The effect of body size on male courtship success in a plethodontid salamander

Sexual selection and male-biased size dimorphism in a lineage of lungless salamander (Ampibia: Plethodontidae)

Evolutionary biologists have long focused on the patterns and causes of sexual size dimorphism (SSD). While female-biased SSD is common among ectotherms, a few lineages predominately exhibit male-biased SSD. One example is the clade of desmognathans, a monophyletic group of two genera within the Plethodontinae of the lungless salamander family Plethodontidae. Members of these two genera have a unique pattern of SSD: males mature earlier and at smaller sizes than females but reach greater maximum sizes. We used comparative phylogenetic methods to test whether SSD in these salamanders is the result of sexual selection on males. Spatial evolutionary and ecological vicariance analysis indicated a significant divergence in SSD associated with the phylogenetic origin of the desmognathans. Phylogenetic least-squares regression across the two most speciose genera of the subfamily determined a significant relationship between SSD and adult sex ratio. While male desmognathans are not sexually dimorphic in head size, they have a unique head morphology that causes their heads to grow more rapidly as their body size increases as compared with the heads of other salamanders. This pattern of allometric growth combines with a powerful bite force and enlarged premaxillary teeth to create formidable weaponry that probably is more responsive to sexual selection.

THE EVOLUTION OF ASYMMETRY IN SEXUAL ISOLATION: A MODEL AND A TEST CASE

We constructed a model for the evolution of sexual isolation by extending Lande's (1981) model of sexual selection. The model predicts that asymmetric sexual isolation is a transient phenomenon, characteristic of intermediate stages of divergence in sexually selected traits. Unlike the Kaneshiro (1976, 1980) proposal, our model does not depend upon drift and the loss of courtship elements to produce asymmetries in sexual isolation. According to our model, the direction of evolution cannot be predicted from asymmetry in sexual isolation. We tested some features of the model using data from an experimental study of sexual isolation in the salamander Desmognathus ochrophaeus. We tested for sexual isolation between 12 allopatric populations and found significant asymmetry in sexual isolation in about a quarter of the test cases. The highest degrees of asymmetry were associated with intermediate levels of divergence. A curvilinear relationship between isolation asymmetry and divergence was predicted by our model and was supported by statistical analysis of the salamander data.

Size does not matter for male Marbled Salamanders (Ambystoma opacum)

Understanding the phenotypic attributes that contribute to variance in mating and reproductive success is crucial in the study of evolution by sexual selection. In many animals, body size is an important trait because larger individuals enjoy greater fitness due to the ability to secure more mates and produce more offspring. Among males, this outcome is largely mediated by greater success in competition with rival males and (or) advantages in attractiveness to females. Here we tested the hypothesis that large male Marbled Salamanders (Ambystoma opacum (Gravenhorst, 1807)) mate with more females and produce more offspring than small males. In experimental breeding groups, we included males chosen specifically to represent a range of sizes. After gravid females mated and nested freely, we collected egg clutches and genotyped all adults and samples of hatchlings with highly variable microsatellite markers to assign paternity. Size had little effect on male mating and reproductive success. Breeding males were not bigger than nonbreeding males, mates of polyandrous females were not smaller than those of monogamous females, and there was no evidence for positive assortative mating by size. Although body size did not matter for male Marbled Salamanders, we documented considerable fitness variation and discuss alternative traits that could be undergoing sexual selection.

Female sperm storage in amphibians

The three orders of extant amphibians are Gymnophiona, Anura, and Urodela. Although all gymnophionans apparently have internal fertilization and many are viviparous, female sperm storage is unknown. Internal fertilization has convergently evolved in a few anurans, but females of just one species, Ascaphus truei, are known to possess oviductal sperm storage tubules (SSTs). The SSTs of A. truei are similar anatomically to such glands in squamate reptiles. This similarity is convergence due to similar functional adaptations and/or internal design constraints. In salamanders and newts (Urodela), absence of sperm storage in females is the ancestral condition (three families). In the derived condition, sperm storage occurs in cloacal glands called spermathecae, and their possession is a synapomorphy for females in the suborder Salamandroidea (seven families). Salamandroids are the only vertebrates with cloacal sperm storage glands. In this paper, a phenetic analysis of variation in spermathecal characters reveals patterns of convergence in certain spermathecal characters in unrelated taxa that breed in similar habitats. In the family Salamandridae, a role in sperm nutrition for the spermathecal epithelium is questioned, and the widespread occurrence of spermiophagy is related to other reproductive strategies. I propose how the packaging of sperm in structurally different types of spermathecae may influence male paternity.

Variance in male mating success and female choice for persuasive courtship displays

An important function of the courtship displays of male animals is to stimulate, or 'persuade', females to mate. While many studies of courtship success have shown that females choose stimulating males as partners, fewer have demonstrated that the most persuasive males in a population also enjoy the highest mating success (i.e. mate with the greatest numbers of partners). Such a relationship provides a stronger test of the hypothesis that male displays evolve by sexual selection via female choice. We tested this hypothesis in the laboratory using the North American plethodontid salamander, Desmognathus ocoee, in which females only elicit spermatophore deposition if rendered sexually responsive by male courtship. First, we determined variance among individual males in the numbers of females with which they mated across 35 encounters, in the absence of differences in partner encounter rates and direct intermale interactions. Second, we demonstrated that males with high 'historical' scores of mating success performed significantly higher frequencies of persuasive courtship displays, which provide tactile, chemical and visual stimulation to females. We conclude that sexual selection via female choice favours persuasive displays because they confer high mating success on the males that perform them at the highest frequencies. Copyright 1998 The Association for the Study of Animal Behaviour.

Differential effects of mate competition and mate choice on eastern tiger salamanders

Male tiger salamanders, Ambystoma tigrinum tigrinumare slightly larger in body size and have considerably higher and longer tails than females. To determine how these dimorphic traits affected reproductive performance and success, we conducted breeding trials using 12 males and six females per trial and monitored male-female and male-male interactions. Larger males had an advantage in most aspects of mate competition investigated. Males with higher tails had no advantage in either mate competition or mate choice. Males with longer tails also had no advantage in mate competition but were preferred as mates by females. Larger males interrupted courting males more often than smaller males did. The form of male-male interference was conditional on body size and not on either tail dimension. If the intruder was larger than the courting male, it would shove the female away from the courting male and initiate courtship; if the intruder was smaller, it adopted a female mimicry tactic in which it positioned itself between the courting male and female and performed female behaviours to the courting male while simultaneously courting the female. Our trials indicated that the two components of sexual selection may influence the evolution of different male morphological traits in tiger salamanders. Mate competition may favour increased male body length; mate choice may select for greater male tail length.