Determinants of sperm release and storage in a spiny orbweaving spider

Sperm release and sperm storage were examined in the spiny orbweaver, Micrathena gracilis. This species shares characteristics with other spiders that show a first-male advantage for egg fertilization. Male M. gracilis differentially released sperm to virgin females. Sperm release was not related to female age or copulatory duration. In this species, a complete mating between a pair requires two copulations that are separated by a dismount. The second copulation is twice as long as the first. After manipulating the duration of the second copulation, it was found that sperm were released early in the copulation. When a female copulated twice, storage of sperm in the female’s first side was influenced by the prolonged second copulation. Considering that males often do not obtain two copulations, and that a female may alternate copulations when two males are present, it was experimentally determined that the copulatory stimulation provided by a second male does facilitate the storage of the first copulating male’s sperm. It is concluded that sperm release and sperm storage are two separate processes. Copulation does not necessarily result in sperm release by the male. Not all sperm released are stored within the female, and the number stored is influenced by copulatory duration. ? 1997 The Association for the Study of Animal Behaviour The reproductive consequences of a male’s copulatory behaviour are determined in part by the number of his sperm that are released and taken up or stored by the female in relation to those of other males (Parker 1970, 1984; Parker et al. 1990). When one of several males that mate with a female has the advantage in fertilizing her eggs (Smith 1984; Birkhead 1988; Birkhead & Hunter 1990), males could benefit by responding differentially according to female mating history (Watson 1990, 1991a; Keller & Passera 1992; Lorch et al. 1993; references in Simmons et al. 1994; Lewis & Iannini 1995), especially when sperm are limited (Dewsbury 1988; Verrell 1990). For example, when there is a first-male advantage based upon some feature other than number of sperm released, males might be expected to avoid expending gametes on females that have previously mated. Discrimination on the basis of female mating history can occur before (Wicklund & Forsberg 1985; Lewis & Iannini 1995; Riechert & Singer 1995; Watson 1991a) or during copulation (Cordero & Miller 1990; Suter 1990; Lorch et al. 1993; Cigliano 1995). In many species, the advantage in fertilization goes to the male with the greatest number of sperm within the female’s reproductive tract (Martin et al. 1974; Woodhead 1985; Dickinson 1986). Males can adjust their ejaculate size in response to greater risk of cuckoldry (Baker & Bellis 1989; Gage 1991; Gage & Baker 1991) or to maximize the number of females they can inseminate (Dewsbury 1982; Pitnick & Markow 1994). This can be done by lengthening the duration of copulation, since sperm release can be timedependent (Thornhill 1980; Dickinson 1986; Siva-Jothy & Tsubaki 1989; Keller & Passera 1992). A male would also benefit reproductively by facilitating the storage of his own gametes once they are released. Eberhard (1985, 1991, 1994) noted that copulation may serve an ‘internal courtship’ function. Males with particular genitalic configurations might be better able to stimulate the female and facilitate the storage and usage of their sperm. Dewsbury (1988) suggested that tactile stimulation during copulation may affect female neuroendocrine processes that influence Correspondence: T. C. Bukowski, Department of Psychology, Tulane University, New Orleans, LA 70118, U.S.A. 0003–3472/97/020381+15 $25.00/0/ar960329 ? 1997 The Association for the Study of Animal Behaviour