Mating System

The exploratory sojourns of aye-aye males are striking. Travel over Nosy Mangabe's steep slopes using both terrestrial and arboreal locomotion requires a great expenditure of energy, suggesting that there must be strong incentives for males to travel long distances. The fact that individual male home ranges overlapped those of many females may support the prediction that males are distributing themselves to best take advantage of the distribution of females. Lemurs typically exhibit a strictly seasonal pattern of breeding, with a limited number of successive estrous cycles occurring at a particular time of the year, which varies from species to species. The aye-aye, however, is unique among Malagasy primates in the unpredictability of its breeding events throughout the year. Evidence from the wild suggests that aye-ayes do not exhibit reproductive synchrony: females in close proximity to one another neither cycle nor become pregnant at the same time (Gibson, D., personal communication; Sterling, 1994b). Estrus brevity in individual females and asynchrony across females means that a male aye-aye's ability to detect when the female is in estrus is very important and very difficult. Long forays by males and consequent large home ranges may reflect male efforts to locate estrous females. Indeed, male aye-ayes on Nosy Mangabe did encounter estrous females on a number of their extended foraging excursions.

Resource availability and photoperiod are often cited as important factors in reproductive timing in animals, but aye-aye breeding behavior does not appear to be influenced by either variable. Aye-ayes eat all their major food resources across almost all months of the year, and the timing of peak availability of fruits varies from year to year. It is unclear whether the availability of food resources is highly unpredictable, or if there are patterns that existing data cannot detect. Field observations of aye-aye births occurring throughout the year suggest that photoperiod has little effect on reproductive timing. In addition, aye-ayes maintained on different light regimes in two different captive institutions mated and gave birth at approximately the same time, indicating that breeding was not prompted by changes in the light cycle. It seems that whatever factors contribute to reproductive seasonality in most other lemurs may not affect wild aye-ayes (Sterling, 1994b).

Behavioral signs of reproductive activity become apparent in female aye-ayes about 10 days prior to the onset of full estrus. Females increase the frequency of scent-marking and often visit nests occupied by males, a behavior not seen outside the mating season (Sterling, 1993b). Physiologically, female estrus is marked by vulvar and labial swelling, and a color change in the labia from gray to pink or red (Winn, 1994). Prior to and during mating activity, males exhibit testicular swelling and increase scent-marking frequency. During this time, males cluster around the female during both day and night and, like the females, increase their scent-marking frequency. Males generally mate with a female about a week after testicular swelling is first observed.

During each night of estrus, females exhibit a repetitive pattern of moving swiftly over 500-1000 m, and then sitting still for about an hour and emitting long calls that they use only during the mating period. In response to this call, several males converge on the female from all directions. Males chase and fight with one another near the female, and the female repels some mating attempts while accepting others. The accepted male copulates with the female and maintains hold of her for about an hour, while other males chase each other in circles around the pair and try to dislodge whichever male is copulating with the female. When copulation is complete, the female quickly moves another 500 to 1000 m and repeats the pattern. A female may mate with one or more males during each night of estrus, making it impossible to determine which male is the father of the female's offspring without genetic analysis (Sterling and Richard, 1995).

The aye-aye's home range patterns and breeding behavior suggest that the species exhibits scramble competition polygyny, in which females are solitary and males range widely in search of estrous females (Clutton-Brock, 1989). The females' advertisement calls suggest that they have an interest in attracting more than one male, possibly to provide themselves with a choice of several males on each night of estrus. Individual females may benefit by maintaining reproductive asynchrony, because a large number of males are available to respond to any individual female's call on a given night (Kappeler, 1997c). When a female chooses a mate, the male monopolizes her for a long period, either through single or multiple intromissions. In this way, the male temporarily prevents other males from inseminating the female while increasing his chances for fathering the offspring. Because females mate with more than one male during each period of estrus, Daubentonia has a multi-male - multi-female breeding system (Sterling, 1993b). The aye-aye was one of the first lemurs noted to exhibit scramble competition polygyny, but subsequent studies have pointed to other nocturnal lemurs with this system, such as Coquerel's dwarf lemur (Mirza coquereli) (Kappeler, 1997c) and the gray mouse lemur (Microcebus murinus) (Radespiel, 2000). These species also display similar home range patterns to those found in Daubentonia, providing further support for the connection between larger male home ranges and scramble competition polygyny.

Polygynous species often display pronounced sexual dimorphism, with larger, stronger males able to outcompete other males for available females. Sexual size dimorphism is characteristic of many solitary primates, including the Lorisidae, most Galagidae, and Pongo pygmaeus (Kappeler, 1997a). However, Malagasy primates, including the aye-aye, generally lack sexual size dimorphism, and some lemur species even show a trend toward larger female size (Jolly, 1998). The wide variety of mating systems and patterns of sexual dimorphism found in lemurs are often seen as a challenge to the general predictions of sexual selection theory (Kappeler, 1997a; Müller and Thalmann, 2000).

In scramble competition polygyny, a male's primary challenge is to locate sexually receptive females. Male attributes such as mobility, perceptiveness, and spatial memory are likely to aid a male in finding estrous females in these systems (Schwagmeyer, 1988). These traits may benefit a male more than would large body size or other defensive traits that are characteristic of males of species that engage in direct combat for females. Studies of other species that use a scramble competition polygyny mating system may help to explain the lack of sexual dimorphism in aye-ayes and other lemurs. Eberle and Kappeler (2004) found that body mass was a poor predictor for mating success in male gray mouse lemurs (Microcebus murinus), whereas a high level of spatial familiarity improved mating success. Similarly, the mating success of male thirteen-lined ground squirrels is closely linked to the number of estrous females he finds, while his ability to dominate over his competitors is a poor predictor of mating success (Schwagmeyer, 1988).

Scramble competition polygyny in aye-ayes may correlate with improved spatial ability and mobility in males, as seems to be the case in other species with this type of mating system. Unlike aye-ayes, however, gray mouse lemurs and thir-teen-lined ground squirrels tend to exhibit seasonal reproduction, which may limit direct contest competition between males and alleviate the need for males to have a large body size. The lack of seasonality in the estrus cycles of female aye-ayes would be expected to promote direct (contest) competition between males, presumably leading to sexual size dimorphism in addition to improved male spatial ability. The prolonged intromissions and enlarged testes observed in aye-ayes may be a strategy to manage competition. Males of other lemur species with polygynous mating systems exhibit increased testis size, either permanently or through testicular swelling during the mating season (Kappeler, 1997b). Another successful strategy used by males of many polygynous species is the use of copulatory plugs, although this possibility has not yet been explored in aye-ayes. All of these may increase a male's chance of fathering the offspring of a female that mates with additional males (Kappeler, 1997c; Parga, 2003; Schwab, 2000). Clearly, much remains to be understood about Daubentonia's mating system and correlations with male and female morphology and behavior.

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