Fitness consequences of the timing of metamorphosis in a freshwater crustacean

Metamorphosis is a common life-cycle transition in organisms as diverse as amphibians, insects, fishes and crustaceans, and the timing of this transition often affects an individual's fitness. Here, we measured age and size at metamorphosis in laboratory-reared individuals of the freshwater copepod, Diaptomus leptopus , and then followed individuals over their entire life cycle to assess the fitness consequences of variation in age and size at metamorphosis. In 3 separate experiments, individuals were raised in different food conditions: low food (0.2 μg C/ml) switched to high food (0.7 μg C/ml), or high food switched to low food, at several different larval and juvenile stages. Control individuals were reared on high or low food concentrations over their entire life cycles. For each individual, we measured age and size at metamorphosis and age and size at maturity; for females, we also measured total lifetime egg production, longevity, and calculated a composite fitness measure, λ. Statistical analyses showed no significant effects of age or size at metamorphosis on these same traits measured at maturity, or on the fitness components we estimated. The first individuals to mature had the highest total egg production and individual fitness; differences in body size at maturation explained none of the variation observed in fitness components. Our results show that metamorphosis was uncoupled from maturity and from fitness components by growth and development achieved during the juvenile phase of the life cycle, and support the conclusion that fitness consequences of metamorphosis depend fundamentally on the organization of an organism's life cycle. They also suggest that body size plays a different life-history role in these organisms than is recognized in most poikilotherms, and suggest the hypothesis, based on laboratory experiments, that selection may act primarily on juvenile developmental rates in field populations.     

神农架川金丝猴投食群的母婴关系

灵长类动物母亲的育婴行为不仅对后代的存活起着关键作用,而且对婴猴的发育和成年后的社会交往有着重大影响,因此母-婴关系一直是灵长类学研究的热点之一。为了解川金丝猴的母-婴关系,探讨川金丝猴母-婴关系个体差异的影响因素,我们于2013年3月至2014年10月,以母-婴间相对距离表示照料强度,对神农架川金丝猴的母-婴照料关系以及母亲照料强度个体差异的影响因素进行了定量研究。结果表明：母亲照料行为强度、母亲对阿姨行为的容忍与婴猴年龄相关,限制行为与婴猴年龄不相关。身体接触与婴猴年龄不相关;腹部接触以及母-婴相对距离小于1 m与婴猴年龄呈负相关;相对距离1 m至5 m、相对距离5 m至10 m和相对距离大于10 m与婴猴年龄呈正相关;母亲对阿姨行为的拒绝与婴猴年龄呈负相关;母亲对婴猴的限制行为与年龄不相关。总的来说,即随着婴猴成长,母婴间距离越来越远,母亲也逐渐减少对婴猴的保护。母亲的生育经验、婴猴性别、婴猴出生先后和社会单元大小对神农架川金丝猴母婴关系没有显著影响,补食群丰富的食物资源和群内雌性友好的关系可能是导致本结果的原因。     

Density-range size relationships in French riverine fishes

We examined the relation between the local density of species and the size of the geographic range for French riverine fishes. As for most other taxonomic groups, a positive interspecific relationship is found for this group. This relationship is robust to the confounding effects of phylogeny and is not a priori a product of other potential mechanistic artefacts. We formally tested two of the principal biological mechanisms already proposed (i.e. the niche breadth hypothesis and the resource availability hypothesis). We found no support for the niche breadth hypothesis. In contrast, we found consistent support for the closely related resource availability hypothesis. Species utilising resources (habitats) that are marginal tend to appear at low density and to have narrow distribution whereas species utilising widespread habitats tend to be more abundant and more widely distributed. Using data on body size and reproductive traits we explored the potential influence of these variables in explaining significant variation around the density-range size relationship. Only body size explains significant variation around the relationship, being negatively correlated with local density and positively correlated with range size.     

