Abstract Although most birds are monogamous, theory predicts that greater female parental investment and fe- male-biased adult sex ratios will lower the polygyny threshold. This should result in polygynous mating, un- less obligate biparental care or the spatial and temporal distribution of fertilizable females constrains a male’s ability to take advantage of a lowered polygyny thresh- old. Here we present data on the extent of male sexually dimorphic plumage, adult sex ratios and breeding season synchrony in three populations of a socially monoga- mous seabird, the brown booby Sula leucogaster. For one of these populations, San Pedro Mártir Island, we also present data on differences in male and female pa- rental investment, mortality and probability of pairing. The extent of plumage dimorphism varied among popu- lations. Sex ratios were female biased in all populations. On San Pedro Mártir Island, parental investment was fe- male biased, females failed more often than males to find a mate, but there was no polygyny. We suggest that on San Pedro Mártir: (1) a period of obligate biparental care coupled with a relatively synchronous breeding season constrained the ability of males to take advantage of a high environmental polygamy potential and (2) the re- sulting socially monogamous mating system, in combi- nation with the female-biased adult sex ratio, caused fe- males to be limited by the availability of males despite their greater parental investment. Key words Sula leucogaster · Polygyny · Parental investment · Sexual dimorphism · Sex ratios · Brown booby · Breeding synchrony · Gulf of California Introduction Two fundamental questions in behavioural ecology are the reasons variance in reproductive success is typically higher in males than in females, and the conditions under which males will provide parental care. Trivers (1972) predicted that the sex with greater parental investment, typically females, would limit the reproductive success of the other sex and have lower variance in reproductive success. This is because, if individuals of the limiting sex are able to survive and garner sufficient resources to produce offspring, they should have little trouble finding a mate willing to provide less than half the parental in- vestment for a 50% share of the resulting reproductive success. For the sex with lower parental investment, typ- ically males, variance in reproductive success is general- ly higher because individuals must not only survive and garner sufficient resources for gamete production, they must also attract a mate willing to provide more than half the parental investment. Consequently, more low- quality individuals of the limited sex fail to attract a mate, while some high-quality individuals may attract more than one mate. In the limited sex, the effect of greater variance in re- productive success is expected to heighten intrasexual competition for mates. This, in turn, leads to the devel- opment of secondary sexual characteristics and life his- tory strategies that result in higher mortality. Emlen and Oring (1977) showed that variance in the reproductive success of the limited sex is influenced by the operation- al sex ratio (the ratio of fertilizable females to sexually active males). The operational sex ratio is a function of differential mortality by sex, the spatial distribution of mates, and the synchrony of the breeding season. Theory predicts that female-biased parental investment will be associated with increased variance in male reproductive success, which, in turn, will heighten male intrasexual conflict for access to female mates and result in in- creased male mortality. Initial asymmetries in gamete investment and certain- ty of parentage predispose males to provide less parental Communicated by J. Höglund B.R. Tershy Section of Neurobiology and Behavior, Cornell University, Ithaca, NY 14853, USA B.R. Tershy ( ✉ ) · D.A. Croll Institute of Marine Sciences. University of California, Santa Cruz, CA 95064 USA e-mail: tershy@cats.ucsc.edu Behav Ecol Sociobiol (2000) 48:52–60 © Springer-Verlag 2000 ORIGINAL ARTICLE Bernie R. Tershy · Donald A. Croll Parental investment, adult sex ratios, and sexual selection in a socially monogamous seabird Received: 18 November 1999 / Accepted: 24 January 2000