Mass of weaned elephant seal pups in areas of low and high human presence

On sub-Antarctic Macquarie Island, we examined pup weaning mass of southern elephant seals in relation to human presence. Pup weaning mass was previously found to be positively associated with 1st-year survivorship. Weaned pups were weighed in a remote area, Middle Beach, and in an area of relatively high human presence, Isthmus East. The areas were reasonably similar in beach topography, wind and surf conditions, numbers of seals present per kilometre of coastline, and numbers of males and females present in harems. For a sub-sample of measured pups, data on the respective maternal size were collected using photogrammetry. Both male and female weaned pups on Middle Beach were significantly heavier than those on Isthmus East. Estimated length of mothers was significantly higher on Middle Beach. In proportion to their own size, mothers in both areas produced weaners of similar mass, indicating no direct effect of human disturbance on the efficiency of lactation. It remained unclear whether the area differences in maternal and pup size were due to natural or human-related factors.     

Brain size in neonatal and adult Weddell seals: Costs and consequences of having a large brain

Little is known about the ontogeny of brain size in pinnipeds despite potential functional implications of brain substrate (glucose, oxygen) requirements for diving, fasting, growth, and lactation strategies. We measured brain mass (brM) and cranial capacity (CC) in newborn and adult Weddell seals. Neonatal Weddell seals had brM that represented ~70% of adult brM. Weddell seals have the largest neonatal brain, proportional to adult brain, reported for any mammal to date, which is remarkable considering the relatively small size of Weddell seal pups at birth (6%–7% of maternal body mass) compared to neonates of other highly precocial mammals. Provision of sufficient glucose to maintain the large, well‐developed brain of the neonatal Weddell seal has a nontrivial metabolic cost to both pup and mother. We therefore hypothesize that this phenomenon must have functional significance, such as allowing pups to acquire complex under‐ice navigation skills during the period of maternal attendance.     

Differential mobilization of blubber fatty acids in lactating Weddell seals: evidence for selective use

A major source of energy during lactation in mammals is provided through the mobilization of blubber fatty acids (FAs). We investigated the extent to which FAs were mobilized to support both maternal metabolic requirements and milk production in the Weddell seal and how this was reflected in the FA composition of the pup's blubber at the end of lactation (EL). FA composition of postpartum female blubber was similar in the 2 yr of study (2002 and 2003) but differed markedly by EL. Pup blubber FAs (at EL) were also different between years and did not match that of the mother's milk or blubber. Milk FA composition changed during lactation, which may have been a reflection of an increase in pup energy demands at different stages of development. In addition, there was evidence of feeding by some females during lactation, with higher levels of some FAs in the milk than in the blubber. Our results indicate that differential mobilization of FAs occurred in lactating Weddell seals and that this was related to total body lipid stores at postpartum. Furthermore, growing pups did not store FAs unmodified, providing evidence that selective use does occur and also that using FA composition to elucidate dietary sources may be problematic in growing individuals.     

Effects of maternal age and condition on parturition and the perinatal period of Antarctic fur seals

The effect of maternal age and condition on the date of parturition and the duration of the perinatal period of Antarctic fur seals at Bird Island, South Georgia, were investigated over three consecutive breeding seasons. Females rear young during a four-month lactation period in a highly seasonal but predictable environment. Although females may first pup at three years of age, they did not attain full adult size until six years of age; older females (≥ 6 years) tended to be heavier, longer, and in better condition than younger females (3–5 years). Older females returned to breeding beaches earlier and could occupy the most suitable pupping sites, and gave birth when densities of animals on the beaches were low (i.e. more favourable for pup survival). Females that arrived earlier were able to remain ashore longer with their pups prior to departing on their first foraging trips but this was unrelated to either maternal age or condition. Younger females returned later in the pupping season, possibly as a result of late implantation due to smaller energy reserves than older and larger females. In 1990 all females arrived late, were in poorer condition, gave birth to lighter pups, and had shorter perinatal periods. This suggests that not only was implantation late but that females returned to an area of low food availability prior to parturition.     

