A Review of Reproductive Rates in Sticklebacks in Relation to Parental Expenditure and Operational Sex Ratios

The expenditures of time and energy on parental activities of female and male threespine sticklebacks, Gasterosteus aculeatus L., are reviewed in the context of sex differences in competition for and choice of mates. The female’s parental contribution is mainly cytoplasmic in the form of provisioned eggs. In energy units each clutch spawned represents about 200% of the daily maintenance ration of a sexually mature female. Each egg in a clutch represents about 2% ofthat ration. The ‘time out’, during which the female is sexually unreceptive between spawnings, varies from about 3 to 15 days, and is inversely correlated with the rate of food consumption. The male’s parental contribution is primarily behavioural. The daily energy expenditure of a parental male is equivalent to a daily ration of about 6% of the male’s body weight, a value not dissimilar to that of a reproductively active female. The pattern of ‘time-outs’ of males differs from that of females, with the periods of sexual receptivity and unreceptivity both longer. A crude simulation model of the effect of sex differences in the pattern and length of ‘time outs’ suggested that the operational sex ratio (OSR) would usually be male-biased favouring inter-male competition for mates and mate choice by females. However, both environmental factors such as food availability, and demographic factors such as sex differences in mortality rates, together with the sex differences in ‘time out’ could cause the OSR to vary within breeding seasons, between breeding seasons and between populations. This leads to the prediction that there will be corresponding variations in the intensities of male-male and female-female competition for mates and hence in the development of secondary sexual characteristics.