Evolution of sexually dimorphic characters in peccaries (Mammalia, Tayassuidae)

Abstract

Cladistic analysis of osteological and dental characters in a monophyletic group of Miocene and younger tayassuids demonstrates a pattern of changes in the degree of sexual dimorphism in canine tooth diameter and zygomatic arch width, and in phenotypic correlations between these characters. Primitively, tayassuids have canine teeth that are sexually dimorphic and discretely bimodal in size, and zygomatic arches that are narrow in both sexes. Many late Miocene and Pliocene tayassuids have broad, winglike zygomatic processes. In some species, these processes are large in both sexes, but in others, those of females are much smaller than those of males. The presence of large processes in both sexes is primitive relative to the condition of strong sexual dimorphism. In five separate clades, the zygomatic processes of both sexes become reduced in size, and the degree of sexual dimorphism in canine size becomes reduced as well. The pattern is congruent with predictions derived from a theoretical model of the evolution of sexual dimorphism, and it further indicates the emergence of a new phenotypic correlation between two previously uncorrelated characters, canine size and zygoma size. The advent of this new correlation coincides with the advent of pronounced sexual dimorphism in zygomatic processes. Although such a pattern could be explained by genetically modifying phenotypic expression of homologous characters in one sex or the other, an epigenetic modification of expression is equally plausible: the evolution of sexual dimorphism in homologous characters could be accomplished by placing phenotypic expression of an originally monomorphic character under the control of steroid sex hormones. This hypothesis is consistent with evidence from many vertebrate groups, and it provides testable predictions.

Sexual Size Dimorphism as a Correlated Response to Selection on Body Size: An Empirical Test of the Quantitative Genetic Model

“…This type of response is not supported by the results of the present study, which showed that dimorphism changed even more slowly than predicted by the models. Support for the Lande model of slow SSD evolution (as opposed to the fast response of the sex-limited genes hypothesis) has previously been found in the fossil record: Wright (1993) concluded that skeletal elements of extinct peccaries followed the expectations of the Lande model in that sexually selected structures were found in both sexes before they were slowly reduced in females. In the only other test of the model, Rogers and Mukherjee (1992), analyzing data on the rate of change in dimorphism in three human traits, concluded that the Cheverud mechanism of stronger response in the sex with the higher heritability was not sufficient to account for the existing relationships seen between dimorphism and body size throughout the primate order.…”

“…Although Lande's paper is generally interpreted as predicting that SSD will be slow to evolve (e.g., Rogers and Mukherjee 1992;Wright 1993), this is not a fundamental prediction of the model itself, but rather a consequence of the assumptions made about parameter values. Cheverud et al (1985), again using the standard formulae (eqs 3a,b), showed algebraically that, given assumptions of equal selection intensities and heritabilities in each sex but unequal phenotypic variances, SSD could evolve rapidly and that its magnitude is actually enhanced by high genetic correlations.…”

Sex-Limited Mutations and the Evolution of Sexual Dimorphism

“…Thus, assumptions concerning the inheritance of quantitative traits may account for the mediocre predictive power of previous models, even when genetic parameters are accurately estimated (Reeve and Fairbairn 1996). Although sex chromosome mutations and sex-limited mutations on the autosomes (Mackay et al 1992) may allow sexual dimorphism to evolve relatively freely over much longer periods of time (Wright 1993;Fry et al 1995), the rate of such mutations remains virtually unknown. Consequently, the evolution of sexual dimorphism may be constrained, even when the genetic correlation between the sexes is less than perfect.…”

“…After some tedious algebra, it was determined that equilibrium allele frequencies are a complex function of sex-specific selection and the presence of other alleles at the same locus (results not shown). Thus, I use Wright's adaptive landscape to illustrate mean population fitness as a function of different patterns of sex-specific selection and different allele frequencies (Wright 1932). I use the product of the sex-specific mean fitnesses (i.e., W ϭ W F ϫ W M ) as the measure of mean population fitness because it is the denominator for all terms on the right hand side of equations (4), (5), and (6).…”

Record of tayassuids in ?late Pliocene to Quaternary deposits in Venezuela

“…Based on cranial and dental characters, Wetzel et al (1975) and Wetzel (1977) proposed that Tayassu and Pecari are more closely related to each other than to Catagonus. In contrast, based on his analysis using osteological and dental characters Wright (1989Wright ( , 1993Wright ( , 1998 considered Pecari and Catagonus to be more closely related, whereas Tayassu was considered to be a member of a separate clade along with other extinct species.…”