Feast or famine: evidence for mixed capital–income breeding strategies in Weddell seals

Evolved patterns of resource expenditure for reproduction have resulted in a life history continuum across species. A strictly capital-breeding strategy relies extensively on stored energy for reproduction, whereas income breeding uses energy acquired throughout the reproductive period. However, facultative income breeding has been shown in some classically capital-breeding animals, and was originally thought to provide a nutritional refuge for smaller females incapable of securing sufficient reserves during pre-partum foraging. We examined milk composition and milk output for the Weddell seal to determine to what degree lactation was aided by food intake, and what factors contributed to its manifestation. Milk composition was independent of maternal post-partum mass and condition, but did change over lactation. Changes were most likely in response to energetic and nutritional demands of the pup at different stages of development. During early lactation, females fasted and devoted 54.9% of total energy loss to milk production. Later in lactation 30.5% more energy was devoted to milk production and evidence suggested that larger females fed more during lactation than smaller females. It appears that Weddell seals may exhibit a flexible strategy to adjust reproductive investment to local resource levels by taking advantage of periods when prey are occasionally abundant, although it is restricted to larger females possessing the physiological capacity to dive for longer and exploit different resources during lactation. This supports the assumption that although body mass and phylogenetic history explain most of the variation in lactation patterns (20–69%), the remaining variation has likely resulted from physiological adaptations to local environmental conditions. Our study confirms that Weddell seals use a mixed capital–income breeding strategy, and that considerable intraspecific variation exists. Questions remain as to the amount of energy gain derived from the income strategy, and the consequences for pup condition and survival. Electronic supplementary material The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

Reproductive performance in grey seals: age-related improvement and senescence in a capital breeder

1. Three hypotheses have been advanced to account for age-related improvement in performance: the selection hypothesis predicts improved due to the loss of lower quality phenotypes, the constraint hypothesis predicts individuals improve function, and the restraint hypothesis predicts younger individuals forego or reduce effort because of mortality risks. A decline in age-related performance (i.e. senescence) is predicted by mutation accumulation, antagonistic pleiotropy and disposable soma (wear and tear) hypotheses. 2. Using five measures of performance - birth rate, maternal and pup birth mass, pup weaning mass, weaning success and lactation length - we tested these hypotheses concerning age-related change in reproduction in 279 female grey seals (Halichoerus grypus), ages 4-42 years, over a 23-year period between 1983 and 2005 on Sable Island, Nova Scotia. These females produced 2071 pups. 3. Although body mass of primiparous females increased with age (4-7 years) birth mass of their pups did not, but pup weaning mass did. Second- and third-parity females of the same age as primiparous females gave birth to and weaned heavier pups. However, parity and age were dropped from models when maternal body mass was included. 4. The proportion of females giving birth varied significantly with maternal age, increasing in young females and then declining late in life. Weaning success rate also increased rapidly to about 8 years and subsequently declined in females > 32 years. 5. Generalized additive models indicated nonlinear changes in 3 day body mass (i.e. approximately birth mass) and weaning mass of pups as a function of maternal age, after accounting statistically for the effects of maternal body mass. Mixed-effects, repeated-measures models fitted to longitudinal data further supported the conclusion that pup birth mass and weaning mass vary nonlinearly with maternal age and indicated nonlinear changes in lactation duration. 6. We found some support for the constraint hypothesis, but our findings were not consistent with the selection hypothesis or the restraint hypothesis as the basis for improvement in reproductive performance. 7. Senescence was evident in multiple female and offspring traits, indicating the degeneration in function of several physiological systems as predicted by the disposable soma hypothesis.     

Factors affecting weaning weight in Antarctic fur seals Arctocephalus gazella at South Georgia

Summary Relationships between size, body condition, age and feeding-attendance patterns during pup rearing of female Antacrtic fur seals Arctocephalus gazella and their effects on the timing of birth and weaning, pup weight, growth and condition were studied at South Georgia in 1981–1982. Twenty-seven (6 male, 21 female) mother-pup pairs were followed from birth to weaning. The analysis of maternal effects was limited to female pups because of the small sample size of male weaners. High weaning weight was associated with those female pups whose mothers spent more time ashore attending their offspring. Weaning weight showed no relationship with perinatal duration, number of feeding trips to sea, days at sea or date of weaning. A further 63 mother-pup pairs were analysed for the effects of maternal body condition (weight/length), age and timing of birth on offspring body weight and condition. Pup weight and condition were weakly correlated with maternal age in female pups. Male pups born earlier in the season were heavier and in better condition. Maternal and offspring body weight and condition were unrelated. For the Antarctic fur seal population at South Georgia where the food supply was apparently not limiting in summer, maternal condition and foraging time were subordinate to maternal care on land (as expressed by attendance duration) in determining offspring weight at weaning.     

