Causes and consequences of variation in offspring body mass: meta‐analyses in birds and mammals

Early survival is highly variable and strongly influences observed population growth rates in most vertebrate populations. One of the major potential drivers of survival variation among juveniles is body mass. Heavy juveniles are better fed and have greater body reserves, and are thus assumed to survive better than light individuals. In spite of this, some studies have failed to detect an influence of body mass on offspring survival, questioning whether offspring body mass does indeed consistently influence juvenile survival, or whether this occurs in particular species/environments. Furthermore, the causes for variation in offspring mass are poorly understood, although maternal mass has often been reported to play a crucial role. To understand why offspring differ in body mass, and how this influences juvenile survival, we performed phylogenetically corrected meta‐analyses of both the relationship between offspring body mass and offspring survival in birds and mammals and the relationship between maternal mass and offspring mass in mammals. We found strong support for an overall positive effect of offspring body mass on survival, with a more pronounced influence in mammals than in birds. An increase of one standard deviation of body mass increased the odds of offspring survival by 71% in mammals and by 44% in birds. A cost of being too fat in birds in terms of flight performance might explain why body mass is a less reliable predictor of offspring survival in birds. We then looked for moderators explaining the among‐study differences reported in the intensity of this relationship. Surprisingly, sex did not influence the intensity of the offspring mass–survival relationship and phylogeny only accounted for a small proportion of observed variation in the intensity of that relationship. Among the potential factors that might affect the relationship between mass and survival in juveniles, only environmental conditions was influential in mammals. Offspring survival was most strongly influenced by body mass in captive populations and wild populations in the absence of predation. We also found support for the expected positive effect of maternal mass on offspring mass in mammals (r_pearson = 0.387). As body mass is a strong predictor of early survival, we expected heavier mothers to allocate more to their offspring, leading them to be heavier and so to have a higher survival. However, none of the potential factors we tested for variation in the maternal mass–offspring mass relationship had a detectable influence. Further studies should focus on linking these two relationships to determine whether a strong effect of offspring size on early survival is associated with a high correlation coefficient between maternal mass and offspring mass.     

Body size,litter size,timing of reproduction,and juvenile survival in the Unita ground squirrel,Spermophilus armatus

The timing of reproduction affected litter size, offspring mass, and offspring survival in the Uinta ground squirrel, Spermophilus armatus, in Grand Teton National Park, Wyoming. Survival of juvenile females to yearling age varied negatively with date of weaning and positively with individual offspring mass. At the same time, juveniles weaned early in the season were lighter, and juveniles weaned later in the season were heavier. The coefficient of variation for juvenile body mass, originally measured at weaning, significantly decreased by the time juveniles entered hibernation, indicating that individuals weaned early and light caught up in body mass to individuals weaned later and heavier. From the perspective of the mother's investment in the litter, litter size (corrected for mother's mass) decreased with later wcaning dates, while the relationship of weaning date to litter mass (corrected for mother's mass) was significant in only one year. Maternal allocation of resources in litters changed over the season so that mothers produced many, small offspring early in the season, and fewer, large offspring late in the season.     

Investigating life‐history traits of Steller sea lions with multistate hidden Markov mark–recapture models: Age at weaning and body size effects

The duration of offspring care is critical to female fitness and population resilience by allowing flexibility in life‐history strategies in a variable environment. Yet, for many mammals capable of extended periods of maternal care, estimates of the duration of offspring dependency are not available and the relative importance of flexibility of this trait on fitness and population viability has rarely been examined. We used data from 4,447 Steller sea lions Eumetopias jubatus from the Gulf of Alaska and multistate hidden Markov mark–recapture models to estimate age‐specific weaning probabilities. Maternal care beyond age 1 was common: Weaning was later for animals from Southeast Alaska (SEAK) and Prince William Sound (PWS, weaning probabilities: 0.536–0.648/0.784–0.873 by age 1/2) compared with animals born to the west (0.714–0.855/0.798–0.938). SEAK/PWS animals were also smaller than those born farther west, suggesting a possible link. Females weaned slightly earlier (+0.080 at age 1 and 2) compared with males in SEAK only. Poor survival for weaned versus unweaned yearlings occurred in southern SEAK (female survival probabilities: 0.609 vs. 0.792) and the central Gulf (0.667 vs. 0.901), suggesting poor conditions for juveniles in these areas. First‐year survival increased with neonatal body mass (NBM) linearly in the Gulf and nonlinearly in SEAK. The probability of weaning at age 1 increased linearly with NBM for SEAK animals only. Rookeries where juveniles weaned at earlier ages had lower adult female survival, but age at weaning was unrelated to population trends. Our results suggest the time to weaning may be optimized for different habitats based on long‐term average conditions (e.g., prey dynamics), that may also shape body size, with limited short‐term plasticity. An apparent trade‐off of adult survival in favor of juvenile survival and large offspring size in the endangered Gulf of Alaska population requires further study.     

