Badger, J. | Search bioRxiv

bioRxiv2021

Consistent differences among individuals more influential than energetic trade-offs in life history variation in grey seals (Halichoerus grypus)

Life history variation is thought to be mainly a result of energetic trade-offs among fitness components; however, detecting these trade-offs in natural populations has yielded mixed results. Individual quality and environmental variation may mask expected relationships among fitness components because some higher quality individuals may be able to acquire more resources and invest more in all functions. Thus, life history variation may be more affected by variation in individual quality than varying strategies to resolve energetic trade-offs, e.g. costs of reproduction. Here, we investigated whether variation in female quality or costs of reproduction is a larger factor in shaping differences in life history trajectories by assessing the relationship between survival and individual reproductive performance using a 32-year longitudinal data set of repeated reproductive measurements from 273 individually marked, known-aged female grey seals (Halichoerus grypus) from the Sable Island breeding colony. We defined individual reproductive performance using two traits: reproductive frequency (a females probability of breeding) and provisioning performance (provisions given to young measured by offspring mass), computed using mixed effects models separately for (1) all reproductive events, and (2) an age-class specific reproductive investment. Individual differences contributed a large portion of the variance in reproductive traits, with individuals displaying a range in individual reproductive frequencies from 0.45 to 0.94, and a range of average pup weaning masses from 34.9 kg to 61.8 kg across their lifetime. We used a Cormack-Jolly-Seber open-population model to estimate the effect of these reproductive performance traits on adult survival probability. Our approach estimated a positive relationship between reproductive performance and survival, where individuals that consistently invest well in their offspring survive longer. The best supported model estimated survival as a function of age-class specific provisioning performance, where late-life performance was quite variable and had the greatest impact on survival, possibly indicating individual variation in senescence. There was no evidence to support a trade-off in reproductive performance and survival at the individual level. These results suggest that in grey seals, individual quality is a stronger driver in life history variation than varying strategies to mitigate trade-offs among fitness components.

ecology↗

bioRxiv2021

Reproductive success is influenced by early development: the covariation of natal body size and reproductive performance in grey seals (Halichoerus grypus)

An individuals size when young may be an important source 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 natal size influences subsequent reproductive performance in grey seals (Halichoerus grypus) using mark-recapture and repeated measures reproductive data on a marked sample of 363 females that were measured for standard length at roughly 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 (the rate at which a female returns to breed), modeled using mixed-effects multistate mark-recapture models. After accounting for female age, experience, and offspring sex, we found a positive association between natal length and our measures of reproductive performance. Mothers with the longest natal lengths produced pups nearly 8 kg heavier and were 20% more likely to breed in a given year than mothers with the shortest natal lengths. Because correlation in individual body length between natal and adult life stages is weak (animals that were longer than average as pups do not always grow to be longer than average adults), the covariation between natal length and future reproductive performance reported here may act more as a carry-over effect from the size advantages afforded in the juvenile stage that allow for greater adult performance, rather than a physical trait that is maintained throughout life.

ecology↗