Growth and survival of New Zealand sea lions, <Emphasis Type="Italic">Phocarctos hookeri:</Emphasis> birth to 3 months

Offspring birth mass and growth rate represent important life history traits, which influence many vital population and individual characteristics, while offspring survival is a key factor in variation in female reproductive success. For a threatened population of pinnipeds, such as New Zealand sea lions, Phocarctos hookeri, (Grey, 1844, NZ sea lions), understanding individual life history parameters and population dynamics is vital for their management and conservation. This is the first study of the behaviour of females during parturition, pup birth mass and growth, and pre-weaning survival of NZ sea lions, Enderby Island, Auckland Islands during austral summer breeding seasons, 2001/2002 to 2003/2004. Pregnant females arrived ashore 2.1 ± 0.16 days prior to giving birth. After parturition, mothers suckled their pups for 8.6 ± 0.16 days before leaving on their first foraging trip. Male pups were born significantly heavier than female (males 10.6 ± 1.4 kg, females 9.7 ± 0.9 kg). Pups lost on average 48 ± 0.14 g per day mass during the early postpartum period (between birth and mothers first foraging trip). Pup mortality did not vary by pup sex, birth mass, date of birth or any maternal characteristics however it varied significantly between years due to a bacterial infection epidemic (Pup mortality at 60 days: 2001 32%; 2002 21%; 2003 12%). The absolute growth rate per day for pups was 151 g/day over all years. Pup growth rate measured as the slope of linear line fitted to pup mass by age was consistently higher for pups with heavier birth mass, male pups and during the 2002 season. High offspring mortality and slow growth rates coupled with maternal foraging behaviour at their physiological limits may reflect a threatened species which has limited ability for population growth in an environment which is at the extreme of their historical range and impacted upon by fisheries.     

Age-related reproductive variation in a wild marine mammal population

Life history theory predicts a change in reproduction success with age as energy resources are limited and must be allocated effectively to maximize reproduction and survival. In this study, we use three reproductive performance measures, maternal expenditure, offspring weaning mass, and first-year survival, to investigate the role that maternal age plays in successful reproduction. Long-term uninterrupted life history data available for Marion Island’s southern elephant seals and mass change estimates from photogrammetry data allow for assessment of age-related reproduction performance and trade-offs. Known-aged adult females were photographed for photogrammetric mass estimation (n = 29) and their pups weighed at weaning during the 2009 breeding season. Maternal age and proportional mass loss positively influenced pup weaning mass. In turn, first-year pup return rates (as a proxy for survival) were assessed through the intensive mark–recapture program. Pup survival increased with female age and weaning mass. Pups of young females aged 3–6 years have a lower first-year survival probability compared with pups of older and larger females.     

Effects of reproductive strategies on pollutant concentrations in pinnipeds: a meta‐analysis

In marine mammals such as pinnipeds (seals, sea lions and walruses), reproductive strategies reveal how species acquire, store and allocate energy to offspring. During lactation, females can allocate energy acquired from concurrent resources (income breeding); or utilize energy stored prior to reproduction (capital breeding). Mothers transfer a large proportion of energy to their pups via lipid rich milk, meaning that pollutants such as polychlorinated biphenyls (PCBs) and mercury (Hg) will also transfer, raising concern for negative health effects. To quantify the effect of reproductive strategy on maternal pollutant transfer in pinnipeds, we developed a proxy for income and capital breeding by focusing on the lactational component of reproduction, and arranged species along this gradient. We found a strong positive relationship between lipid content in milk and degree of capital breeding. We tested this gradient against maternal pollutant transfer expressed as a concentration ratio by meta‐analysis. In mother‐pup pairs, the concentration ratio of PCBs was one order of magnitude higher than for mercury. PCB concentrations in pups and juveniles were similar to adult females, but mercury was always lower in young offspring than in females. We found no effect of reproductive strategy between studies investigating mother‐pup pairs and non‐related females and juveniles (< 1 year old), however data were strongly biased towards capital breeders. Our results suggest that either: 1) reproductive strategy does not affect pollutant bioaccumulation; or 2) a lack of income breeder data prevents us from testing the overall effect of reproductive strategy on maternal pollutant transfer. The finding that PCB concentrations in juveniles are similar to females is of concern due to early life stage exposure. We recommend data collection from income breeding species such as the sea lions to elucidate whether reproductive strategy, and potentially other life history traits, has an overall effect on maternal pollutant transfer.     

