Courtship Persistence and Female-Guarding as Male Time Investment Strategies

Abstract

AbstractTime investment strategy is defined as the optimum allocation of times spent on given activities so as to achieve maximum reproductive success. Selective pressures on males to increase time invested in encountering females would be least in sessile and communally spawning species, and maximum in mobile species which spawn at low density and those which copulate. The present paper concerns male time investment in courtship persistence and female-guarding. Staying with a given female reduces the rate at which new females are encountered. Females are often unreceptive for some time after mating. Males often court unreceptive conspecific females; they can achieve a gain if the female rejection reactions can be overcome (rape) and the ejaculate can compete in the fertilization of the ova. Courtship of unreceptive females of closely related sympatric species is also considered adaptive. Though female unreceptivity will be favoured if hybrids are disadvantageous, males may gain by attempting rape if the fitness of hybrid offspring is high enough and the time investment favourable. A model is constructed to explain how optimum persistence durations are determined. This depends on 1) how the cumulative probability of insemination changes through time invested, 2) the encounter frequency, 3) the ejaculate cost (measured as feeding time investment/ejaculate), and 4) is modified by the pattern of gain from other types of female. Females can adapt to male persistence either by acceptance, by increasing rejection effectiveness, or by dispersing into another area where it is disadvantageous for males to search. This last solution may have been especially important in sympatric speciation. Male courtship duration with potentially receptive conspecific females may also be optimized. Variation in male persistence time may be due to assessment of particular situations. Female guarding has commonly evolved as a male time investment strategy. Precopulatory guarding appears to function to stake a claim to a female (or females) until she becomes receptive. This poses two problems : at what point in the female's reproductive life does it become advantageous for the male to guard, and how is guarding time optimized? Optimum guarding duration can be determined with the same model as for courtship persistence. If males adopt a given cue for closeness to receptivity for the onset of guarding, females showing the cue become scarce and selection may favour drive for earlier and earlier cues. This could be stabilized by the opposing selective pressures of 1) chances of finding a female closer to mating high enough, 2) female distribution suitably non-random with respect to mating, and 3) guarding investment more costly than searching investment in terms of male future reproductive success. Postcopulatory guarding appears to function to prevent loss in gain to a male due to sperm competition from other males. Such behaviour could evolve in conditions of high female receptivity and high encounter rate during an adequate overlap period (time per female during which ejaculates from different males can compete for fertilization of the ova), since males which guard after mating may waste less time and sperm than non-guarders. Its advantage is increased by a male-biassed sex ratio during the overlap period. The behaviour depends on the fact that second matings can compete in the fertilization of the ova, and postcopulatory guarding has its higlest advantage when the last male to mate fertilizes most eggs. Optimum guarding duration can be determined with basically the same model as before, and depends mainly on how sperm utilization is distributed within the overlap period.