A COMPARATIVE TEST OF ADAPTIVE HYPOTHESES FOR SEXUAL SIZE DIMORPHISM IN LIZARDS

Allometric analysis of sexual dimorphism and morphological variation in two chromosome races of the Sceloporus grammicus complex (Squamata: Phrynosomatidae) from Mexico

Sexual dimorphism is a widespread feature in the Animal Kingdom. In lizards of the Sceloporus grammicus complex, studies of sexual dimorphism that analyze the allometric trajectories of body traits remain unexplored. Here we investigate sexual dimorphism in key phenotypic traits, including body size (snout-vent length, SVL) as well as head length (HL), head width (HW), and forearm length (FL). We use an allometric approach to detect differences in scale relationships among body parts in the S. grammicus complex in Mexico. We focus on two chromosomal races within this complex, F5 (2n = 34) and FM2 (2n = 46). In the complex, we found that males are larger than females in all morphological variables, and this pattern was confirmed in both races. We determined negative allometric trajectories (SVLvs.HL and HW), isometry (SVLvs.FL) and intersexual differences in the slopes of the SVLvs.HL and HW; the males showed steeper slopes. Thus, the growth of the head is more pronounced in males than females. Additionally, we found between-race differences in these trajectories (SVLvs.FL) and in all morphological variables (F5 lizards are larger than those of the FM2 race), which correlate with their chromosomal divergence. We discuss biological implications of our findings in relation to sexual selection and natural selection.

Patterns of intraspecific aggression inferred from injuries in an aquatic turtle with male-biased size dimorphism

Patterns of sexual size dimorphism (SSD) in turtles are correlated with ecological mode, and it has been hypothesized that mating systems are also shaped by ecological mode. Male combat and coercive mating are competing explanations for male-biased SSD, but are difficult to assess empirically in aquatic species with cryptic behaviour. We quantified SSD and compiled observations of putative combat wounds collected from over 500 captures of Snapping Turtles (Chelydra serpentina (L., 1758)) in Algonquin Provincial Park, Ontario, to test hypotheses of mate competition and coercion. We found that both sex and body size were important predictors of risk of wounding, consistent with the hypothesis that male–male sexual competition is the primary driver of intraspecific aggression. Low wounding rates among females suggests that resource competition and coercive mating are not important causes of injuries. The risk of wounding increased monotonically with body size in adult males but not in adult females, and small males were less likely to be injured, suggesting that they employ a risk-averse strategy by avoiding direct competition for mates. There was no evidence of asymptotic or decreasing wounding probability in the largest males, which is consistent with the hypothesis that large males compete most intensively to monopolize mates.

Sex-specific ecomorphological variation and the evolution of sexual dimorphism in dwarf chameleons (Bradypodion spp.)

Natural selection can influence the evolution of sexual dimorphism through selection for sex-specific ecomorphological adaptations. The role of natural selection in the evolution of sexual dimorphism, however, has received much less attention than that of sexual selection. We examined the relationship between habitat structure and both male and female morphology, and sexual dimorphism in size and shape, across 21 populations of dwarf chameleon (genus Bradypodion). Morphological variation in dwarf chameleons was strongly associated with quantitative, multivariate aspects of habitat structure and, in most cases, relationships were congruent between the sexes. However, we also found consistent relationships between habitat and sexual dimorphism. These resulted from both differences in magnitude of ecomorphological relationships that were otherwise congruent between the sexes, as well as in sex-specific ecomorphological adaptations. Our study provides evidence that natural selection plays an important role in the evolution of sexual dimorphism.

Male body size varies with latitude in a temperate lizard

To examine the evolution of sexual size dimorphism (SSD), interspecific studies are often performed to generate hypotheses for the origin and maintenance of SSD. Although these methods are invaluable to our understanding of the evolution of SSD, they often quantify SSD for a species based on few populations. We found a significant sex-specific latitudinal cline in Plestiodon fasciatus (L., 1758) (=  Eumeces fasciatus (L., 1758)), a species that was previously considered to be monomorphic for body size. Male body size significantly increased with increasing latitude, whereas female body size was relatively constant. Our findings argue for the importance of increased understanding and appreciation of intraspecific variation in SSD. We suggest that a more integrated approach to SSD be employed, where both intraspecific and interspecific variation is considered. We provide a foundation for posing hypotheses of the causes and consequences of SSD in P. fasciatus and perhaps other members of the species group.