Lactating Females Do Not Discriminate Between Their Own Young and Unrelated Pups in the Communally Breeding Rodent, Octodon degus

Females in numerous rodent species engage in communal nesting and breeding, in which they share one or more nests to rear their young. A potential cost of communal nesting and breeding is that mothers divert resources to unrelated offspring. One way mothers could avoid this cost is to recognize and favour their own young over unrelated offspring when allocating maternal effort. We assessed whether female degus (Octodon degus), a communally nesting and breeding caviomorph rodent, discriminate between their own and unrelated offspring during lactation. Female degus previously have been shown to distinguish between their own and unrelated pups when exposed to odours from both. We measured pup discrimination based on differences in the retrieval behaviour of females that were in early or intermediate lactation directed towards their own and unrelated offspring; offspring presented were of similar or different age. Before any event of pup retrieval, lactating females spent similar amounts of time and interacted to a similar extent with their own and unrelated pups. During pup retrieval, all lactating females transported both pups to the nest. Neither relatedness to pups, nor pup‐age differences, influenced the order in which pups were retrieved to the nest. Dams waited similar amounts of time before retrieving the first pup when the first transported young was their own or unrelated. Likewise, females waited similar amounts of time before retrieving the second pup when the pup transported first was their own or unrelated. The time between first and second pup transport events was longer when dams were in early when compared with intermediate lactation, but only when pups were of similar age. All experimental subjects nursed unrelated pups after they were retrieved. Collectively, our results do not support the hypothesis that communally breeding female degus use their recognition ability to discriminate against unrelated offspring in favour of their own young.     

Variation in milk production and lactation performance in grey seals and consequences for pup growth and weaning characteristics

Phocid seals are one of the few groups of mammals capable of sustaining the energetic demands of lactation entirely through body nutrient stores while fasting. Lactation performance of the female in turn influences the rate and pattern of pup growth. We examined variation in and patterns of milk composition and production, maternal energy output, and pup growth and energy deposition over the entire lactation period in 18 grey seal mother-pup pairs using hydrogen isotope (3H2O and D2O) dilution. Milk composition was independent of maternal mass and nutrient stores, indicating dependence on other physiological and genetic factors. Heavier females lactated longer (r2=0.653, P<0.001), had higher total milk outputs (r2=0.652, P<0.001), and produced larger pups at weaning (r2=0.417, P=0.005). While fatter females lactated for longer periods of time (r2=0.595, P<0.001), females with a larger lean body mass at parturition produced more milk (r2=0.579, P<0.001). Total milk energy output was the strongest predictor of pup weaning mass, which, along with the pup's efficiency of energy storage, accounted for 91% of the variation in weaning mass. Nevertheless, there was sufficient plasticity in milk composition and energy output that some smaller females produced relatively large pups. Few females appeared to deplete body nutrients to the point where it might limit the duration of lactation.     

Long-term evaluation of body mass at weaning and postweaning survival rates of Weddell seals in Erebus Bay, Antarctica

Variability in juvenile survival rate is expected to be an important component of the dynamics of long-lived animal populations. Across a range of species, individual variation in juvenile body mass has been shown to be an important cause of variation in fates of juveniles. Our goal in this paper was to estimate age-specific apparent survival rates for Weddell seals ( Leptonychotes weddellii ) in Erebus Bay, Antarctica, and to investigate hypotheses about relationships between body mass at weaning and apparent survival rate for juveniles. Mark–resighting models found average apparent juvenile survival rate (survival from weaning to age 3) was similar between males and females, and revealed positive relationships between body mass at weaning and apparent juvenile survival rate. The effects of body mass on juvenile survival rate differed between the sexes and the relationship between body mass and survival rate was stronger in males. These results indicate that the magnitude of energy transferred from mother to pup during lactation likely has important consequences on offspring survival rate and maternal fitness.     

Milking strategy in subantarctic fur seals Arctocephalus tropicalis breeding on Amsterdam Island: evidence from changes in milk composition.

