<p>Indri indri is unusual in that it is only one of three prosimian species to use a monogamous mating system—one which may have resulted due to both ecological and biological factors. Ecological constraints influencing the mating system of indris include spatio-temporal dispersion of mates. Indris live in groups of two adults, one male and female, along with their recent offspring (Pollock 1975). This grouping pattern along with the territorial behavior of indris creates a spacing pattern that prevents females from grouping into clusters which could be monopolized by males to form a polygynous mating system. Additionally, indris have been observed to give birth during the month of May after a gestation period of 4-5 months (Pollock 1975). Thus, there is a rather narrow window of opportunity for males to mate with reproductively active females. Indispensable parental care of developing indris by both sexes may also greatly influence the mating system of the Indri. Parents can live with their offspring for several years depending on when the young will leave their parental home ranges (Macdonald 2001). Infant indris are not even capable of independent movement until after eight months of being carried either ventrally or dorsally (Garbutt 1999). The aid of males in raising young seems a possible factor in the monogamous mating system of indris based on the slow development of the young. In agreement with this hypothesis, Pollock (1979) observed males sharing at times in the carrying duties of their offspring. While no single cause for the mating behavior of indris can be identified, the ecological and behavioral factors influencing indris fit the general pattern of constraints forcing indris into their observed monogamous mating system.</p><p>Garbutt, N. 1999. Mammals of Madagascar. Sussex: Pica Press.</p><p>Macdonald, David. 2001. The New Encyclopedia of Mammals. Oxford: Oxford University Press.</p><p>Pollock, J.I. 1975. “Field observations on Indri indri: a preliminary report.” In Lemur Biology, ed. I. Tattersall and R.W. Sussman. New York: Plenum Press, pp. 287- 311.</p><p>Pollock, J.I. 1979. “The ecology and sociology of feeding in Indri indri.” In The study of prosimian behavior, ed. G.A. Doyle and R.D. Martin. New York: Academic Press, pp. 359-409.</p>