UVM Theses and Dissertations
Format:
Print
Author:
Hudman, Stephen P.
Dept./Program:
Biology
Year:
2005
Degree:
PhD
Abstract:
Sexual selection arises when there is intra-sexual variation in reproductive success and acts on traits associated with acquiring mates. Mate choice has received considerable attention in the sexual selection literature, but relatively little attention has been directed at intra-sexual contests. Intra- and inter-sexual processes, however, are not likely to act independently. Thus, understanding their relative contribution is essential to understanding sexual selection. For this research, I three lab-based experiments to investigate intra- and inter-sexual mechanisms of sexual selection in the fathead minnow (Pimephales promelas): (1) I used an open aquarium experiment and a computer simulation to estimate the opportunity for sexual selection in this species, (2) I used an open aquarium experiment to determine the morphological and behavioral phenotypes associated with male reproductive success, and (3) I used an open aquarium experiment to test the hypothesis that female fathead minnows gain direct fitness benefits from their choice of relatively large males. The results from these projects are summarized below.
First, I used an aquarium experiment to measure variance in male mating success (Vm) and to test the hypothesis that female fathead minnows distribute their eggs randomly among nests when there are no eggs present to act as a cue for mate choice. I used a computer simulation of random and selective mating models to generate expected distributions of Vm for comparison with the results from the aquarium experiment. Female fathead minnows mated non-randomly in all aquaria, and the observed Vm in each aquarium was more consistent with a model of selective mating rather than a model of random mating.
Second, I used an open aquarium design to examine the association of male body size, secondary sex characters, and behavior with mating success during initial spawning. Large body size was a strong predictor of success during intra-sexual contests, and female fathead minnows preferred relatively large males when making their initial spawning decisions. Females preferred large males independent of past reproductive success and dominance status. Secondary sex characters apparently had little influence on the outcome of male-male competition, but banding pattern was strongly associated with mating success during female choice. Although males that were successful during malemale competition were more aggressive than their subordinate counterparts there was no relationship between body size and aggression levels. Males that spawned successfully were not more attentive to their nests nor did they court females more often. Taken together, these results suggest that intra- and inter-sexual processes of sexual selection both favor larger male body size in the fathead minnow.
Third, I used an open aquarium experiment to examine the potential for direct fecundity benefits of female preference for relatively large males. Reproductively successful males were housed with egg predators (10 juvenile fathead minnows) over four days and egg survivorship was monitored daily. There was a positive relationship between egg survivorship and relative male body size, but no relationship between relative size and male paternal behaviors. Body size did not appear to deter juveniles from intruding into the nest and eating eggs. Although females gain direct fitness benefits from choosing large males, the benefit does not appear to arise because of an increased egg defense. Rather, larger males may have relatively high energy reserves and therefore cannibalize fewer of their own eggs.
First, I used an aquarium experiment to measure variance in male mating success (Vm) and to test the hypothesis that female fathead minnows distribute their eggs randomly among nests when there are no eggs present to act as a cue for mate choice. I used a computer simulation of random and selective mating models to generate expected distributions of Vm for comparison with the results from the aquarium experiment. Female fathead minnows mated non-randomly in all aquaria, and the observed Vm in each aquarium was more consistent with a model of selective mating rather than a model of random mating.
Second, I used an open aquarium design to examine the association of male body size, secondary sex characters, and behavior with mating success during initial spawning. Large body size was a strong predictor of success during intra-sexual contests, and female fathead minnows preferred relatively large males when making their initial spawning decisions. Females preferred large males independent of past reproductive success and dominance status. Secondary sex characters apparently had little influence on the outcome of male-male competition, but banding pattern was strongly associated with mating success during female choice. Although males that were successful during malemale competition were more aggressive than their subordinate counterparts there was no relationship between body size and aggression levels. Males that spawned successfully were not more attentive to their nests nor did they court females more often. Taken together, these results suggest that intra- and inter-sexual processes of sexual selection both favor larger male body size in the fathead minnow.
Third, I used an open aquarium experiment to examine the potential for direct fecundity benefits of female preference for relatively large males. Reproductively successful males were housed with egg predators (10 juvenile fathead minnows) over four days and egg survivorship was monitored daily. There was a positive relationship between egg survivorship and relative male body size, but no relationship between relative size and male paternal behaviors. Body size did not appear to deter juveniles from intruding into the nest and eating eggs. Although females gain direct fitness benefits from choosing large males, the benefit does not appear to arise because of an increased egg defense. Rather, larger males may have relatively high energy reserves and therefore cannibalize fewer of their own eggs.