Yftftfa Liana HHH Dead wood Bark
I I Live tree/liana ^H Other
Figure 5. Overall percentage of time spent feeding on larvae from different sources by season for aye-ayes on Nosy Mangabe from November 1989 to April 1991. Averages of six individuals were used to calculate seasonal means. See Figure 3 for sample sizes and abbreviations (Sterling, 1994a). Used with permission from S. Karger AG, Basel.
Many of the aye-aye's preferred foods are highly structurally defended. The animal's long anterior teeth and slender middle finger provide access to foods that are difficult for many of its competitors to reach. Feeding on wood-boring larvae requires aye-ayes to use both of these special morphological features, as an individual must gnaw through live or dead wood and then extend its middle finger into the cavity to hook larvae. The pericarp surrounding Canarium seeds is harder than any fruits or seeds that are broken open by primates in South America (Kinzey and Norconk, 1990), but the aye-aye is able to break it open with its strong incisors. The prevalence of these two foods in the aye-aye's diet underscores the species' specializations for, and ability to reach, structurally defended foods.
While the aye-aye's morphology provides access to a variety of food sources that would otherwise be inaccessible, it does not restrict the animal's diet to only structurally defended foods. Nectars and cankers have no known structural defense. Ants may be defended by chitin, but unless the digestive system of the aye-aye contains chitinase, they have no better access to ants than do other animals. Aye-aye specializations and foraging patterns demonstrate that although many ecological specializations may be associated with morphological adaptations, these adaptations are not necessarily associated with ecological specialization. Indeed, in the case of the aye-aye, morphological specialization may confer ecological generalization by allowing the animal to gain access to structurally defended foods in addition to those that are more easily reached.
Although aye-ayes exploit a wide variety of food types, such as seeds, nectar, and larvae, the number of species eaten within each food type is quite small (Table 2). Most primates, including other lemur species, eat a greater variety of species within food types. The reasons for the aye-aye's exploitation of so few species within each food type remain unknown. It is possible that a high dietary diversity exists among the larvae, but sampling techniques have not been able to measure larval diversity. Alternatively, since aye-ayes on Nosy Mangabe specialize on resources that are structurally defended, their choices may be limited if only a few species of structurally defended species grow on the island. However, Nosy Mangabe does not appear to be lacking in structurally defended resources: there are at least five plant species on the island that produce hard-coated seeds, and wood-boring larvae are quite common throughout the island.
The specific factors contributing to Daubentonia's dietary patterns have yet to be fully understood, but there is no question that the aye-aye's morphological adaptations play an important role in its foraging behavior. The aye-aye's hand, which extends up to 45% of its trunk length, is proportionately longer than the hand of almost any other primate — only Tarsius equals Daubentonia in relative hand length — and exhibits a number of structural modifications that are used in locating and consuming food (Figure 6). The middle finger of the hand differs from the other fingers in its relatively gracile construction and
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