Heads or tails? Sexual dimorphism in helodermatid lizards

We tested the hypothesis that helodermatid lizards (Gila monsters, Heloderma suspectum Cope, 1869, and beaded lizards, H. horridum (Wiegmann, 1829)) show sexual dimorphism in morphological traits related to male–male agonistic behaviors. Male–male combat in helodermatid lizards involves repeated sequences of ritualized grappling. Male Gila monsters use their heads in attempts to gain or maintain a superior position during repeated combat bouts that may last for hours. Pairs of fighting male beaded lizards form spectacular body arches, with abdomens adpressed and snouts, forelimbs, and tail tips contacting the ground. We measured body size, head size, and tail length in 208 preserved H. suspectum, and body size and tail length (but not head size) in 79 live H. horridum, then tested for sexual dimorphism using analysis of covariance. Male Gila monsters had proportionately larger heads than females but did not differ in tail length or body size. Male beaded lizards had proportionately longer tails than females and were larger in body size only when the largest individuals were included in the analysis. Differences in head dimensions (in H. suspectum) and tail length (in H. horridum) are likely the result of sexual selection acting through male–male agonistic behaviors in this unique lizard taxon.

A test of the reproductive cost hypothesis for sexual size dimorphism in Yarrow's spiny lizard Sceloporus jarrovii

1. Trade-offs between reproduction and growth are central assumptions of life-history theory, but their implications for sexual size dimorphism (SSD) are poorly understood. 2. Adult male Yarrow's spiny lizards Sceloporus jarrovii average 10% larger than adult females. In a low-altitude (1700 m) population, this SSD develops because males grow more quickly than females during the first year of life, particularly during the first female reproductive season. This study tests the hypothesis that SSD develops because female growth is constrained by energetic costs of reproduction. 3. To test for a growth cost of reproduction, I compared growth rates of free-living females that differed, either naturally or experimentally, in reproductive status. Females that naturally delayed reproduction until their second year grew more quickly than females that reproduced as yearlings, and ovariectomized yearlings grew more quickly and to larger sizes than reproductive controls. 4. To determine whether SSD develops in the absence of this inferred reproductive cost, I also studied a high-altitude (2500 m) population in which all females delay reproduction until their second year. Sex differences in growth trajectories were similar to those observed at low altitude, such that males averaged 10% larger than females even prior to female reproduction. 5. Although female growth may be constrained by reproduction, multiple lines of evidence indicate that this cost is insufficient to explain the full magnitude of SSD in S. jarrovii. First, differences in growth of reproductive and nonreproductive females are not observed until the final month of gestation, by which time SSD is already well developed. Second, the growth benefit accruing from experimental inhibition of reproduction accounts for only 32% of the natural sex difference in body size. Finally, SSD develops well in advance of female reproduction in a high-altitude population with delayed maturation.

SEXUAL SELECTION IN THE GIFT-GIVING DANCE FLY, RHAMPHOMYIA SULCATA, FAVORS SMALL MALES CARRYING SMALL GIFTS

In some species of insects males transfer a gift to females during courtship or copulation. In the dance flies these nuptial gifts vary from nutritious prey items to inedible tokens such as a leaf, stone, or silk balloon. Nuptial gifts in dance flies are presumed to increase male mating success. We examined the strength and form of sexual selection on male Rhamphomyia sulcata, an empidid in which males provide females with a nutritious prey item as a nuptial gift. We found that whereas large males carried large gifts, neither large males nor gifts were targets of sexual selection. Indeed, correlational selection analysis and nonparametric examination of the fitness surfaces revealed that small males carrying small gifts were the most successful. Males may be more maneuverable or flight efficient with small gifts, or small males with large gifts may be unable to carry both a large gift and a female in the paired descent flight. These results suggest carrying constraints may be an important factor in determining selection on nuptial gift size. The largest target of sexual selection was old males. Old males were also paired with the largest and most fecund females, highlighting the role mate quality can further contribute to selection on males. Correlational selection analysis also revealed selection for an increase in covariance between male wing length and body size, and for an increase in slope between these traits. Males who deviate away from the optimal phenotypic relationship for two tightly related morphological traits, such as tibia and wing length, may have overall reduced performance. These findings highlight the role correlational sexual selection can play in optimizing nonsexual male morphology and scaling relationships. This study questions the role of the nuptial gift in dance flies as a resource for females.