Introduction Detailed investigations of mating systems in diverse species have long provided a focal point in delineating the balance between natural and sexual selection as first out- lined by Darwin (1871). Recent applications of highly variable nuclear DNA markers have transformed specula- tions of behavioural observations into reality by provid- ing powerful tests of parentage in natural populations (Packer et al. 1991; Craighead et al. 1995; Keane et al. 1997). Surprising results have been uncovered for some species indicating that distribution of paternity may be contrary to expectations, and new insights have directed the rethinking of selective pressures (Kempenaers et al. 1992; Oring et al. 1992; Jones & Avise 1997). Of particular note has been the development of theories regarding female mate choice (see Birkhead & Møller 1993), sperm competi- tion, and the sometimes conflicting interests of male and female mating strategies. Much theoretical interest has centred on the extent and consequences of multiple matings by females, after a long period of ignoring this as an advantageous component of female mating strategies. Benefits to females are often specific to particular mating systems, and a major focus has been on monogamous bird species and extrapair cop- ulations. Fewer studies have been made of promiscuous mating systems but, in these, natural selection should promote multiple matings more strongly in males than females. This is because a male’s fitness may increase with the number of mates, whereas females have physiological limits to the number of young they can produce so that multiple matings may not appreciably increase fitness (but see Jones & Avise 1997). However, several hypothe- ses propose female gains from multiple mating that are of relevance to promiscuous breeders, including: (i) the pro- curement of ‘good genes’ if a better male is encountered subsequently and his sperm is successful (Kempenaers et al. 1992; Otter & Ratcliffe 1996); (ii) fertility assurance if some males have poor-quality sperm (Wagner 1992); (iii) increased offspring viability via sperm competition if out- competing sperm produce hatchlings with higher viabil- ity (Travis et al. 1990; Madsen et al. 1992); (iv) increased genetic diversity of offspring if sperm from several males are successful (see Westneat et al. 1984); and (v) reduced Molecular Ecology (1998) 7, 575–584 © 1998 Blackwell Science Ltd Single paternity of clutches and sperm storage in the promiscuous green turtle (Chelonia mydas) N. N. FITZSIMMONS Centre for Conservation Biology, Department of Zoology, University of Queensland, Brisbane, Queensland 4072, Australia Abstract Paternity of 22 green turtle (Chelonia mydas) clutches from 13 females of the southern Great Barrier Reef breeding population was determined through microsatellite analyses at five loci, including the analysis of successive clutches for nine of the females. A large number of alleles per locus (10–40) provided probabilities of detecting multiple paternity that were quite high, particularly at all loci combined (99.9%). Although green turtles are promiscuous breeders and there was an expectation of finding extensive multiple paternity, only two clutches were multiply sired and, in these, very few eggs had been fertilized by a secondary male. The rarity of multiple paternity may reflect either a low proportion of multiple matings by females in this population, or sperm competition, possibly resulting from a first-male sperm preference. Additionally, the analysis of > 900 offspring provided data on mutations, which included 20 mutation events that were observed in 27 offspring and involved both maternal and paternal lineages. Most muta- tions (n = 16) occurred at a single highly variable locus and their presence emphasizes the need to use multiple loci in paternity studies. Keywords: marine turtle, mating system, microsatellite, mutation, sperm competition Received 23 June 1997; revision received 27 October 1997; accepted 28 October 1997 Correspondence: N. N. Fitzsimmons. Fax: +61-7-3365-1655; E-mail: NFitzSimmon@zoology.uq.edu.au