Sperm competition occurs when two or more males copulate with the same female and their sperm compete to fertilize the female's available eggs. For most species of mammals, sperm competition is prevalent among males. The aim of this proposal is to examine how the persistent occurrence of sperm competition has modified the physiology, morphology, and copulatory behavior of males in mammals. A major hypothesis throughout the proposal is that male mammals use chemical signals to assess the presence of nearby male conspecifics and respond to particular risks of sperm competition. Experiment 1 will test whether copulatory behavior and sperm investment are modified in relation to different risks of sperm competition. Specifically, do male voles alter their sperm investment by adjusting the number of sperm they allocate in each ejaculation? Alternatively, do males alter their sperm investment by adjusting the total number of ejaculations? The main hypothesis in Experiments 2 to 5 is that when a risk of sperm competition initially inexistent arises, males will resume copulation with the same female to increase their sperm investment. Experiments 6 and 7 will examine whether male body size affects the outcome of sperm competition. Do larger and smaller males have different or similar sperm investment strategies? Do larger and smaller males respond differently to the same risk of sperm competition? Does a larger male sire more offspring than a smaller male when they have simultaneous access to the same female and both copulate with her? This proposal will be the first in which different risks of sperm competition will be experimentally manipulated by using odors from conspecifics. Thus, for the first time an association is presented between olfactory communication and sperm competition theory. This approach is ecologically relevant. Most mammals rely on olfactory signals for same-sex competition and for attracting mates. Most importantly, the experimental approach introduced in this proposal may be applied to many other animal taxa to expand current knowledge on sperm competition and sexual selection.
Broader Impacts: The project will employ several undergraduates including women and those from under-represented groups. Students from UMEB and BRIDGE programs have worked on this project in the past and will in the future. Several prior students have won competitions for best poster and presentation at conferences. One current under represented student recently won a university wide competition for undergraduate achievement. The research synthesizes physiology, behavior, and evolution questions and has already drawn the attention of the media with popularized versions of the project in Science News and on several internet sites.
