Introduction

The aye-aye is one of the most unique primates in the world. In 1863, Richard Owen, foreshadowing contemporary intelligent design arguments, posited that the aye-aye's unique qualities provided clear evidence that Darwin's theory of natural selection must be wrong (Owen, 1863). Owen, the most eminent British anatomist of his time, detailed the aye-aye's distinctive dental and digital morphology, briefly described how naturalists at the time thought the animal uses these morphological features to acquire food, and concluded that only God could have created an animal so well adapted to its environment. Indeed, the aye-aye has a number of morphological traits that set it apart from other primates and allow it to exploit resources unavailable to most other animals in Madagascar (Figure 1). It also exhibits behavioral characteristics that distinguish it from most other lemurs. Recent research on aye-ayes has begun to overcome obstacles to observing these animals and has started to shed light on the mysterious social habits of this species. As we learn more about the aye-aye, we find more ways in which it is similar to other lemur species, as well as the ways in which it is different.

The aye-aye's unusual morphological characteristics generated a century of controversy, beginning with its introduction to Western science in the 1780s (Sonnerat, 1782), on whether to place Daubentonia within the primates, the rodents, or even the marsupials (Sterling, 1994c). Owen's definitive study of aye-aye anatomy (Owen, 1866) finally quelled the debate over the species' taxonomic

Eleanor J. Sterling • Erin E. McCreless • Center for Biodiversity and Conservation, American Museum of Natural History, New York, NY 10024

Figure 1. Drawing of a female aye-aye, based on an animal living in the Gardens of the Zoological Society of London, 1862. Drawing by J. Wolf (Owen, 1866).

position, focusing attention away from the animal's rodentlike anterior teeth and towards its primatelike characteristics, such as a postorbital bar, stereoscopic vision, and an opposable hallux (Figure 2). Although its placement within the primates is still being debated, Daubentonia is considered a member of the family Indridae (Schwartz, 1986); as a sister taxon to the other Malagasy primates (Pastorini et al., 2002, 2003; Yoder et al., 1996a,b); and as the most basal branch of the strepsirrhines (Delpero et al. 2001; Groves, 1990).

The only living representative of the family Daubentoniidae, the aye-aye is the only primate to have claws on all digits but the thumb, a nictitating membrane ("third eyelid"), and abdominal mammary glands. With a length of 80 cm from nose to tail and a weight of 2.5-3 kg, Daubentonia is the largest nocturnal primate species in the world. A distinctive dental formula of 1/1 incisors, 0/0 canines, 1/0 premolars, and 3/3 molars includes incisors that grow continuously like those of a rodent. The aye-aye is probably best known for its slender middle finger, in which modifications to the metacarpal provide extra flexibility in the joint and make the finger appear especially long.

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