth (if this is necessary) and then extracts the larvae with its finger. The process is sometimes known as tap foraging and is shown in the video below. The elongated finger is also used to scoop the pulp out of fruit and the yolk out of eggs.
The aye-aye has a large brain in proportion to its skull size in comparison to the situation in other lemurs. It's thought that this may be due to the necessity to analyze sound coming from the animal's tree tapping behaviour.
Duke University has a colony of aye-ayes and has shared a lot of information about the animals on its website. The information includes facts about their reproduction. These facts may or may not apply to the wild animals.
In captivity, the aye-aye breeds at any time of the year. Gestation lasts for about 170 days. The animal produces just one offspring at a time. The Duke University scientists say that in the wild nursing may end when the baby is as young as seven months. In captivity, it may last for twice as long. The captive animals breed every two to three years.
Aye-ayes have lived for as long as twenty-four years in captivity. Their lifespan may be considerably less in the wild where more dangers may be encountered.
The Pseudothumb of an Aye-Aye
Researchers at North Carolina State University used seven aye-aye bodies in their pseudothumb research—six adults and one juvenile. All of the animals had lived in captivity and all of them died of natural causes. They weren't killed for the research.
The Radial Sesamoid
The pseudothumb develops from a bone called the radial sesamoid. The outer of the two bones running down our arm (the one on the side with the thumb) is called the radius. In some mammals, an extra bone is located where the radius joins the wrist bones. This bone is the radial sesamoid. Aye-ayes have the extra bone. Humans sometimes have it, but its presence is rare in us. If it is present in our body, it's generally a small bone.
The pseudothumb of the aye-aye contains an enlarged radial sesamoid bone that looks like an elongated knob. A dense extension of cartilage extends from its tip. (The structure is shown in an animation in the video below.) Three muscles are attached to the radial sesamoid bone via tendons, which could potentially allow movement in multiple directions.
The scientists believe that the aye-aye uses the pseudothumb as an extra digit. This digit is shorter than the other ones and is located within the hand instead of extending from it as the fingers do, but it's believed to be useful.
The Skin Pad
A fleshy pad covers the structures that make up the pseudothumb and is visible on the aye-aye's palm. The skin on the pad has a distinct "fingerprint", or a distinct dermatoglyph as the pattern of skin ridges is technically called.
The scientists say that the long and spindly fingers of the aye-aye help them to get food, but they aren't very good at gasping branches. The pseudothumb and its skin pad likely provide extra gripping ability. The investigators have discovered that some closely-related lemurs don't have a pseudothumb. So far, it appears to be unique to aye-ayes.
A Surprising Discovery
A pseudothumb has been found in a few other mammals, including giant pandas, but it's never been found in a primate before. It seems that no one else has traced the pathway of muscles and tendons to the aye-aye's radial sesamoid bone. This factor may have prevented people from realizing that it might be used as a digit. As the title of the video above says, the pseudothumb is a "secret" finger.
An aye-aye has other pads on its palm besides the one over the pseudothumb. These pads may also be helpful in gripping objects, though not to the extent of the one covering the pseudothumb.
The muscles associated with the aye‐aye pseudothumb are anatomically positioned to enable abduction, adduction, and opposition of this digit relative to the palm.— Adam Hartstone-Rose et al, American Journal of Physical Anthropology
Aye-Aye Population Size
The IUCN (the International Union for Conservation of Nature) classifies the aye-aye as an endangered animal and says that its population is decreasing. Threats include logging and the consequent habitat loss, hunting, and trapping.
The last population assessment was performed in 2012. The number of scientists studying the animal has significantly increased since that time. As scientists have searched for the animal and found it, they've discovered that aye-ayes appear to be more numerous than was thought at the time of the last assessment.
The aye-aye is a fascinating animal. I hope more is learned about its behaviour, its pseudothumb, and its population size. The suspicion that its numbers are higher than was recently believed could be good news. It's important that we know whether the animal needs additional help in order for the species to survive in the wild. Hopefully a new population assessment will be performed soon.
- Aye-aye information from the Duke Lemur Center
- Some facts about the animal from the Encyclopedia Britannica
- Aye-aye facts from Science Direct via journal and book excerpts
- The aye-aye entry on the IUCN Red List
- A bizarre primate with a newly-discovered digit from National Geographic
- Researchers discover the aye-aye's extra finger from North Carolina State University
- The anatomy of the pseudothumb of Daubentonia madagascariensis from the American Journal of Physical Anthropology
Questions & Answers
© 2019 Linda Crampton