Spring-loaded reproduction: effects of body condition and population size on fertility in migratory caribou (Rangifer tarandus)

In many ungulates, female fecundity is affected by body condition and has important effects on population dynamics. In some species, females adopt a conservative strategy, reducing reproductive effort when population density is high. We investigated what factors affect the probability of gestation in adult female caribou (Rangifer tarandus (L., 1758)) from the Rivière-George herd in northern Quebec and Labrador over 5 years that spanned various population sizes and trends. Similar to other populations of migratory caribou, the probability that a female was pregnant in spring increased with body mass and percent body fat. The probability of gestation appeared to be reduced by high infestation of warbles (Hypoderma tarandi (L., 1758)). The proportion of females pregnant varied between years and was lower at high population size. Females of similar mass, however, were pregnant regardless of whether the population was increasing at low density, had reached a peak, or was declining. Compared with other ungulates that reduce maternal expenditure at high density, female caribou of the Rivière-George herd may have a risk-prone reproductive strategy.

Maternal strategies of the New Zealand fur seal: evidence for interannual variability in provisioning and pup growth strategies

Maternal attendance, nursing behaviour, pup growth and weaning age were investigated in the New Zealand fur seal (Arctocephalus forsteri) at Kangaroo Island, South Australia. Foraging trips to sea increased throughout lactation (5.9–9.8 days), while the duration of shore attendance bouts (1.7 ± 0.7 days) remained unchanged, resulting in time ashore declining from 24% to 14% throughout lactation. Mothers with sons made longer foraging trips than mothers with daughters throughout lactation and, as a consequence, maternal time ashore was less for sons (21%) than for daughters (28%). Pups maintained an overall sucking-rate of 1.4 h day–1 throughout their mothers’ lactation, by sucking more frequently and for longer periods as their mother became less available. Because mothers with daughters were ashore more frequently, the overall sucking-rate of females (1.7 h day–1) was greater than that of male pups (1.1 h day–1). The growth patterns of male and female pups from two cohorts were similar, but males grew faster and weaned heavier in one of the years. Age at weaning was ~285 days (9.4 months). This study indicates some significant differences in the nature of maternal expenditure in male and female pups, and the growth strategies used by pups both within and between years.

Body mass and survival of bighorn sheep

In ungulates, body mass is often positively correlated with juvenile survival, but little is known of whether body mass affects survival of other age-classes. We studied two marked populations of bighorn sheep (Ovis canadensis) in Alberta, Canada, to determine if body mass affected the survival of different sex –age classes. Chest girth at weaning was correlated (P < 0.0001) with survival of bighorn lambs in the Sheep River population. In the Ram Mountain population, body mass in mid-September had a stronger effect upon survival than mass in early June, mass gain in summer, or mass loss in winter. Body mass at weaning was correlated with lamb survival (P = 0.004). In both study areas, relationships between size and survival of lambs were similar for the two sexes. At Ram Mountain, survival of yearling and adult males seemed to be independent of body mass. Light yearling females were less likely to survive than heavy yearling females. Among females aged 3 – 6 years, body mass had no effect on survival. Among females 7 years of age and older, mass in mid-September had a weak but significant (P = 0.03) effect on survival. Females were slightly lighter in mid-September in their last year of life than in the rest of their adult life. Maternal expenditure is unlikely to affect the survival of prime-age ewes, but may have a detrimental effect on the survival of older ewes.