Nonlinear growth cost in Menidia menidia: theory and empirical evidence

Juvenile growth is submaximal in many species, suggesting that a trade-off with juvenile growth must exist. In support of this, recent studies have demonstrated that rapid growth early in life results in decreased physiological performance. Theory clearly shows that for submaximal growth in juveniles to be optimal, the cost of growth must be nonlinear. However, nearly all of the empirical evidence for costs of growth comes from linear comparisons between fast- and slow-growing groups. It is consequently unclear whether any known cost can account for the evolution of submaximal juvenile growth. To test whether the cost of growth exhibits the logically necessary nonlinearity, we measured critical swimming speed (Ucrit), the maximum speed sustained in incremental velocity trials, in Atlantic silversides, a species for which the costs and benefits of growth are well studied. To increase our ability to detect a nonlinear relationship between Ucrit, a proxy for juvenile fitness, and growth, we manipulated ration levels to produce a broad range of growth rates (0.16 mm/day(-1) to 1.20 mm/day(-1)). Controlling for size and age, we found that Ucrit decreased precipitously as growth approached the physiological maximum. Using Akaike's information criterion, we show that swimming performance decreases with the square of growth rate, providing the first demonstration of a nonlinear cost of growth.     

Selective and genetic constraints on the evolution of body size in a stream-dwelling salmonid fish

To examine constraints on evolution of larger body size in two stunted populations of brook charr (Salvelinus fontinalis) from a single river in Cape Race, Newfoundland, Canada, we measured viability selection acting on length-at-age traits, and estimated quantitative genetic parameters in situ (following reconstruction of pedigree information from microsatellite data). Furthermore we tested for phenotypic differentiation between the populations, and for association of high juvenile growth with early maturity that is predicted by life history theory. Within each population, selection differentials and estimates of heritabilities for length-at-age traits suggested that evolution of larger size is prevented by both selective and genetic constraints. Between the populations, phenotypic differentiation was found in length-at-age and age of maturation traits, whereas early maturation was associated with increased juvenile growth (relative to adult growth) both within and between populations. The results suggest an adaptive plastic response in age of maturation to juvenile growth rates that have a largely environmental basis of determination.     

State-dependent shifts between nocturnal and diurnal activity in salmon

Animal species have usually evolved to be active at a specific time of the daily cycle, and so are either diurnal, nocturnal or crepuscular. However, we show here that the daily timing of activity in juvenile Atlantic salmon is related to the life-history strategy that they have adopted (i.e. the age at which they will migrate to the sea) and their current state (body size/relative nutritional state). Salmon can detect food more easily by day than by night, but the risk of predation is greater. Nocturnal foraging should generally be preferred, but the greater the need for growth, the greater should be the shift towards diurnal activity. In line with this prediction, all fish were predominantly nocturnal, but salmon preparing to migrate to the sea, which would experience size-dependent mortality during the forthcoming migration, were more diurnal than fish of the same age and size that were delaying migration for a further year. Moreover, the proportion of activity by day was negatively correlated with body size within the intending migrants. It has previously been shown that overwinter survival in fish delaying migration is maximized not by growth but by minimizing exposure to predators. As predicted, daytime activity in these fish was correlated with the prior rate of weight loss, fish being more diurnal when their risk of starvation was greater. To our knowledge, these are the first quantitative demonstrations of state-dependent variation in the timing of daily activity.     

Tail growth tracks the ontogeny of prehensile tail use in capuchin monkeys (Cebus albifrons and C. apella)

Physical anthropologists have devoted considerable attention to the structure and function of the primate prehensile tail. Nevertheless, previous morphological studies have concentrated solely on adults, despite behavioral evidence that among many primate taxa, including capuchin monkeys, infants and juveniles use their prehensile tails during a greater number and greater variety of positional behaviors than do adults. In this study, we track caudal vertebral growth in a mixed longitudinal sample of white-fronted and brown capuchin monkeys (Cebus albifrons and Cebus apella). We hypothesized that young capuchins would have relatively robust caudal vertebrae, affording them greater tail strength for more frequent tail-suspension behaviors. Our results supported this hypothesis. Caudal vertebral bending strength (measured as polar section modulus at midshaft) scaled to body mass with negative allometry, while craniocaudal length scaled to body mass with positive allometry, indicating that infant and juvenile capuchin monkeys are characterized by particularly strong caudal vertebrae for their body size. These findings complement previous results showing that long bone strength similarly scales with negative ontogenetic allometry in capuchin monkeys and add to a growing body of literature documenting the synergy between postcranial growth and the changing locomotor demands of maturing animals. Although expanded morphometric data on tail growth and behavioral data on locomotor development are required, the results of this study suggest that the adult capuchin prehensile-tail phenotype may be attributable, at least in part, to selection on juvenile performance, a possibility that deserves further attention.     