Yellow-bellied marmots do not compensate for a late start: the role of maternal allocation in shaping life-history trajectories

Yellow-bellied marmots, Marmota flaviventris, are obligate hibernators with a relatively short active season. Animals born earlier in the season have higher chances of fulfilling the energetic requirements to survive the long winter. Therefore, the onset of breeding should have a profound impact on juvenile survival. However, there are different ways to compensate for a late start. Late breeders might allocate more resources to late born offspring, making up for the bad start, or juveniles might show compensatory growth. They are not exclusive hypotheses and both can lead to juveniles entering hibernation with a similar body condition. We used data from a long term study in and around the Rocky Mountain Biological Laboratory in Colorado to test these two hypotheses. Animals are individually marked and trapped regularly. We compared mass at weaning, mass at the end of the season, growth rates and survival between animals born earlier and later in the season. We found no evidence of increased maternal input: late breeders had lighter offspring at weaning than early breeders, and late born juveniles did not increase their growth rates. Therefore, late born individuals ended the season with lower body mass, and were thus, less likely to survive the winter. In conclusion, life-history trajectories of juvenile yellow-bellied marmots were determined by maternal allocation, whereas post-weaning decisions did not modify their fate.     

The cost of growing large: costs of post‐weaning growth on body mass senescence in a wild mammal

Individual body mass often positively correlates with survival and reproductive success, whereas fitness costs of growing large are rarely detected in vertebrates in the wild. Evidence that adult body mass progressively declines with increasing age is accumulating across mammalian populations. Growing fast to a large body can increase the cellular damage accumulated throughout life, leading body growth in early life to be negatively associated with the rate of body mass senescence. Moreover, the onset of mass senescence may strongly depend on both sex‐specific reproductive tactics and environmental conditions. Assessing the timing and the rate of body mass decline with increasing age thus offers an opportunity to look for costs of having grown fast, especially after a poor start during early life, in both sexes and in different environments. Using a unique dataset including 30 years of longitudinal data on age‐specific body mass collected in two roe deer Capreolus capreolus populations subjected to contrasted environmental conditions, we looked for potential costs of high post‐weaning growth rate in terms of steeper rate of body mass senescence. Our analyses of body mass senescence accounted for the potential variation in the onset of senescence and allowed explicit comparisons of this variable between sexes and populations. Higher growth rates late in the growing period (after weaning) were associated with a steeper rate of body mass senescence, regardless of early mass (gained before weaning), but at different extents depending on sex and environmental conditions. Body mass senescence occurred earlier in males than in females, especially in the population facing limiting resources. In the wild, although heavy individuals generally survive better than small ones, the costs of growing large late in the growing period only became apparent late in life through mass senescence.     

