Postcopulatory guarding strategy in a finite mating period

Abstract

A simple model is presented for finding which male strategy is advantageous for fertilization of eggs, guarding a specific female, or searching for many females. The model assumes that there is a finite mating period in which the number of virgin females decreases temporally. When more than one male copulates with the same female, the last copulating male can displace a constant proportion of the sperm injected the other males. Two oviposition patterns of females (single lump oviposition after the mating period and continuous ovipositions through the period) are analyzed. The model predicts (i) that when the sperm displacement rate is 1, the guarding strategy (GS) is more advantageous than the searching strategy (SS) if and only if the sex ratio (males/females) is larger than 1, independent of the searching efficiency and the oviposition pattern, and (ii) that when the sperm displacement rate is smaller than 1, GS is more advantageous than SS for higher values of sex ratio, sperm displacement, and searching efficiency. The results of the current non-stationary model are compared with those of our previous stationary model of postcopulatory guarding ( Yamamura, 1986, Theor. Pop. Biol., 29, 438–455). Both models suggest that there is a common tendency toward coexistence of guarding and searching strategies in the same population, and that the product of the sex ratio and the sperm displacement rate represents a rough criterion for an optimal strategy in more general cases.