Since the reproductive success of many plants hinges upon animal pollen vectors, variation in floral visitation behaviors by these vectors potentially has important consequences for plant fitness. Most work investigating pollinator quality has, however, focused on differences among rather than within pollinator species. Sex-associated variation drives a number of large and discrete differences in the behavior and morphology of many animal species, and may generate consistent foraging differences among floral visitors. The degree to which sex differences drive foraging patterns across taxa or influence the services floral visitors provide to plants is less clear.
We surveyed the literature for reports of sex differences in the behavior of floral visitors, comparing taxonomic groups. We also explored how the reported patterns might influence plant reproduction by examining how and how frequently plant-reproductive consequences of observed differences were directly investigated. We then explored these patterns further by comparing pollen loads collected by male and female hawkmoths collected in southern Arizona field sites over 5 years. Specifically, we asked whether differences reported in the literature predicted patterns in the pollen loads we collected, and whether these patterns were consistent across time within our long-term data set.
Results/Conclusions
We show that sex-associated differences in pollinator foraging behavior and morphology are common across pollinator taxa, but that their significance has gone largely unrecognized: they have significant potential to influence the reproductive services visitors provide to plants. First, across many taxa, offspring provisioning behaviors such as feeding brood, provisioning eggs, or searching for oviposition sites are primarily performed by females, and are associated with higher reward removal per visit, higher floral constancy, and differences in floral preference during foraging. Second, mate searching behaviors, primarily performed by males, are associated with lower patch residence times, higher inter-flower travel distances, lower floral constancy, and lower visitation rates during foraging. Finally, sex-associated differences in feeding morphology influence pollen deposition per visit. In our own data we find that female hawkmoths carry larger and richer pollen loads than males. Our study is unique in showing consistency in these differences across years, suggesting that sex-associated differences in foraging can be stable and predictable. Given how little data is available on the consequences of sex-differences for plant reproduction, our results generate many new questions for investigation. We conclude that these differences are important in their broad generality and cannot be treated simply as random variation.