Seasonal Adjustment of Growth Rate and Adult Body Weight in Ringtailed Lemurs

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Abstract

Photoperiodic cues provide nonequatorial animals a reliable schedule for adaptive changes in metabolism, developmental rate, reproductive effort, and behavior. This study discovered seasonal adjustments of growth rate and adult body weight in semi-free-ranging ringtailed lemurs (Lemur cafta) that were correlated with changes in rate of hair growth, thermoregulatory behavior, and social behavior but independent of changes in food availability. Infants increased body weight with 3 to 12g/day, accelerating growth rate annually just after summer solstices. Adult males and females gained weight one to two months later, just before longterm suppressions of growth rate in immatures. Correlations between this set of results and the annual schedule of rainfall in Madagascar led me to propose a new model of life-history strategy for the large-bodied lemurs. Harsh but predictable seasonality of nutritive resources seems to have led these primates to evolve life histories comprehensively geared toward the conservation of energy, including suppression of metabolic rate for much of the Malagasy dry season. This life-history tactic would be analogous to diverse torpid states that enhance survival and reproduction in nonprimate mammals living in harshly seasonal environments. The overall life-history strategy, however, requires rates of infant growth that are uncommonly high among group-living primates relative to maternal size. Like many other animal parents, lemur mothers suffer high costs of investment to help offspring meet a deadline: lemur infants must grow large and perhaps store energy before the dry season, so they can safely reduce metabolic rate and grow little during the dry season. The high cost of lactation is suggested to have promoted wide-spread evolution of female dominance and male-committed infanticide among lemurid and indrid species. Continued research on the environmental regulation of life histories will be required before a deep understanding of primate natural history can be achieved.