In most mammalian species, mothers bear all the direct costs associated with producing offspring, which include those for gestation, birth, lactation and parental care. In long-lived iteroparous animals, the success of any one reproductive event may be influenced by efforts in previous seasons (Clutton-Brock, Guinness & Albon 1983; Newton 1989). The extent to which the reproductive effort in any one season influences future reproductive success depends on the availability of resources, their variability and the animal's response to that variability. Population parameters may be influenced by intrinsic differences between individuals, such as age, condition, fecundity or individual quality, but these need to be considered along with extrinsic effects such as spatial or temporal variation in weather or food supply. While the dynamics of a population can be described by the changes in the average life table parameters, individual reproductive performances must be examined to understand how these demographic changes are brought about. Furthermore, accounting for the way that resources are allocated to reproduction over the course of female lifetimes offers insights into the factors which shape and constrain maternal reproductive strategies with their corresponding fitness consequences.
For a number of mammalian species, including some pinnipeds, there are demonstrable costs to producing young (Clutton-Brock, Guinness & Albon 1983; Huber et al. 1991; Reiter & Le Boeuf 1991; Sydeman et al. 1991; (but see also Sydeman & Nur 1994)). For example, a cost of pupping in any year for Weddell seals (Leptonychotes weddelli Lesson) was a reduction of 0·05 in the probability of pupping the following year (Testa 1987). Antarctic fur seals Arctocephalus gazella Peters showed a reduction in adult female survival as a result of pregnancy and there was an age-related decline in pregnancy rate above the age of 8 years (Boyd et al. 1995).
Some components of each reproductive episode are variable and depend critically on maternal state which is influenced by environmental factors such as food availability, or may reflect individual responses to circumstances, such as maternal control of expenditure. Studies have considered the effects of limited resource availability on maternal allocation of resources (e.g. Anderson & Fedak 1987a,b; Campagna et al. 1992; Iverson et al. 1993; Festa-Bianchet, Jorgensen & Wishart 1994; Fedak, Arnbom & Boyd 1996). Survival of offspring to the weaning stage has been related to maternal investment in a number of mammalian species (red deer Cervus elaphus L., Clutton-Brock, Albon, Guinness 1987a; Clutton-Brock 1991; Soay sheep Ovis aries L. Clutton-Brock et al. 1992; bighorn sheep Ovis canadensis L. Festa-Bianchet et al. 1994, Antarctic fur seals, Lunn, Boyd & Croxall 1994; Arnbom, Fedak & Boyd 1997). For example, factors affecting birth mass in mammals have been shown to include maternal age, parity, body condition and social status as well as date of birth and offspring sex (Guinness, Clutton-Brock & Albon 1978; Reiter, Panken & Le Boeuf 1981; Clutton-Brock, Albon & Guinness 1984; Boyd & McCann 1989; Michener 1989; Trites 1991). Different females may vary in condition from each other but the same female may vary in condition from one breeding season to the next and may vary her expenditure in response to her state at parturition from year to year. Her state in any one year may depend on her previous expenditure or foraging success. Furthermore, animals may vary in the extent of their adjustment in expenditure and there may be a systematic variation in adjustment of expenditure in relation to condition at parturition across animals of different sizes. Thus, in a longitudinal study of the type reported here, we may expect a complex pattern of variation in maternal expenditure both across animals and within animals over different years (Trillmich 1996). This aspect of maternal variation has been partially accounted for in previous longitudinal studies of pinnipeds by using age-specific cohorts of animals (e.g. Trillmich 1986; Testa 1987; Reiter & Le Boeuf 1991; Sydeman et al. 1991; Boyd et al. 1995). However, the effects of maternal body size and expenditure in one season on that in the next, both within and between individuals demands examination.
Maternal expenditure (the resources used to rear offspring) can be difficult to measure in species which have complex life histories, a long rearing period or large brood size. However, phocid seals have life history characteristics which make measurement of maternal postpartum reproductive performance relatively straightforward. Mothers face a simplified version of the major trade-off between their reproductive effort and its allocation between offspring because they produce a single pup at each episode. Grey seal mothers do not feed during lactation, so their mass and body composition at parturition set limits to what they may expend on their pups. This stored reserve can be viewed as an index of the individual's capacity to tolerate environmental variability. Because the size of stored reserves is positively related to the mother's postpartum mass, the absolute and relative measure of reserves expended should also be related to postpartum mass (Arnbom, Fedak & Boyd 1997). Thus in common with other mammals, variations in maternal condition at parturition may account for a significant portion of preweaning and possibly postweaning offspring mortality (Guinness et al. 1978; Clutton-Brock et al. 1983; Clutton-Brock et al. 1992; Bowen et al. 1994; Arnbom et al. 1997).
In the eastern Atlantic, female grey seals Halichoerus grypus Fabricius come ashore to pup in the autumn. Timing of reproduction is highly synchronous and occurs in large aggregations of animals on remote islands and coasts. Although individual seals show fidelity to breeding sites, females may pup each year or miss some years (Pomeroy et al. 1994; Twiss et al. 1994). Pups are weaned after about 18 days (Bonner 1972), but the breeding season may last from 5 to 10 weeks depending on site. Adult male grey seals are typically two or three times the mass of females, but the postpartum masses of breeding females may range from 120 to 250 kg.
Longitudinal mass change data from a population of known individual grey seals at North Rona, Scotland was collected to investigate the consequences of inter- and intra-individual variation in maternal body size on reproductive expenditure and pupping success. Maternal expenditure is affected by postpartum body size and the duration of lactation, while pup mass at weaning provided a measure of the resultant of maternal expenditure. This basic relationship may be modified by associated factors such as pup sex, birth date, maternal mass, age and length as well as the year. Specifically we examined (i) long-term variation in natality; (ii) maternal postpartum mass and reproductive expenditure, assessed by mass change during lactation; (iii) the relationships between factors contributing to maternal expenditure and its product, the weaned pup; (iv) the roles of previous maternal parity, size and expenditure in mothers’ subsequent success in rearing pups to weaning.