Sexual selection and canine dimorphism in new world monkeys

Abstract

Social and ecological factors are important in shaping sexual dimorphism in Anthropoidea, but there is also a tendency for body-size dimorphism and canine dimorphism to increase with increased body size (Rensch's rule) (Rensch: Evolution Above the Species Level. London: Methuen, 1959.) Most ecologists interpret Rensch's rule to be a consequence of social and ecological selective factors that covary with body size, but recent claims have been advanced that dimorphism is principally a consequence of selection for increased body size alone. Here we assess the effects of body size, body-size dimorphism, and social structure on canine dimorphism among platyrrhine monkeys.Platyrrhine species examined are classified into four behavioral groups reflecting the intensity of intermale competition for access to females or to limiting resources. As canine dimorphism increases, so does the level of intermale competition. Those species with monogamous and polyandrous social structures have the lowest canine dimorphism, while those with dominance rank hierarchies of males have the most canine dimorphism. Species with fission-fusion social structures and transitory intermale breeding-season competition fall between these extremes.Among platyrrhines there is a significant positive correlation between body size and canine dimorphism However, within levels of competition, no significant correlation was found between the two. Also, with increased body size, body-size dimorphism tends to increase, and this correlation holds in some cases within competition levels.In a n analysis of covariance, once the level of intermale competition is controlled for, neither molar size nor molar-size dimorphism accounts for a significant part of the variance in canine dimorphism. A similar analysis using body weight as a measure of size and dimorphism yields a less clear-cut picture: body weight contributes significantly to the model when the effects of the other factors are controlled. Finally, in a model using head and body length as a measure of size and dimorphism, all factors and the interactions between them are significant. We conclude that intermale competition among platyrrhine species is the most important factor explaining variations in canine dimorphism. The significant effects of size and size dimorphism in some models may be evidence that natural (as opposed to sexual) selection also plays a role in the evolution of increased canine dimorphism.It is generally recognized that there are tion. Explanations that invoke phyletic inertia (Cheverud et al., 1985a,b) depict dimany "causes" for sexual dimorphism in primates and other mammals. Four sorts of mechanisms have been proposed to explain dimorphism: phyIetic 'Orrelated re-

Sexual dimorphism in primate evolution

“…However, there are other factors including developmental, phylogenetic, and genetic constraints, natural selection, and epiphenomenal factors that can influence the expression of dimorphism in species (Crook, 1972;Clutton-Brock et al, 1977;Leutenegger and Kelly, 1977;Harvey et al, 1978;Cheverud, 1982, 1985;Gaulin and Sailer, 1984;Cheverud et al, 1985;CluttonBrock, 1985;Gautier-Hion and Gautier, 1985;Leutenegger and Lubach, 1987;Shea, 1986;Oxnard, 1987;Kay et al, 1988;Ely and Kurland, 1989;Kappeler, 1990Kappeler, , 1991Kappeler, , 1996Greenfield, 1992aGreenfield, ,b, 1996Leigh, 1992Leigh, , 1995aPlavcan and van Schaik, 1992, 1997bFord, 1994;Martin et al, 1994;Hayes et al, 1995;Leigh and Shea, 1995;Mitani et al, 1996;Plavcan, 1998Plavcan, , 1999. The relative importance of these factors to the expression of dimorphism among primates has been at times a topic of heated debate.…”

“…Mate competition has long been thought to be the primary mechanism favoring the development of sexual dimorphism in primates (Darwin, 1871;Crook, 1972;Leutenegger and Kelly, 1977;CluttonBrock, 1985;Kay et al, 1988;Kappeler, 1990Kappeler, , 1991Plavcan and van Schaik, 1992, 1997bFord, 1994;Martin et al, 1994;Lindenfors and Tullberg, 1998). In easily observed species, such as baboons, males are largely intolerant of one another, often engaging in spectacular fights and threat displays.…”

Patterns of size sexual dimorphism in Australopithecus afarensis: Another look

“…In primates, there is a general finding of associations between monogamous species and low sexual dimorphism, and between polygynous species and high sexual dimorphism (Alexander et al, 1979;Clutton-Brock and Harvey, 1977;Ford, 1994;Kay et al, 1988;Leutenegger and Cheverud, 1985;Leutenegger and Kelley, 1977;Lindenfors, 2002;Lindenfors and Tullberg, 1998;Mitani et al, 1996;Plavcan, 2000Plavcan, , 2001Plavcan and Cope, 2001;Plavcan and van Schaik, 1992. If this relationship is to be held true for the past, one can draw inferences about the mating system in fossil primates (Andrews, 1983;Fleagle et al, 1980;Greenfield, 1972;Kay, 1982a, b;Kelley, 1986;Kelley and Xu, 1991;Pan et al, 1989;Wu and Oxnard, 1983a, b;Zhang, 1982).…”

Sexual dimorphism in canine length of woolly spider monkeys (Brachyteles arachnoides, E. Geoffroy 1806)

“…Previous workers who found no sexual dimorphism in canine lengths, (Zingeser, 1973;Milton, 1985a, Kay et al, 1988Rosenberger and Strier, 1989), may have dealt only with northern populations or combined specimens, regardless of their geographical origins. In addition to sexual dimorphism in canine length, we also observed other differences between northern and southern populations.…”

“…Previous studies of sexual dimorphism documented no interpopulational difference in dentition (Zingeser, 1973, Kay et al, 1988. We present data on sexual dimorphism in canine length, based on measurements of museum specimens and live monkeys in the field.…”