Bimorphism in Male Verreaux’s Sifaka in the Kirindy Forest of Madagascar

Abstract

Male primates in species with pronounced secondary sexual adornments can exhibit reversible or irreversible bimorphism, i.e., striking variation in the degree to which males express the adornments. Verreaux's sifaka (Propithecus verreauxi verreauxi) use scent marking as a form of communication and exhibit sex differences in scent glands. Some males exhibit a pronounced brown staining around their sternal gland, whereas others do not. We studied morphological and behavioral characteristics of males in 6 social groups in Kirindy Forest, Madagascar, from November 2000 to March 2002 to evaluate the hypotheses that the bimorphism in male sifaka chest status represents alternative mating tactics and is a badge of status. Males are clearly divided into 2 categories: clean and stained chests, with rare, but informative, intermediate males. The chest staining probably results from the males scent marking with their sternal glands, because stained-chested males scent marked significantly more often than clean-chested males. Though sample sizes are small, chest status did not appear to depend on body size. Chest status is reversible and related to dominance rank. In each group, only 1 male, the dominant, was stained-chested, whereas all other (subordinate) males were clean-chested. These findings suggest that stained chests are visual and olfactory signals of dominance rank and that clean chests signal lack of competitive intent. Thus, 159 0164-0291/07/0200-0159/0 C 2007 Springer Science+Business Media, LLC 160 Lewis and van Schaik this bimorphism may reflect alternative mating tactics used by males to maximize their reproductive success based upon their social environment.

Mate-Guarding as a Male Reproductive Tactic in Propithecus verreauxi

“…This conclusion is in concordance with previous descriptions of the behavior in sifakas of this and other populations (Brockman 1999;Lewis and van Schaik 2007). In comparison to both non-natal subordinate and natal male-female dyads, female-dominant male dyads spent more time in close proximity during the RP and these bouts of proximity were both primarily initiated and maintained by the dominant male.…”

“…Although we could not test statistically changes in male-male aggression rates, the data suggest that there may be an increase in aggression towards non-natal subordinate males during the RP that is not associated with feeding competition. Even so, the overall rate of aggression in the Kirindy population is low (Lewis and van Schaik 2007), which suggests that males are not physically fighting for access to females.…”

“…Although Brockman (1999) and Lewis and van Schaik (2007) described mateguarding in sifakas, here we attempt to quantify it for the first time. To determine whether dominant males use mate-guarding as a proximate mechanism to exclude rival males from reproduction, we tested the predictions that 1) females are receptive asynchronously within groups; 2) males increase their olfactory behavior when females are receptive; 3) dyads consisting of a female and the dominant male spend more time in proximity during the receptive period than in the mating season but there is no change in proximity between natal or non-natal subordinate males and females; 4) dominant males are responsible for both the initiation and maintenance of proximity with females; 5) in relation to the costs associated with mate-guarding, an increase in dominant male aggression rate toward rival males within the group occurs but not toward natal males, and a decrease in the total time dominant males spent feeding and their feeding bout lengths while females are receptive; and 6) if both males and females can enhance their mating opportunities through increased contact with neighboring groups, an increase in intergroup encounter rate when females are receptive occurs.…”

Color in competition contexts in non-human animals

“…Red skin occurs in the Cercopithecinae (including in Hamadryas baboons, macaques, vervets, geladas, mandrills and drills) and Colobinae (including in proboscis monkeys), as well as in New World monkeys (including in some species of howler monkeys, spider monkeys, sakis and uacaris), while blue skin has evolved at least twice, in Mandrillus and Cercopithecus (both are Cercopithecine genera, Prum & Torres, 2004). I am not aware of 6 any evidence of cosmetic or adventitious colours in any non-human primates, unlike humans and birds (Montgomerie, 2006a), with the possible exception of chest-staining as a byproduct of scent-marking in sifaka (Lewis & van Schaik, 2007).…”

The lemur syndrome unresolved: extreme male reproductive skew in sifakas (Propithecus verreauxi), a sexually monomorphic primate with female dominance

“…Infanticide by strange males following group takeovers is another male reproductive strategy (Lewis et al 2003). One male in each group is dominant, has a large chest gland, and has significantly elevated testosterone levels during the mating season (Kraus et al 1999;Lewis and van Schaik 2007), but there is no directional selection on male body mass or canine size (Lawler et al 2005). However, because access to receptive females is independent of the occurrence or outcome of male aggression and may be mediated in part by sperm competition (see Pochron and Wright 2005), the sifaka mating system has been characterized as female-controlled polygyny (Richard 1992).…”