Does home range use explain the relationship between group size and parasitism? A test with two sympatric species of howler monkeys

Group size is related to parasite infections in primates. This relationship probably reflects the fact that group size is associated with body contact between group members and with contact with contaminated items in the environment. The latter is highly associated with range use. In the present study we hypothesized that if infection by directly transmitted parasites (DTP) is mainly determined by the exposure of individuals to parasites that accumulate in the environment, and group size correlates negatively with the intensity of home range use, then smaller groups should be more infected by DTP. Additionally, groups that share a higher proportion of their home range with other groups should be more infected. To test our hypothesis we observed and collected fecal samples of two groups of Alouatta palliata (large group size) and two groups of A. pigra (small group size) that live sympatrically in a forest fragment located in Macuspana (Mexico). Group size was positively correlated with range area size and negatively correlated with the intensity of home range use. Range use variables were not related to either DTP prevalence or load. However, there were significant differences in DTP loads between groups, which were positively correlated with group size. Our results suggest that the intensity of home range use is a poor predictor of DTP infection parameters in groups with marked differences in size. Therefore, it is possible that the individual or combined effects of other ecological (e.g., microclimate), social (e.g., contact rate), or physiological (e.g., immune function) factors are more important in the dynamics of DTP in free-ranging primates.     

Playing it cool: Characterizing social play,bout termination,and candidate play signals of juvenile and infant Tibetan macaques(Macaca thibetana)

Play behaviors and signals during playful interactions with juvenile conspecifics are important for both the social and cognitive development of young animals.The social organization of a species can also influence juvenile social play. We examined the relationships among play behaviors, candidate play signals, and play bout termination in Tibetan macaques(Macaca thibetana) during juvenile and infant social play to characterize the species play style. As Tibetan macaques are despotic and live in groups with strict linear dominance hierarchies and infrequent reconciliation, we predicted that play would be at risk of misinterpretation by both the individuals engaged in the play bout and by those watching, possibly leading to injury of the players. Animals living in such societies might need to frequently and clearly signal playful intent to play partners and other group members to avoid aggressive outcomes. We gathered video data on 21 individually-identified juvenile and infant macaques(one month to five years of age)from the Valley of the Wild Monkeys, Mt. Huangshan,China. We used all-occurrence sampling to record play behaviors and candidate play signals based on an ethogram. We predicted that play groups would use multiple candidate play signals in a variety of contexts and in association with the number of audience members in proximity to the players and play bout length. In the 283 playful interactions we scored,juvenile and infant macaques used multiple body and facial candidate play signals. Our data showed that juvenile and infant Tibetan macaques use a versatile repertoire of play behaviors and signals to sustain play.     

Testing ecological and developmental hypotheses of mean and variation in adult size in nephilid orb-weaving spiders

Fecundity selection has been hypothesized to drive the evolution of female gigantism in the orb-weaving family Nephilidae. Several species of these spiders also exhibit large amounts of variation in size at maturity in one or both sexes. In this article, we attempt to detect correlations of mean and variation in adult size at a phylogenetic scale between the sexes and with latitude. We tested six predictions derived from three broad developmental, ecological, and age structure hypotheses, using independent contrasts and a recent species-level nephilid phylogeny as well as least squares and other conventional statistics: 1. In both sexes, species with larger mean size will have greater variation in size; 2. Males and females will show correlated changes in mean size and of variation in size; 3. In both sexes, mean size will be negatively correlated with the midpoint of the latitudinal range; 4. In both sexes, tropical species will be more variable; 5. In both sexes, more widespread species will be more variable; 6. Variation in male size will be positively correlated with mean female size. In no cases were male and female development correlated, suggesting that in this lineage male and female body size evolve independently. The only significant trend detected was a positive phylogenetic correlation between variation in female size and latitude, the opposite of prediction 4. Power tests showed that in all tests of the ecological hypothesis, sample sizes were more than adequate to detect significant trends, if present. Our results suggest that evolutionary trends in juvenile development among species are too weak to be detectable in such data sets.     

