At least half of the world’s primate species are currently threatened with extinction. Slow life histories combined with rapid habitat loss and hunting in recent years has heightened the extinction risk for many species, including the two largest extant lemurs, the diademed sifaka (Propithecus diadema) and the indri (Indri indri). Both species belong to the taxonomic family Indriidae, have similar adult weights, and occur in sympatry in certain areas of the montane rainforests of eastern Madagascar. Both species are adapted for folivory however I. indri spend considerably more time feeding on leaves than do P. diadema resulting in several energy-saving adaptations in I.indri. In this dissertation, I explore infant development and maternal strategies of these critically endangered primates with the goal of increasing our knowledge of reproduction and ontogeny in both species. Although previous studies have elucidated key differences in adult behaviour, there is a dearth of information on infants and lactating females in either of these two species. Between June and December of 2011 and 2012, I collected continuous time focal animal data, in Maromizaha forest, to examine behavioural patterns of 12 infants and their mothers from 0 – 33 weeks. In addition, I developed a framework to define and quantify the weaning process and facilitate comparisons across different species and studies. P. diadema infants developed feeding competency and independent locomotion faster than did I. indri infants however both species were consistently feeding independently more than they were suckling by week 20. The process of feeding ontogeny in I. indri was likely accelerated by coprophagy, as all infants of this species consumed their mother’s feces regularly from 10 – 15 weeks old. Lactating females of both species spent more time feeding in mid-lactation when maternal investment was the highest. The prolonged inter-birth interval in I. indri is suggested as another adaptation that reduces energetic expenditures. In addition, the protracted period of close contact with their mother may offer infant I. indri more time for social learning of the mother’s diet and the group song and for developing competency in vertical clinging and leaping without a tail for balance and support. / Graduate

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