Sperm depletion may not limit male reproduction in a capital breeder

Size-assortative mating in explosive breeders: a case study of adaptive male mate choice in anurans

Exploration of size-assortative mating (SAM) in animals has led to a near consensus that it arises through constraints in choice, such as preference for large females combined with a large male advantage during intrasexual competition. Although such ‘apparent’ SAM is well explored, whether SAM arises because of specific preferences for size-matched mates has been less thoroughly considered. We tested for ‘preference-based’ SAM in an explosively breeding frog (Rana sylvatica), quantifying how male and female sizes affected fertilization and if males preferred size-matched females. We found that size mismatch severely reduced fertilization. Furthermore, males preferred size-matched, not larger, females in mate choice trials. Because males that mated with much larger females fertilized fewer eggs overall than they would have with size-matched females, male preference for size-matched females may be adaptive. Our results expand understanding of the mechanisms underlying SAM, suggesting that multiple mechanisms may simultaneously cause size-assortative mating patterns to emerge.

Fitness costs of mating with preferred females in a scramble mating system

Little is known about the operation of male mate choice in systems with perceived high costs to male choosiness. Scramble mating systems are one type of system in which male choice is often considered too costly to be selected. However, in many scramble mating systems, there are also potentially high rewards of male choosiness, as females vary dramatically in reproductive output and males typically mate once per season and/or per lifetime. Using scramble mating wood frogs (Rana sylvatica), we tested whether males gain fitness benefits by mating with preferred females. We conducted choice trials (1 male presented simultaneously with 2 females) and permitted males to mate with their preferred or nonpreferred female. Offspring of preferred and nonpreferred females were reared in the laboratory and field, and we quantified various fitness-relevant parameters, including survivorship and growth rates. Across multiple parameters measured, matings with preferred females produced fewer and lower-quality offspring than did those with nonpreferred females. Our results are inconsistent with the idea that mate choice confers benefits on the choosing sex. We instead propose that, in scramble systems, males will be more likely to amplex females that are easier to capture, which may correlate with lower quality but increases male likelihood of successfully mating. Such male choice may not favor increased fitness when the operational sex ratio is less biased toward males in scramble mating systems but is, instead, a bet-hedging tactic benefitting males when available females are limited.

Does repeated human handling of study animals during the mating season affect their offspring?

Minimizing disturbance of study animals is a major consideration in ethological and ecological research design. One nearly universal type of disturbance is the handling of study animals as a component of trial setup. Even low to moderate levels of handling can be a substantial stressor to study animals, which may negatively affect their offspring via maternal effects. Understanding how routine human handling and manipulation may affect the outcome of research studies is therefore critical for interpreting study outcomes. We tested whether repeatedly handling and manipulating (i.e., manually disengaging) amplexed pairs of wood frogs (Rana sylvatica [Lithobates sylvaticus]), which have an explosive breeding season, would affect their reproductive output and offspring fitness. Handling and manipulation did not alter any parameter that we measured: reproductive timing, hatching success, and offspring larval duration, survival, and size at metamorphosis. These results suggest that handling and manipulation by researchers may have a negligible effect on wood frog reproduction and offspring fitness. It is possible that many species that are commonly used in reproductive studies because they suppress behavioral and physiological responses during the mating season are likewise unaffected by human handling. Nevertheless, researchers should examine possible consequences of methodological interventions on their study species in order to determine any potential influence on their results. Having a broad understanding of these effects on species that have robust or dampened stress responsiveness during the breeding season would be useful for making generalizations about potential effects.