Ring-tailed lemurs (Lemur catta)
My research on ring-tailed lemurs (in collaboration with Dr. Timothy Keith-Lucas at The University of the South) examines the evolution of female dominance and the role of male behavioral strategies in reproductive success. It has been hypothesized that female dominance in some lemur species, such as the ring-tailed lemur (Lemur catta), can be related to the high energy demands and relatively inefficient reproductive physiology of these primates in the highly seasonal environment of Madagascar. When food is limited and feeding competition is intense, females would not be able to gain sufficient food to successfully reproduce without feeding priority over males. As females are dominant in many species of lemurs, they gain first access to food resources. This female feeding priority is thought to be crucial to the adaptive significance of this social system.
Female feeding priority is, therefore, expected to be advantageous where feeding competition is strong. It is expected that as feeding competition increases, the greater will be the advantage of female feeding priority and the stronger will be the dominance hierarchy. If female dominance has evolved as hypothesized, variation in levels of feeding competition should produce variation in levels of female dominance and feeding priority.
Our feeding experiments with groups free-ranging ring-tailed lemurs show strong relationships between the degree of female dominance and the level of feeding competition. However, we also found that there is a major cost to male rank in that food access decreases with increased male rank. This cost for high ranking males is, however, not a consequence of female aggression. Instead, high ranking males appear to defer to those females that have reached maturity. By doing so, these males avoid the aggressive interactions that are instead directed primarily at lower ranking males. Females are not, however, choosing “wimpy” males, as has been suggested in some cases, as these males score highest in agonistic interactions among the males. Although these high-ranking males receive less food, the major advantage appears to be that they are preferred mates.
Ruffed Lemurs (Varecia variegata)
Ruffed lemurs present a unique set of female behavioral and ecological traits. As specialized frugivores, individuals typically have large, defended home ranges. The female reproductive strategy involves keeping infants in nests that the female guards for the first few weeks of life, thus restricting her ranging during this time. This strategy of shareable care (guarding) as opposed to non-shareable care (carrying offspring) has allowed Varecia to produce up to four infants per litter instead of the more typical single births of primates of this size.
The social organization of this species is especially interesting as it is first characterized by highly flexible sociality: groups vary from stable pair-bonds to what have been described as fission-fusion communities and inter-individual cohesion is highly variable. I have found that female-female relationships are unusual in this species, in that maturing daughters consistently outrank their mothers and may even evict them from social groups if no other options are available. This behavior may actually reflect a strategy of female inheritance of territories that is tied to the invasion by the mother of an established territory elsewhere. Established territories are preferred due to the need for a breeding male to guard offspring, primarily against infanticide. This strategy of invasion and inheritance explains the significantly biased sex-ratio at birth observed for females of different ages. Over time this strategy would expand a pair-bonded group to produce the fission-fusion network of related females with neighboring territories that has been found for some populations.