Milk composition was investigated throughout the 10-mo pup-rearing period in subantarctic fur seals (Arctocephalus tropicalis) breeding on Amsterdam Island. The mean milk composition was 42.8% +/- 5.7% lipid, 12.1% +/- 1.5% protein, and 42.6% +/- 7.3% water. Subantarctic fur seals breeding on Amsterdam Island produced one of the richest milks ever reported in otariids (20.4 +/- 2.9 kJ/g), with lipid content contributing 85% of total gross energy. The high lipid levels measured in the milk of subantarctic fur seals breeding on Amsterdam Island is consistent (i) with the relatively long time lactating females spend at sea, due to the relatively poor local trophic conditions near the colony that necessitate that they travel long distances to reach the foraging grounds, and (ii) with the consequently short time mothers spend with their pups ashore. Milk composition changed according to the time mothers were fasting ashore: milk produced during the first 2 d spent ashore, when more than 80% of milk transfer occurred, had higher levels of lipids, proteins, and gross energy than milk produced later during the visit ashore, suggesting that the pups were fed with two types of milk during a suckling period. Throughout the year, mothers in good condition produced milk of higher lipid content than others, suggesting that individual foraging skills contribute to enhance milk quality. Milk lipid and gross energy content varied with pup age, according to quadratic relationships, increasing during the earlier stages of lactation before reaching asymptotic values when pups were 180 d old. The stage of lactation appears to be a better predictor of milk lipid content than the duration of the preceding foraging trip, suggesting that either changes in the nutritional requirements of the pup and/or seasonal changes in trophic conditions act on milk composition. These changes in milk quality may also be related to changes in maternal care; lactating subantarctic fur seals apparently reallocate their body reserves toward gestation rather than lactation at the end of the pup-rearing period.     

Offspring kidnapping with subsequent shared nursing in Antarctic fur seals

Non-offspring maternal care should be common in phocids, but their occurrence would be uncommon among otariids due to the high costs of raising offspring, particularly lactation, and an efficient recognition system that allow for accurate recognition during the frequent mother–pup re-associations. However, non-offspring maternal care has been documented in some otariid species. While the phenomenon in general is not novel among the colonially breeding seals, the exclusive care to a single pup by two lactating females for an extended time is a behavior scarcely documented in natural population. In an extension of this allonursing care, we document the first case of the kidnapping of a pup with subsequent shared nursing in Antarctic fur seal including data on the effect of this interaction on the pup’s growth. While all other lactating females nursed exclusively their own pups, the shared nursing was advantageous for the pup because he grew noticeably larger (in weight and axillary girth) than other of his cohort, particularly after 50 days. This advantage would have been influenced by the asynchrony of the foraging cycle of the biological and foster mother, which resulted in a higher attendance on shore than any other male pups. Although several explanations have been hypothesized for allolactation in mammals, our observations suggest a misguided parental care, associated with recognition errors by the foster mother whose pup was stillborn.     

Factors influencing and consequences of breeding dispersal and habitat choice in female grey seals (<Emphasis Type="Italic">Halichoerus grypus</Emphasis>) on Sable Island,Nova Scotia

Selection of breeding location can influence reproductive success and fitness. Breeding dispersal links habitat use and reproduction. This study investigated factors affecting breeding dispersal and its reproductive consequences in grey seals (Halichoerus grypus) on Sable Island, Nova Scotia. Breeding dispersal distance was determined in 692 individually marked, known-age female grey seals observed from 2004 to 2014. We used generalized linear mixed-effects models to test hypotheses concerning environmental and demographic factors influencing breeding dispersal distance and the consequences of dispersal distance on offspring weaning mass. Grey seal females rarely exhibited fidelity to previous breeding sites. Median dispersal distance between years was 5.1 km. Only 2.9% of females returned to a previous breeding site. Breeding dispersal distance was affected by parity and density, but effects were small and are presumably of no biological significance. Variation in dispersal distance among adult females was large. Dispersal distance had no significant influence on offspring weaning mass; however, as previously found, pup sex and maternal age did. Although breeding location was not important, heavier pups were born in habitats with no tidal or storm-surge influence indicating that breeding habitat type did influence offspring size at weaning. The lack of site fidelity in grey seals on Sable Island is associated with an unpredictable and changing landscape (sand dunes) that could make it difficult for females to locate previous breeding locations. Although breeding location within habitat type had small consequences on offspring weaning mass, we detected no evidence that breeding site selection within the habitat had consequences to females.     