Basal metabolic rate in carnivores is associated with diet after controlling for phylogeny

Studies of basal metabolic rate (BMR), the minimum metabolic rate of postabsorptive, inactive endotherms while in their rest phase and thermal neutral zone, have contributed significantly to our understanding of animal energetics. Besides body mass, the main determinant of BMR, researchers have invoked diet and phylogenetic history as important factors that influence BMR, although their relative importance has been controversial. For 58 species within the Carnivora, we tested the hypothesis that BMR is correlated with home range size, a proxy for level of activity, and diet, using conventional least squares regression (CLSR) and regression based on phylogenetic independent contrasts (PIC). Results showed that BMR of Carnivora was positively correlated with home range size after controlling for body mass, regardless of the statistical method employed. We also found that diet and mass-adjusted home range size were correlated. When we simultaneously tested the effect of diet and mass-adjusted home range on mass-adjusted BMR, home range size was insignificant because of its colinearity with diet. Then we eliminated home range size from our model, and diet proved to be significant with both CLSR and PIC. We concluded that species that eat meat have larger home ranges and higher BMR than species that eat vegetable matter. To advance our understanding of the potential mechanisms that might explain our results, we propose the "muscle performance hypothesis," which suggests that selection for different muscle fiber types can account for the differences in BMR observed between meat eaters and vegetarian species within the Carnivora.     

Adult Male Density Influences Juvenile Microhabitat Use in a Territorial Lizard

Habitat choice often has strong effects on performance and fitness. For many animals, optimal habitats differ across age or size classes, and individuals shift habitat use through ontogeny. Although many studies document ontogenetic habitat shifts for various taxa, most are observational and do not identify the causal factor of size‐specific habitat variation. Field observations of the brown anole lizard (Anolis sagrei) show that juveniles perch on shorter and thinner vegetation than adults. We hypothesized that this variation is due to adult males forcing smaller juveniles to less preferred habitat. To test this assertion, we manipulated adult male densities in mesh enclosures with artificial trees to examine the response of juvenile microhabitat choice. We found that adult male density had strong effects on juvenile perch height, perch width, and substrate use, suggesting that age‐class competition contributes to the observed ontogenetic differences in habitat choice. We also found that time of day significantly affected juvenile perch height and substrate use. In many cases, our results suggest that juveniles distance themselves from adults using different microhabitats from those used in our control ‘no‐adult’ treatment. However, these findings were often body size dependent and varied depending upon time of day. This study highlights the complexity of juvenile perching behavior and demonstrates the role of intraspecific interactions in shaping habitat use by juvenile animals.     

Functional Morphology and Biochemical Indices of Performance: Is there a Correlation Between Metabolic Enzyme Activity and Swimming Performance?

Comparative physiologists and ecologists have searched for aspecific morphological, physiological or biochemical parameterthat could be easily measured in a captive, frozen, or preservedanimal, and that would accurately predict the routine behavioror performance of that species in the wild. Many investigatorshave measured the activity of specific enzymes in the locomotormusculature of marine fishes, generally assuming that high specificactivities of enzymes involved in aerobic metabolism are indicatorsof high levels of sustained swimming performance and that highactivities of anaerobic metabolic enzymes indicate high levelsof burst swimming performance. We review the data that supportthis hypothesis and describe two recent studies we have conductedthat specifically test the hypothesis that biochemical indicesof anaerobic or aerobic capacity in fish myotomal muscle correlatewith direct measures of swimming performance. First, we determinedthat the maximum speed during escapes (C-starts) for individuallarval and juvenile California halibut did not correlate withthe activity of the enzyme lactate dehydrogenase, an index ofanaerobic capacity, in the myotomal muscle, when the effectsof fish size are factored out using residuals analysis. Second,we found that none of three aerobic capacity indices (citratesynthase activity, 3-hydroxy-o-acylCoA dehydrogenase activity,and myoglobin concentration) measured in the slow, oxidativemuscle of juvenile scombrid fishes correlated significantlywith maximum sustained speed. Thus, there was little correspondencebetween specific biochemical characteristics of the locomotormuscle of individual fish and whole animal swimming performance.However, it may be possible to identify biochemical indicesthat are accurate predictors of animal performance in phylogeneticallybased studies designed to separate out the effects of body size,temperature, and ontogenetic stage.     

