Male strategies for changing group membership in Verreaux's sifaka
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Males of many group-living mammals disperse to avoid inbreeding and improve their mating opportunities. Different strategies may exist for immigrants, such as replacing the alpha male or entering a group as a subordinate. Verreaux's sifaka (Propithecus verreauxi) are highly seasonally breeding lemurs characterized by male-biased dispersal and high within-group reproductive skew. We hypothesized that (1) males time transfers to pursue immediate mating opportunities (i.e., that they enter groups in the pre-mating and mating seasons), (2) males prefer groups with greater reproductive opportunities (i.e., with greater numbers of females), (3) entrances with partners more often result in alpha male replacement, (4) male competitive ability affects immigration strategy, and (5) male competitive ability affects alpha male tenure length. To assess male dispersal strategies, we examined seven years of demographic, morphological, and behavioral data for five social groups of Verreaux’s sifaka in the Kirindy Mitea National Park in western Madagascar. Contrary to expectations, we detected no seasonal pattern in immigrations. Males did generally join groups with favorable sex ratios and, to a lesser extent, high numbers of sexually mature females. Transfers occurred individually and in pairs, and a trend existed for partner presence to increase the likelihood of replacing an alpha male. Pronounced activity of the sternal scent gland (a proxy for testosterone) -- but not body mass, canine size, or potential correlates of leaping ability -- significantly influenced immigration strategy. Our results suggest that male immigration strategies are affected by group composition and prior dominance status but not reproductive season or morphological indicators of competitive ability. Competitive ability may instead rely on a combination of morphological and behavioral attributes, such as personality, social skillfulness, or coalitionary support. Additionally, fluid group boundaries may allow mating success without establishment in a social group before the mating season.