Estimating survival and adoption rates of dependent juveniles

Population growth and fitness are typically most sensitive to adult survival in long‐lived species, but variation in recruitment often explains most of the variation in fitness, as past selection has canalized adult survival. Estimating juvenile survival until age of independence has proven challenging, because marking individuals in this age class may directly affect survival. For Greater Sage‐grouse, uniquely marking juveniles in the first days of life likely results in adverse effects to survival, detection of juveniles is not perfect, and females adopt juveniles from other parents. These challenges are encountered by researchers studying avian and mammalian species with similar life histories, yet methods do not exist that explicitly estimate all these components of the recruitment process. We propose a novel data collection method and demographic model to simultaneously estimate rates of detection, survival, and adoption of juvenile individuals. Using multiple cameras to film the beginning of juvenile activity on specific days, we obtained counts of juveniles associated with marked females. Increases of juveniles to broods provided information that enabled us to estimate rates of adoption that can be applied at the population level. Losses from broods informed apparent survival. These losses could be attributed to death, or they could be chicks that were adopted by other females. We found evidence that apparent survival of juveniles was influenced by localized weather patterns when chicks were young. Similarly, we found that young chicks were more susceptible to the adverse effect of attending females being flushed by an observer. Both of these patterns diminished quickly as chicks aged. We provide the first‐ever estimates of interval‐specific adoption rates. Our results suggest that researchers should be cautious when designing studies to estimate juvenile survival. More importantly, they provide insight into adoption, a behavior that has been known to exist for decades.     

Energetic and endocrine mediation of natal dispersal behavior in Belding's ground squirrels

Natal dispersal, the permanent departure of an individual from its birth site, is sex biased in most mammals, with males dispersing at higher rates or over greater distances than do female conspecifics. Because dispersal movements may be energetically expensive, their occurrence should theoretically be influenced by energy availability. Moreover, the male bias typical of mammalian dispersal suggests that this behavior might be mediated by gonadal androgens. Using free-living Belding's ground squirrels (Spermophilus beldingi) as subjects, we provisioned juveniles with extra food to evaluate energetic influences on male dispersal behavior. Provisioning increased body mass and body fat of juvenile males and caused them to disperse at younger ages, but did not affect blood glucose levels. Dispersing males were fatter than same-aged males that had not yet dispersed. Moreover, body fat of provisioned and unprovisioned males did not differ when evaluated relative to the week during which they dispersed, suggesting that there may be a fat threshold for dispersal. In a second experiment, we measured plasma concentrations of testosterone (T) in provisioned and unprovisioned, free-living juveniles to evaluate the hypothesis that male dispersal behavior is activated by concurrent high levels of T. We observed no increase in plasma T associated with dispersal by juvenile males, no sex differences in circulating T among juveniles, and no effects of food provisioning on juvenile T levels. In a third experiment with free-living S. beldingi, we concurrently altered early androgen exposure by treating females with T at birth and manipulated energy availability by food provisioning. Perinatal T-treatment increased the likelihood of dispersal among juvenile females. Provisioning increased body mass and body fat of juveniles and caused males and T-treated females to disperse at significantly younger ages than either their unprovisioned counterparts or the few provisioned control females that dispersed. These results suggest that early T exposure in this species determines the probability of dispersal, whereas the amount of energy an individual has stored as fat strongly influences the timing of dispersal. Early T exposure also appears to cause the timing of dispersal to respond to energy availability and body fat in a male-typical way, possibly by organizing masculine mass and fat thresholds for dispersal.     

Seychelles warblers with silver spoons: Juvenile body mass is a lifelong predictor of annual survival,but not annual reproduction or senescence

The environment experienced during development, and its impact on intrinsic condition, can have lasting outcomes for individual phenotypes and could contribute to variation in adult senescence trajectories. However, the nature of this relationship in wild populations remains uncertain, owing to the difficulties in summarizing natal conditions and in long‐term monitoring of individuals from free‐roaming long‐lived species. Utilizing a closely monitored, closed population of Seychelles warblers (Acrocephalus sechellensis), we determine whether juvenile body mass is associated with natal socioenvironmental factors, specific genetic traits linked to fitness in this system, survival to adulthood, and senescence‐related traits. Juveniles born in seasons with higher food availability and into smaller natal groups (i.e., fewer competitors) were heavier. In contrast, there were no associations between juvenile body mass and genetic traits. Furthermore, size‐corrected mass—but not separate measures of natal food availability, group size, or genetic traits—was positively associated with survival to adulthood, suggesting juvenile body mass is indicative of natal condition. Heavier juveniles had greater body mass and had higher rates of annual survival as adults, independent of age. In contrast, there was no association between juvenile mass and adult telomere length attrition (a measure of somatic stress) nor annual reproduction. These results indicate that juvenile body mass, while not associated with senescence trajectories, can influence the likelihood of surviving to old age, potentially due to silver‐spoon effects. This study shows that measures of intrinsic condition in juveniles can provide important insights into the long‐term fitness of individuals in wild populations.     