Maturation of guinea pig tracheal strip stiffness

Previously, we showed the shortening velocity of guinea pig tracheal strips was the greatest in juvenile (3-wk-old) compared with infant (1-wk-old) and adult animals (3-mo-old). The greatest shortening velocity was associated with the least resistance to shortening calculated from force-velocity curves among the three age groups. It remained to be verified if the stiffness of tracheal tissue, a measure of tissue response to geometrical deformations, is different among the three age groups. We hypothesized that stiffness of intact tracheal strips is lowest in the juvenile group and that this can explain the ontogeny of airway smooth muscle resistance to shortening and shortening velocity. Static stiffness measured through stepwise deformations showed no age-related differences. Evaluation of tissue response to oscillatory deformations showed that the dynamic stiffness of unstimulated tracheal strips was 8.35 +/- 0.88, 4.15 +/- 1.09, and 8.21 +/- 1.57 kPa, and the phase angle was 10.3 +/- 2.93, 2.46 +/- 0.67, and 7.87 +/- 1.77 degrees in infant, juvenile, and adult, respectively. Unstimulated juvenile strips were significantly lower in dynamic stiffness and phase angle compared with unstimulated infant or adult strips. This maturational profile was independent of muscle strip preset length or oscillation mode/amplitude but was abolished at peak of contraction to either carbachol or electric field stimulation. These results suggest that the noncontractile components of tracheal strips are less stiff and contain fewer viscous/frictional elements in juvenile than in other age groups. This may provide a functional basis for reduced resistance to length changes in juvenile airway smooth muscle.     

The Ontogeny of Encephalization: Tradeoffs Between Brain Growth,Somatic Growth,and Life History in Hominoids and Platyrrhines

This study examines variation in brain growth relative somatic growth in four hominoids and three platyrrhines to determine whether there is a trade-off during ontogeny. I predicted that somatic growth would be reduced during periods of extensive brain growth, and species with larger degrees of encephalization would reach a smaller body size at brain growth completion because more energy is directed towards the brain. I measured cranial capacity and skeletal size in over 500 skeletal specimens from wild populations. I calculated nonlinear growth curves and velocity curves to determine brain/body growth allometry during ontogeny. In addition, I calculated linear regressions to describe the brain/body allometry during the postnatal period prior to brain size reaching an asymptote. The results showed that somatic growth is not substantially reduced in species with extensive brain growth, and body size at brain growth completion was larger in species with greater degrees of encephalization. Furthermore, large body size at brain growth completion was not correlated with interbirth interval, but was significantly correlated with prolonged juvenile periods and late age at maturity when data were corrected for phylogeny. These results indicate that neither reduction in body growth nor reproductive rate are compensatory mechanisms for the energetic costs of brain growth. Other avenues for meeting energetic costs must be in effect. In addition, the results show that somatic growth in encephalized species is particularly slow during the juvenile period after brain growth at or near completion, suggesting that these growth patterns are explained by reasons other than energetic costs.     

Body Size Distribution of the Dinosaurs

The distribution of species body size is critically important for determining resource use within a group or clade. It is widely known that non-avian dinosaurs were the largest creatures to roam the Earth. There is, however, little understanding of how maximum species body size was distributed among the dinosaurs. Do they share a similar distribution to modern day vertebrate groups in spite of their large size, or did they exhibit fundamentally different distributions due to unique evolutionary pressures and adaptations? Here, we address this question by comparing the distribution of maximum species body size for dinosaurs to an extensive set of extant and extinct vertebrate groups. We also examine the body size distribution of dinosaurs by various sub-groups, time periods and formations. We find that dinosaurs exhibit a strong skew towards larger species, in direct contrast to modern day vertebrates. This pattern is not solely an artefact of bias in the fossil record, as demonstrated by contrasting distributions in two major extinct groups and supports the hypothesis that dinosaurs exhibited a fundamentally different life history strategy to other terrestrial vertebrates. A disparity in the size distribution of the herbivorous Ornithischia and Sauropodomorpha and the largely carnivorous Theropoda suggests that this pattern may have been a product of a divergence in evolutionary strategies: herbivorous dinosaurs rapidly evolved large size to escape predation by carnivores and maximise digestive efficiency; carnivores had sufficient resources among juvenile dinosaurs and non-dinosaurian prey to achieve optimal success at smaller body size.     

Intergenerational effect of juvenile hormone on offspring in <Emphasis Type="Italic">Pogonomyrmex</Emphasis> harvester ants

Parents can influence the phenotypes of their offspring via a number of mechanisms. In harvester ants, whether female progeny develop into workers or daughter queens is strongly influenced by the age and temperature conditions experienced by their mother, which is associated with variation in maternal ecdysteroid deposition in fertilized eggs. In many insects, juvenile hormone (JH) is antagonistic to ecdysteroid release, suggesting that seasonal and age-based variation in maternal JH titers may explain maternal effects on offspring size and reproductive caste. To test this hypothesis, we artificially increased maternal JH titers with methoprene, a JH analog, in laboratory colonies of two Pogonomyrmex populations exhibiting genetic caste determination. Increasing maternal JH resulted in a 50% increase in worker body size, as well as a sharp reduction in total number of progeny reared, but did not alter the genotype of progeny reared to adulthood. The intergenerational effect of JH manipulation was not mediated by a reduction in ecdysteroid deposition into eggs; instead, changes in egg size, trophic egg availability or brood/worker ratio may have altered the nutritional environment of developing larvae. Egg ecdysteroid content was significantly negatively correlated with natural variation in worker body size, however, suggesting that there are multiple independent routes by which queens can modify offspring phenotypes.     