O' mother where wert thou? Maternal strategies in the southern elephant seal: a stable isotope investigation

Maternal effects are widespread in ecology and can alter the dynamics of a population. We investigated the impact of maternal foraging strategies on offspring weaning mass—a proxy of maternal foraging success and of offspring survival—in southern elephant seals on îles Kerguelen. Using 4 years of data, we modelled pup weaning mass as a two-component mixture and used blood stable isotope values to discriminate between maternal foraging strategies previously identified from bio-logging studies. Carbon isotope ratio was a strong predictor of weaning mass, but the relationship was non-monotonic in contrast to a priori expectations. Females foraging in the interfrontal zone weaned pups with a smaller mass compared with females foraging in Antarctic waters. Pup mass was positively correlated with a proxy of global primary production in the interfrontal zone for small weanlings. Maternal effects, via a poor foraging efficiency in the 1970s, may help explain the large population decrease observed at that time on îles Kerguelen because of an overall decrease in pup weaning mass, survival and subsequent recruitment.     

VOCAL INDIVIDUALITY OF IN-AIR WEDDELL SEAL (LEPTONYCHOTES WEDDELLII) PUP "PRIMARY" CALLS

As a result of selective pressures faced during lactation, vocal recognition may play a crucial role in maintaining the phocid mother–pup bond during the period of dependence. To investigate this possibility, we examined whether Weddell seal ( Leptonychotes weddellii ) pups produce individually distinctive "primary" calls. One temporal, nine fundamental frequency features, and two spectral characteristics were measured. A discriminant function analysis (DFA) of 15 Vestfold Hills pups correctly classified 52% of calls, while the cross-validation procedure classified 29% of calls to the correct pup. A second DFA of 10 known-age McMurdo Sound pups correctly classified 44% of "test" calls. For novel calls, the probabilities of attaining such classification rates by chance are low. The relationship between age and call stereotypy indicated that pups 2 wk and older may be more vocally distinctive. Overall, findings suggest that Weddell seal pup "primary" calls are moderately distinctive and only exhibit sufficient stereotypy to aid maternal recognition by approximately two weeks of age.     

Primiparous females do not exhibit reduced maternal care in gray seals (Halichoerus grypus)

We compared the behaviors of primiparous and multiparous gray seal (Halichoerus grypus) females over the course of lactation to examine whether poorly developed maternal behaviors may play a role in the reduced lactation performance observed in primiparous females. Overall, primiparous females spent as much time interacting with their pups as multiparous females. The proportion of time spent nursing their pup increased significantly between early and peak lactation in both primiparous and multiparous females. Although there was no significant difference in the duration of nursing bouts as a function of reproductive status, primiparous females nursed significantly more frequently (bouts/hour) and, therefore, spent a significantly greater proportion of time nursing than multiparous females throughout lactation. Primiparous gray seal females were also significantly more active than multiparous females, however, the difference in activity represented only a small proportion of the overall time budget. We conclude that poorly developed maternal behaviors resulting from a lack of prior reproductive experience are unlikely to account for lower levels of milk energy transfer to pups in primiparous gray seals.     

Dynamic influence of maternal and pup traits on maternal care during lactation in an income breeder, the antarctic fur seal

Life-history theory predicts that selection will favor optimal levels of parental effort that balance benefits of current reproduction with costs to survival and future reproduction. The optimal level of effort depends on parental traits, offspring traits, and provisioning strategy. Additionally, how these factors influence effort may differ depending on the stage of reproduction. The relative importance of maternal and offspring traits on energy allocation to offspring was investigated in known-age Antarctic fur seals Arctocephalus gazella across four stages of reproduction, using birth mass and milk-consumption measurements. Maternal traits were important during three of the four stages investigated, with larger females giving birth to larger pups and investing more in pups during perinatal and molt stages. Pup mass influenced maternal effort during the premolt stage, and provisioning strategy influenced postnatal maternal effort at all stages. Energy provided to the offspring during an attendance visit was positively related to the duration of the foraging-trip/visit cycle; however, when investment was controlled for trip/visit cycle duration, the overall rate of energy transfer was similar across trip durations. In addition to strong effects of maternal mass, pup traits affected energy allocation, suggesting that pup demand is important in determining maternal care. These findings emphasize the importance of considering state variables in life-history studies and suggest that timing of measurements of effort in species with long provisioning periods may influence conclusions and our ability to make comparisons of reproductive effort among species.