Functions and consequences of play behaviour in juvenile Belding's ground squirrels

We evaluated hypotheses suggesting that play behaviour in free-living juvenile Belding's ground squirrels, Spermophilus beldingi, helps to regulate energy balance and promotes development of motor skills. We also examined the possibility that play behaviour and motor development influence important early life events in S. beldingi. We regularly measured body fat of juveniles using nondestructive methods, observed their behaviour, conducted motor skills tests, monitored dispersal status of males, and evaluated weaning success of females during their first effort to reproduce as yearlings. Rates of feeding were greater among juveniles who engaged in both social and nonsocial play behaviour at low compared to high rates, suggesting that play does not help young animals expend excess energy that they consume. By contrast, body fat was greater among juveniles who engaged in social play at high compared to low rates, supporting the idea that energetic variables such as body fat limit the expression of play behaviour. Motor skills of juvenile S. beldingi improved throughout the period in which juveniles engaged in play behaviour. Improvement in motor skills was greater in juveniles who engaged in social play at high compared to low rates, suggesting a link between social play and motor development. Motor skill levels at the end of the play period were greater among males who dispersed by the end of the juvenile summer than among males still residing in their natal areas. Rates of social play and improvement in motor skills over the course of the play period were greater in females who successfully weaned a litter as yearling than in females who did not. These last two results raise the possibility that play behaviour and motor development during the play period influence important events in the early lives of S. beldingi, which may ultimately influence long-term reproductive success.     

Paternal effects on juvenile growth and survival in spring peepers (Hyla crucifer)

Summary In 1986 and 1987 we crossed severalHyla crucifer females to males that obtained mates in nature and to large and small body sized males that did not do so. Offspring resulting from these crosses were raised as pairs of juveniles in 1986, or in isolation in 1987 to assess sire effects on juvenile growth and survival in a competitive and a non-competitive context. When raised as part of a pair of juveniles, individuals sired by natural mates had lower growth rates than their fullsibs raised alone. The reduction in growth was of a similar magnitude whether the cohabiting juvenile was sired by a large or small bachelor. When raised together, individuals sired by natural mates had higher growth rates (indicative of competitive superiority) than their halfsibs sired by large bachelor males, but competed as equals with the progeny of small bachelor males. Progeny of large bachelors competed with progeny of small bachelors as equals. When raised individually in 1987, juveniles sired by natural mates had lower growth rates than their halfsibs sired by bachelor males. Paternal size or mating status had marginally non-significant effects on juvenile survival in 1986 and no effect in 1987. The larger individual in a pair was more likely to survive in 1986. In neither year was juvenile growth rate correlated with prior growth rate as a larva. These experiments suggest that the mating system can affect the distribution of juvenile growth rates (and possible survival rates), and thereby have important fitness consequences long after pair formation. These effects are, however, inconsistent either among sets of individual males or among juvenile environments.     

Changes in body condition and body size affect breeding and recruitment in fluctuating house mouse populations in south‐eastern Australia

Changes in body condition and body size in field populations of house mice, Mus domesticus, were examined to investigate why mouse populations do not increase rapidly in some years when favourable environmental and demographic conditions indicate they might. Mice had repeated seasonal patterns each year in breeding, growth rates and body condition that reflected the seasonal availability of food, but mean levels for each parameter varied among years. In most years mice lost body condition during summer, breeding declined and population growth slowed. Rapid population growth occurred when body condition was generally high and was maintained throughout summer. Female mice with large body length were more likely to breed than smaller mice, at all times, but changes in body condition accounted for most of the variability in female breeding activity between years and between habitats, and for the seasonal changes in the importance of body length. During rapid population growth, the recruitment rate of juveniles relative to the number of breeding females was 150–300% higher than in other years but adult survival rates were not higher. The data indicate that the ability of mice to maintain body condition, particularly when subject to moisture stress in summer, affects the proportion of females breeding, the number of juveniles weaned and their body condition at weaning, and is promoted by foraging conditions that favour maintenance of juvenile body condition after weaning. These factors, in turn, greatly affect juvenile recruitment rates and eventual population density of mice. Low juvenile survival is suggested as a reason that numbers of house mice in southern Australian cereal‐growing areas do not increase rapidly in some years when other parameters are favourable. Similar processes are likely to play a role in regulating other rodent populations.     