The evolution of non-maternal care in anthropoid primates: a test of the hypotheses

The amount of non-maternal care (allocare) found in primates varies greatly from species to species. Our paper examines this variation and focuses on possible reasons why mothers in some anthropoid primate species are prepared to relinquish their infants to other carers whereas others are not. We use data collected from the literature to test a number of hypotheses that attempt to explain the observed variation in non-maternal care. Analyses were carried out using comparative methods that control for the influence of both body size and phylogeny on life-history and ecological variation. The results support previous studies of both primates and other mammals in finding a clear link between the amount of allocare provided and female reproductive rates. Species with high allocare levels grow rapidly post-natally and wean their infants at a younger age (but at the same relative size) than species of the same body size with lower allocare levels. This early weaning allows high allocare species to support higher birth rates than low allocare species but does not result in earlier maturation. Our results, therefore, suggest that mothers allow non-maternal care of their infants in order to increase their own reproductive output. It is not clear whether such a strategy also benefits the infant, as we could find no link between the presence of allocare and early maturation (which would increase the infant's reproductive output) or between allocare levels and infant survival (as measured by vulnerability to infanticide). This suggests that mothers and infants might come into conflict over transfer to other carers, as the benefits to the mother may not be accompanied by benefits to the infant. However, although mothers may benefit from allocare in some circumstances, they will not be expected to allow allocare if the costs are high (e.g. if there is a high risk to the infant).     

Park or Ride? Evolution of Infant Carrying in Primates

Few mammalian orders carry their infants clinging to the mother's fur. I investigated the evolution of carrying behavior in primates and the life-history and ecological correlates of infant care patterns. Primates are ideal for the study as there is variation in infant care patterns. Primate infants are left hidden in nests or parked in trees, both of which strategies I term parking, and are carried orally or ride clinging to the mother's fur: riding. Infant carrying has evolved several times in the Primates and, once evolved, it has been conserved. Significant energetic costs of riding are indicated as riding species maintain smaller home ranges than those of non-riders of the same body size. With body size and phylogenetic influences taken into account, riders appear to incur a reproductive cost by weaning and breeding later than parkers. Although riders do not have lower birth rates than those of parkers, their later age at first reproduction leads to their having a lower reproductive rate, measured by the intrinsic rate of population increase. Precociality of infants is not correlated with either riding or nesting behavior. Although non-nesting species have larger litter sizes, their infants are not significantly smaller, nor are their neonatal brains relatively smaller. Although riding may have some energetic and reproductive costs, its repeated evolution in the Primates suggests that it also has some benefits, the most likely being a reduced mortality risk for carried infants.     

Big mice die young: early life body weight predicts longevity in genetically heterogeneous mice

Small body size has been associated with long life span in four stocks of mutant dwarf mice, and in two varieties of dietary restriction in rodents. In this study, small body size at ages 2-24 months was shown to be a significant predictor of life span in a genetically heterogeneous mouse population derived from four common inbred mouse strains. The association was strongest for weights measured early in adult life, and somewhat weaker, though still statistically significant, at later ages. The effect was seen both in males and females, and was replicated in an independent population of the same genetic background. Body size at ages 2-4 months was correlated with levels of serum leptin in both males and females, and with levels of IGF-I and thyroid hormone in females only. A genome scan showed the presence of polymorphic alleles on chromosomes 2, 6, 7 and 15 with significant effects on body weight at 2-4 months, at 10-12 months, or at both age ranges, showing that weight gain trajectory in this stock is under complex genetic control. Because it provides the earliest known predictor of life span, body weight may be usefully included in screens for induced mutations that alter aging. The evidence that weight in 2-month-old mice is a significant predictor of life span suggests that at least some of the lethal diseases of old age can be timed by factors that influence growth rate in juvenile rodents.