Assortative Mating in a Polymorphic Salamander
Reproductive isolation due to divergent selection is thought to be one of the mechanisms that promote speciation in sympatry. A key element of reproductive isolation is assortative mating. We examined a polymorphic population of Eastern Red-backed Salamanders (Plethodon cinereus) for evidence of reproductive isolation through assortative mating. Our study population was made up of two common color morphs, striped and unstriped. In the field, we turned over natural cover objects to find male–female pairs of P. cinereus during peak mating season. We recorded sex, color morphology, and snout–vent length of 112 pairs of salamanders. Estimates of sexual isolation indicated weak assortative pairing in the field with more same-color pairs than expected by chance. Striped females paired with striped males were significantly larger in size than those paired with unstriped males. Intermorph pairs were observed and such pairings, if successful, would interfere with the potential for divergence. Laboratory experiments were conducted to assess the ability of females of each color morph to distinguish between the two phenotypes through fecal odors and male scent. Additionally, mating trials were conducted to examine assortative mating in the laboratory. We found no evidence that scent or natural diet cues (fecal odors) of males contributed to assortative pairing, but females of both phenotypes were more likely to be associated with striped males during mating trials. Our study provides additional evidence that striped males of P. cinereus may be more attractive to females and this may contribute to positive assortative mating in the field. Territoriality and diet may be important factors that influence this pattern.
Size differences between males and females that shared natural cover on the forest floor (n = 100 pairs). Mean index of sexual size dimorphism in the four pair categories is shown. Female salamanders are depicted above males and lines represent standard errors. Different letters associated with bars represent statistically significant differences at α < 0.05. Index values above 100% indicate that, on average, males were smaller than their presumptive mates.
Laboratory experiment 1. Mean nose taps performed by females of Plethodon cinereus when exposed to control fecal pellets, pellets from striped males, and pellets from unstriped males. (A) Responses of striped females. (B) Responses of unstriped females. Different letters associated with bars represent statistically significant differences at α < 0.05. Lines represent standard errors. Females exhibited significantly elevated responses to male fecal pellets, but they failed to distinguish between those of striped and unstriped males.
Laboratory experiment 2. Mean nose taps performed by females of P. cinereus when exposed to controls (no odor), odors of striped males, and odors of unstriped males. (A) Responses of striped females. (B) Responses of unstriped females. Different letters associated with bars represent statistically significant differences at α < 0.05. Lines represent standard errors. Females exhibited significantly elevated responses to male odors, but they failed to distinguish between those of striped and unstriped males.
Contributor Notes
Associate Editor: J. W. Snodgrass.