Sex Ratio and Sexual Size Dimorphism in a Toad-headed Lizard,Phrynocephalus guinanensis

Phrynocephalus guinanensis has sexual dimorphism in abdominal coloration, but its ontogenetic development of sexual size dimorphism(SSD) is unknown. Using mark-recapture data during four days each year from August from 2014 to 2016, we investigated the development of sex ratios, SSD, sex-specific survivorship and growth rates in a population of P. guinanensis. Our results indicated that the sex ratio of males to females was 1:2.8. Males had a lower survival rate(6%) than females(14%) across the age range from hatchling to adult, which supported the discovered female-biased sex ratio potentially associated with the low survival rate of males between hatchlings and juveniles. Male-biased SSD in tail length and head width existed in adults rather than in hatchling or juvenile lizards. The growth rates in body dimensions were undistinguishable between the sexes during the age from hatchling to juvenile, but the growth rate in head length from juvenile to adult was significantly larger in males than females. Average growth rate of all morphological measurements from hatchling to juvenile were larger compared with corresponding measurements from juvenile to adult, but only being significant in tail length, head width, abdomen length in females and snout-vent length in males. We provided a case study to strengthen our understanding of the important life history traits on how a viviparous lizard population can survive and develop their morphology in cold climates.     

Sex- and age-dependent effects of population density on life history traits of red deer Cervus elaphus in a temperate forest

We studied both the short‐ and long‐term effects of density on three life history traits of a red deer population inhabiting a temperate forest. Both male and female body mass increased when population density decreased, but male mass changed to a greater extent than female mass. Density did not influence female survival irrespective of age, however, survival of males was lower at high density for all age classes except the prime‐age class. Pregnancy rates of primiparous females increased markedly with decreasing density, whereas those of adult hinds were fairly constant and unrelated to density. For both sexes, of the studied life history traits we detected a long‐term effect of density at birth (cohort effect) only on body mass. These results suggest that density influences life history traits in the same way as factors of environmental variation such as climate. In this population we did not find any evidence for an influence of climatic conditions on life history traits of red deer. Both mild winters and the absence of summer droughts during the study period could account for such an absence of climatic effects. We interpreted our results to show that 1) as expected for a highly dimorphic and polygynous species such as red deer, male traits showed consistently higher sensitivity to variation in density than female traits, illustrating possible costs caused by sexual selection in males, 2) the female‐based Eberhardt's model according to which increasing density should sequentially affect juvenile survival, reproductive rates of primiparous females, reproductive rates of adults and lastly adult survival was only partly supported because we found that pregnancy rate of primiparous females rather than juvenile survival was the most sensitive trait to variation in density. We propose that including variation in male traits would improve the accuracy of models of population dynamics of large mammals, at least for highly dimorphic species. Because the population we studied was not fenced, we only measured apparent survival. We discuss how dispersal, in relation to the phenotypic quality of young deer, might be a potential regulating factor under such conditions.     

The Effects of Birth Weight and Maternal Care on Survival of Juvenile Steller Sea Lions (Eumetopias jubatus)

