Large offspring have enhanced lifetime reproductive success: Long-term carry-over effects of weaning size in gray seals (Halichoerus grypus)

Abstract

An individual's size in early stages of life may be an important source of individual variation in lifetime reproductive performance, as size effects on ontogenetic development can have cascading physiological and behavioral consequences throughout life. Here, we explored how size-at-young influences subsequent reproductive performance in gray seals (Halichoerus grypus) using repeated encounter and reproductive data on a marked sample of 363 females that were measured for length after weaning, at ~4 weeks of age, and eventually recruited to the Sable Island breeding colony. Two reproductive traits were considered: provisioning performance (mass of weaned offspring), modeled using linear mixed effects models; and reproductive frequency (rate at which a female returns to breed), modeled using mixed effects multistate mark-recapture models. Mothers with the longest weaning lengths produced pups 8 kg heavier and were 20% more likely to breed in a given year than mothers with the shortest lengths. Correlation in body lengths between weaning and adult life stages, however, is weak: Longer pups do not grow to be longer than average adults. Thus, covariation between weaning length and future reproductive performance appears to be a carry-over effect, where the size advantages afforded in early juvenile stages may allow enhanced long-term performance in adulthood.

Keywords: capture‐recapture; carry‐over effects; grey seals; life history theory; mixed‐effects.

FIGURE 1

Density plots of the distribution of weaning lengths of our sample of females by cohort, 1998–2002.

FIGURE 2

Estimated effect of an individual's weaning length on their future provisioning performance, controlling for the effects of age, sex, parity, year, and individual effects not accounted for by weaning length.

FIGURE 3

Estimated effect of weaning length on provisioning performance as a female ages. Lines are 0.025%, 50%, and 97.5% quantiles of weaning lengths corresponding to 95, 110, and 125 cm.

FIGURE 4

There is no evidence for an interactive effect of weaning length and parity–effect of weaning length on pup weaning mass does not taper off (p > .05, Table 1). Boxplots of pup weaning masses for individuals with short (90–105 cm), average (105–115 cm), and tall (115–132 cm) weaning lengths (panels) over the 1st, 2nd, and 3+ parities.

FIGURE 5

Results from the Markov chain multistate model describing probability of breeding, ψ ^kB , as a function of (a) weaning length, and (b) the female's previous state in year t − 1.

FIGURE 6

Cost of reproduction is estimated by finding the difference between reproductive probabilities of nonbreeders and breeders: panels depict posterior distribution of ψ ^BB (breeder to breeder) minus posterior distribution of ψ ^NB (nonbreeder to breeder) for (a) output of the preferred model reported here, estimating reproductive probabilities for females born from 1998 to 2002, and (b) the output from (Badger et al., 2020), a similar model estimating reproductive probabilities for females born 1962, 1969, 1970, 1973, 1974, 1985–87, 1989, and 1998–2002. Note that for (b), the models did not estimate a cost of reproduction in terms of reproductive rate, where ψ ^BB  > ψ ^NB , i.e. current reproduction does not incur a “penalty” to future reproduction. By contrast, our sample of females (a) show a slight cost of reproduction ψ ^BB  < ψ ^NB , where individuals are slightly more likely to breed in a given year if they had skipped reproduction previously.