Sex, mate guarding, and reproductive state as potential modulators of herbivory in an aquatic consumer

Sex-specific nutritional requirements, mating behavior, and parental care can potentially affect food selection and consumption in animals. However, relatively little is known about how sex and reproduction influence food use by aquatic herbivores. We measure male:female ratios in the field and studied sexual differences in algal food choice and feeding rates for the amphipod Gammarus aequicauda from a desert saline lake, where it is the most abundant mesograzer during the colder months. We also assessed the effects of precopulatory pairing and female egg development on feeding behavior. Males were more numerous than females over two sampling periods and significantly preferred one algal species in each of three pairwise food-choice combinations tested. Females were moderately less selective, expressing a preference in only two of these experiments, but showed significantly higher total consumption and feeding rates than males in most assays. When males and females expressed similar preferences, the magnitude of these preferences (% differences in consumption between the algae offered) was statistically equivalent for both sexes. Patterns of compensatory feeding on less nutritious algae were significant and similar for both sexes. Mated pairs had reduced feeding rates compared to unpaired females, but not unpaired males. In contrast, no short-term differences in feeding rates were detected for males and females separated from precopula or between females at two stages of egg development. Variations in population sex ratios and reproductive cycles can potentially modulate interactions between aquatic consumers and their food sources.

Automated summary

Sex and reproduction affect food selection and consumption in aquatic herbivores. Male Gammarus aequicauda preferred specific algal species, while females showed lower selectivity but higher consumption and feeding rates. When both sexes had similar preferences, the magnitude of these preferences was equivalent. Compensatory feeding on less nutritious algae was observed in both sexes. Mated pairs had reduced feeding rates compared to unpaired females, but no short-term differences were detected between males and females separated from precopula or between females at different stages of egg development. Variations in population sex ratios and reproductive cycles can modulate interactions between aquatic consumers and their food sources.