Steller sea lions were listed as endangered following a collapse of the western distinct population beginning in the late 1970s. Low juvenile survival has been implicated as a factor in the decline. I conducted a multistate mark-recapture analysis to estimate juvenile survival in an area of the western population where sea lions are showing signs of recovery. Survival for males and females was 80% between 3 weeks and 1 year of age. Approximately 20% of juveniles continued to be nursed by their mothers between ages 1 and 2 and 10% between ages 2 and 3. Survival for juveniles that suckled beyond 1 year was 88.2% and 89.9% to ages 2 and 3, respectively. In contrast, survival for individuals weaned by age 1 was 40.6% for males and 64.2% for females between ages 1 and 2. Birth mass positively influenced survival for juveniles weaned at age 1 but had little effect on individuals continuing to suckle. Cumulative survival to age 4 was double that estimated during the population decline in this region. Evidence suggests that western Steller sea lions utilize a somewhat different maternal strategy than those in the eastern distinct population. Western adult females generally invest more in their pups during the first year but wean offspring by age 1 more often. This results in better survival to age 1, but greater mortality between ages 1 and 3 compared to the eastern population. Different maternal strategies may reflect density dependent pressures of populations at opposite levels of abundance.     

Quantitative aspects of harbour seal (Phoca vitulina) play

The play behaviour of harbour seals was analysed on videotape, which allowed the determination of rates of play per age class relative to the population of seals of the same age, and sex of the player in 190 episodes. The majority of the results of our earlier study were confirmed in that adult play was unusually common, and most play was solitary rather than social. However, the reason for the apparent predominance of juvenile play in our 1986 study was most likely because of a large number of juvenile seals present in the herd at that time. Similarly, mothers, pups and weaners proved to play at high rates when population size was taken into account. Adults' playing increased during the weaning/mating period, whereas juveniles and subadults played progressively less over time. Mothers and pups played more near weaning, after which time the youngsters played at a steady high rate, though never socially. Adult social play was only evident during the mating period. The play of males and females differed qualitatively and quantitatively.     

Offspring size at weaning affects survival to recruitment and reproductive performance of primiparous gray seals

Offspring size affects survival and subsequent reproduction in many organisms. However, studies of offspring size in large mammals are often limited to effects on juveniles because of the difficulty of following individuals to maturity. We used data from a long‐term study of individually marked gray seals (Halichoerus grypus; Fabricius, 1791) to test the hypothesis that larger offspring have higher survival to recruitment and are larger and more successful primiparous mothers than smaller offspring. Between 1998 and 2002, 1182 newly weaned female pups were branded with unique permanent marks on Sable Island, Canada. Each year through 2012, all branded females returning to the breeding colony were identified in weekly censuses and a subset were captured and measured. Females that survived were significantly longer offspring than those not sighted, indicating size‐selective mortality between weaning and recruitment. The probability of female survival to recruitment varied among cohorts and increased nonlinearly with body mass at weaning. Beyond 51.5 kg (mean population weaning mass) weaning mass did not influence the probability of survival. The probability of female survival to recruitment increased monotonically with body length at weaning. Body length at primiparity was positively related to her body length and mass at weaning. Three‐day postpartum mass (proxy for birth mass) of firstborn pups was also positively related to body length of females when they were weaned. However, females that were longer or heavier when they were weaned did not wean heavier firstborn offspring.     

Larval growth, juvenile size and heterozygosity in laboratory reared mussels, Mytilus edulis

Studies with marine bivalve juveniles have shown a positive correlation between growth and allozyme multi-locus heterozygosity (MLH), and, in some cases, between larval growth and juvenile growth, but there has been little research on the relationship between allozyme heterozygosity and larval growth. Larvae of M. edulis from different mating systems (half-sib families with a single female, or a single male parent, a reciprocal cross of two malesxtwo females and two mass matings of 13x13 and 8x17 females and males, respectively) were reared in the laboratory and selected into fast and slow growing groups when about 10-30% were undergoing metamorphosis. Offspring were reared to the juvenile stage (>3.00 mm) and both groups of each mating were electrophoresed and genotyped at up to 12 allozyme loci. There was generally good agreement with Mendelian inheritance (half-sibs and reciprocal cross) or the Hardy-Weinberg model (mass matings). Null alleles were detected at the Odh and Lap loci but there was no evidence that null allele heterozygotes grew slower than other genotypes. Over all cohorts, juveniles from the fast growing larval group were not significantly larger, or smaller, than juveniles from the slow growing group which suggests that larval growth rate may be independent of juvenile growth rate. This observation agrees with some, but not all, earlier studies and has commercial relevance. Tests of heterozygosity and juvenile shell length indicated no association between average heterozygosity across all allozyme loci and the size of juveniles in any cohort regardless of the mating system used or their larval growth rate. The association between MLH and juvenile growth in bivalves is seldom detected in cohorts from a limited genetic background. The lack of an association between heterozygosity and size might therefore be expected in the half-sib and reciprocal cross cohorts, but not in the mass matings. The results argue against any significant association between heterozygosity and larval size in mussels.     

