RELATIVE GROWTH OF THE LIMBS AND TRUNK IN SIFAKAS - HETEROCHRONIC, ECOLOGICAL, AND FUNCTIONAL CONSIDERATIONS

Limb, trunk, and body weight measurements were obtained for growth ser ies of Milne-Edwards's diademed sifaka, Propithecus diadema edwardsi, and the golden-crowned sifaka, Propithecus tattersalli. Similar measur es were obtained also for primarily adults of two subspecies of the we stern sifaka: Propithecus verreauxi coquereli, Coquerel's sifaka, and Propithecus verreauxi verreauxi, Verreaux's sifaka. Ontogenetic series for the larger-bodied P. d. edwardsi and the smaller-bodied P. tatter salli were comb pared to evaluate whether species-level differences in body proportions result from the differential extension of common pat terns of relative growth. In bivariate plots, both subspecies of P. ve rreauxi were included to examine whether these taxa also lie along a g rowth trajectory common to all sifakas. Analyses of the data indicate that postcranial proportions for sifakas are ontogenetically scaled, m uch as demonstrated previously with cranial dimensions for all three s pecies (Ravosa, 1992). As such, P. d. edwardsi apparently develops lar ger overall size primarily by growing at a faster rate, but not for a longer duration of time, than P. tattersalli and P. verreauxi; this is similar to results based on cranial data. A consideration of Malagasy lemur ecology suggests that regional differences in forage quality an d resource availability have strongly influenced the evolutionary deve lopment of body-size variation in sifakas. On one hand, the rainforest environment of P. d. edwardsi imposes greater selective pressures for larger body size than the dry-forest environment of P. tattersalli an d P. v. coquereli, or the semi-arid climate of P. v. verreauxi. On the other hand, as progressively smaller-bodied adult sifakas are located in the east, west, and northwest, this apparently supports suggestion s that adult body size is set by dry-season constraints on food qualit y and distribution (i.e., smaller taxa are located in more seasonal ha bitats such as the west and northeast). Moreover, the fact that body-s ize differentiation occurs primarily via differences in growth rate is also due apparently to differences in resource seasonality (and juven ile mortality risk in turn) between the eastern rainforest and the mor e temperate northeast and west. Most scaling coefficients for both arm and leg growth range from slight negative allometry to slight positiv e allometry. Given the low intermembral index for sifakas, which is al so an adaptation for propulsive hindlimb-dominated jumping, this sugge sts that differences in adult limb proportions are largely set prenata lly rather than being achieved via higher rates of postnatal hindlimb growth. Our analyses further indicate that the larger-bodied P. d. edw ardsi has a higher adult intermembral index than P. tattersalli and P. verreauxi, thus supporting the allometric argument regarding the inte rspecific scaling of limb proportions in arboreal primates which emplo y vertical postures (Cartmill, 1974, 1985; Jungers, 1978, 1985). Lastl y, additional analyses indicate that P. d. edwardsi exhibits significa nt sexual dimorphism where adult females are larger than adult males i n about one-third of all adult comparisons, whereas P. tattersalli exh ibits significant sexual dimorphism in about one-fifth of all adult co mparisons. Among western sifakas, adult P. v. coquereli exhibit signif icant sex dimorphism in about one-third of all comparisons, whereas ad ult P. v. verreauxi show no significant differences between the sexes. Given that all taxa are ontogenetically scaled, this suggests that se xual dimorphism develops via such processes as well. Interestingly, ou r data indicate that sex dimorphism is allometric, with larger-bodied taxa like P. d. edwardsi being more d