Consequences of maternal size for reproductive expenditure and pupping success of grey seals at North Rona, Scotland


Patrick Pomeroy, Sea Mammal Research Unit, Gatty Marine Laboratory University of St. Andrews, St. Andrews, Fife KY16 8LB, UK. Fax: 01334 462632. E-mail:


1. The reproductive performance of individually marked mothers aged between 4 and 36 years breeding at the established grey seal colony of North Rona, Scotland was studied. Natality rate was between 0·805 and 0·975 for these females during 1979–95 and 57% of females produced 74% of the pups born. Mothers pupped successfully on N. Rona after absences of up to 5 years.

2. The average maternal postpartum mass (MPPM) of mothers was 190 ± 23 (SD) kg, larger than had been recorded from this colony previously. Although annual mean MPPM increased during the study, there were increases and decreases in individuals’ MPPM between years.

3. Pup mass at birth and pup growth rate were related to MPPM and date of parturition. No evidence of differential postpartum expenditure in the sexes was found. Relative pup birth mass decreased with MPPM but relative pup weaning mass remained constant over the range of MPPM.

4. Maternal mass expenditure during lactation averaged 39% of MPPM, and the consequences for MPPM in the year following either high or low relative expenditure were inversely related to relative expenditure in the first year. However, mothers increased their mass after skipping breeding in a year.

5. General Linear Models and REML analyses indicated that expenditures were significantly different between mothers when other variables and factors had been taken into account. In general, maternal expenditure was greater for animals of larger masses and duration of lactation, but corrected maternal expenditure of longer individuals was less than expected. Pup mass at weaning was influenced by mother's identity and year and there was evidence that individual mothers which were longer for their weight raised smaller pups.

6. The life history consequences of reproductive expenditure in any year appeared in subsequent breeding patterns. The cost of breeding for larger animals and larger expenditures was indicated by lower pupping success rates in years following births and skipped breeding years.