Fewer newborn result in superior juveniles in the paternally brooding pipefish Syngnathus typhle L.

In the pipefish Syngnathus typhle L. parental care is exclusively paternal. Males brood embryos in a brood pouch for about a month, providing nutrients and oxygen. The newborn juveniles are free-swimming and no further care is provided. The influence of paternal length and number of newborn on juvenile weight and growth rate, and how in turn the latter relates to juvenile survival, were investigated. It was found, using partial correlations, that paternal length is significantly and positively correlated to weight of newborn, weight of a two-week-old juvenile and juvenile growth rate (weight increment day^-1). Furthermore, number of newborn is correlated negatively to weight of newborn, weight after two weeks and juvenile growth rate. In an experiment in which juvenile pipefish of different sizes were exposed to predation it was shown that larger juveniles survived better. It is concluded that, in S. typhle , large juvenile size and rapid juvenile growth positively influence offspring performance. Offspring performance is positively influenced by paternal length which, however, may be a consequence of larger males receiving larger eggs. The number of newborn, i.e. the number of siblings in the male's pouch, has a negative effect on offspring performance, independently of other factors. Thus, the results show that for males the benefits of having superior juveniles will be at the cost of having fewer offspring.     

Determinants of survival in juvenile Psammodromus algirus lizards

Theories posit that the relative mortality rate of adults and juveniles is a major determinant of population dynamics and life history evolution. Moreover, differential survival of pre-reproductive individuals may be an important source of variation in lifetime reproductive success, and characters that influence survival of juveniles are likely to be under strong selection. We examined survival from hatching to maturity in a natural population of Psammodromus algirus lizards using data from a capture-mark-recapture study. We found that mortality from hatching to maturity was high: only 8% of males and 14% of females that hatched in 1996 survived the entire study period until maturity in spring 1998. The probability of survival was 75% during both the first and second overwinter periods when lizards were inactive most of the time, and about 25% during their first spring to autumn activity season. Our analyses further revealed significant associations between survival and snout-vent length, body condition, sex and microhabitat use. However, the relationship between survival and morphological characters varied among time periods, presumably because the sources of mortality during the activity season were different from those during hibernation. The association between survival and body condition also varied within time periods, both between large and small individuals and between the two sexes. This suggests that the relative importance of different selective agents may change during the life of individuals and vary between males and females due to differences in body size and behaviour. Received: 24 March 1999 / Accepted: 14 February 2000     

Early-life density-dependence effects on growth and survival in subantarctic fur seals

Understanding the regulation of natural populations has been a long-standing research program in ecology. Current knowledge on marine mammals and seabirds is biased toward the adult component of populations and lacking are studies investigating the juvenile component. Our goal was to estimate demographic parameters on the pre-weaning stage of a subantarctic fur seal (Arctocephalus tropicalis) population on Amsterdam Island, suspected to be regulated by density-dependence. The influence of abundance on growth parameters (length and weight) and survival was assessed over a study period spanning 16 years. We evidenced a negative trend in population growth rate when density increased. Density-dependence models were favored for pup body size and mass growth. Abundance had a clear influence on body length at high population-density, pups grew slower and were smaller at weaning than pups born in years with low population density. Abundance partly explained pup body mass variation and a weak effect was detected on pre-weaning survival. The causal mechanisms may be increased competition for food resources between breeding females, leading to a reduction of maternal input to their pups. Our results suggested that pup favored survival over growth and the development of their diving abilities in order to withstand the extreme fasting periods that are characteristic of this fur seal population. This analysis provides significant insight of density-dependent processes on early-life demographic parameters of a long lived and top-predator species, and more specifically on the pre-weaning stage with important consequences for our understanding of individual long-term fitness and population dynamics.