Post-copulatory mate guarding in decorated crickets

Although post-copulatory mate guarding occurs in a variety of crickets, its adaptive significance remains largely unknown. Mate guarding may function to prevent females from prematurely removing the externally attached sperm ampulla, thereby ensuring maximum insemination. This hypothesis was tested in decorated crickets, Gryllodes supplicans, by comparing ampulla retention times of females guarded by their mates with those of unguarded females. There was no difference in ampulla attachment duration between the two groups, thus falsifying the 'ampulla-retention assurance' hypothesis. Two additional hypotheses related to the function of mate guarding were also tested: (I) mate guarding allows a male to remain in close proximity to his mate during the time it takes to produce a new spermatophore and (2) guarding functions to deter rivals from courting the recently mated female. The 'spermatophore renewal' hypothesis was rejected because the average inter-copulatory interval of males greatly exceeded the average guarding duration. The 'courtship reduction' hypothesis was supported by four lines of evidence: (1) guarded females were less likely to be courted by intruders than were females whose mates had been removed, (2) unguarded females mounted intruders significantly more often than guarded females, (3) the ampullae of unguarded females were more likely to be partially dislodged by the copulatory attempts of intruding males than were those of guarded females, and (4) guarded females were more likely to be maximally inseminated than were unguarded females. In various insect species, males guard females after mating and such behaviour usually functions to deter the female from remating with other males (Thornhill & Alcock 1983). Guarding serves to increase a male's fitness by reducing the probability that his sperm will have to compete with the sperm of his rivals for the fertilization of a female's eggs (Parker 1970, 1984). Post-copulatory mate guarding occurs in crickets (Orthoptera: Gryllidae; Khalifa 1950; Alexander & Otte 1967), but its function remains unclear. This study was designed to determine the adaptive significance of postcopulatory mate guarding in the decorated cricket, Gryllodes supplicans (Orthoptera: Gryllidae), a species for which other aspects of copulatory behaviour have been examined in some detail (Sakaluk 1984, 1985). In crickets, copulation is completed when the male transfers a spermatophore, which usually consists of a small sperm-containing ampulla that hangs outside the female's body after mating (Alexander & Otte 1967). The spermatophore is drained as sperm enter the female's genital tract through a narrow spermatophore tube inserted in the female's bursa copulatrix. In most species, the female eventually disposes of the spermatophore by consuming it (Loher & Renee 1978; Sakaluk &Cade 1980, 1983), and often does so before the spermatophore has been emptied of sperm (Sakaluk 1984, 1987; Simmons 1986). Male crickets initiate guarding behaviour immediately after copulation and attempt to remain in close proximity to their mates by employing a variety of tactics, including: (1) passively standing immediately adjacent to the female, (2) frequently antennating the female, (3) searching rapidly whenever the female wanders out of range of the male's antennae, (4) producing aggressive chirps upon any movement by the female or upon intrusion by another male and (5) physically attacking males that intrude (Khalifa 1950; Alexander & Otte 1967). There are at least three hypotheses that could account for the post-copulatory guarding behaviour of male crickets. First, mate guarding may function to prevent the early removal of sperrnatophores by females, thus ensuring that females are maximally inseminated (Alexander 1961, 1962; Loher & Rence 1978; Zuk 1987). In support of this hypothesis, a correlation between guarding time and the duration of spermatophore attachment has 0003-3472/91/020207 + 10 $03.00/0 9 1991 The Association for the Study of Animal Behaviour