SPERM COMPETITION GAMES - INDIVIDUAL ASSESSMENT OF SPERM COMPETITION INTENSITY BY GROUP SPAWNERS

A distinction is made between sperm competition risk (where there is t ypically a low probability of competition between two ejaculates) and sperm competition intensity (where typically two or more ejaculates co mpete). The relation between sperm competition intensity and sperm exp enditure can be radically different across species from that within a species. Across species, the average ejaculate expenditure will increa se with the average intensity of sperm competition. But within a speci es, the reverse trend is generally predicted for greater than two male s competing for the same set of eggs. These effects are demonstrated w ith three sperm competition game models. They are devised mainly for e xternally fertilizing group-spawning species such as many fish, in whi ch males group around a female and ejaculate when the female sheds her eggs. Fertilization is assumed to be instantaneous and each male gain s a proportion of the eggs equal to his sperm number divided by the to tal sperm. In the first model, males cannot assess the number of compe titors, and their ejaculate effort is shaped by the average number of males for the species or locally isolated deme. The proportion of repr oductive effort expended on the ejaculate is predicted to increase as (N-1)/N, where N = the mean number of competing males present at a spa wning. Thus if N is large, ejaculate expenditure dominates reproductiv e effort. In the second model, males can estimate whether there are mo re or less than average numbers of competitors present at a spawning, and in the third model, males can assess the number of competitors exa ctly. As in the first model, these models confirm that the mean ejacul ate effort should increase with the mean number of competitors for the species. However, they predict that males should decrease their sperm expenditure as the estimated number of competitors present at a given spawning increases above two. These conclusions do not apply to sperm competition risk: there is thus no conflict with earlier models based on risk.