Life history consequences of mammal sibling rivalry

Pup Recruitment in a Eusocial Mammal-Which Factors Influence Early Pup Survival in Naked Mole-Rats?

In eusocial insects, offspring survival strongly depends on the quality and quantity of non-breeders. In contrast, the influence of social factors on offspring survival is more variable in cooperatively breeding mammals since maternal traits also play an important role. This difference between cooperative insects and mammals is generally attributed to the difference in the level of sociality. Examining offspring survival in eusocial mammals should, therefore, clarify to what extent social organization and taxonomic differences determine the relative contribution of non-breeders and maternal effects to offspring survival. Here, we present the first in-depth and long-term study on the influence of individual, maternal, social and environmental characteristics on early offspring survival in a eusocial breeding mammal, the naked mole-rat (Heterocephalus glaber). Similarly to other mammals, pup birth mass and maternal characteristics such as body mass and the number of mammae significantly affected early pup survival. In this eusocial species, the number of non-breeders had a significant influence on early pup survival, but this influence was negative-potentially an artifact of captivity. By contrasting our findings with known determinants of survival in eusocial insects we contribute to a better understanding of the origin and maintenance of eusociality in mammals.

Who listens to mother? A whole-family perspective on the evolution of maternal hormone allocation

Maternal effects, or the influence of maternal environment and phenotype on offspring phenotype, may allow mothers to fine-tune their offspring's developmental trajectory and resulting phenotype sometimes long after the offspring has reached independence. However, maternal effects on offspring phenotype do not evolve in isolation, but rather within the context of a family unit, where the separate and often conflicting evolutionary interests of mothers, fathers and offspring are all at play. While intrafamilial conflicts are routinely invoked to explain other components of reproductive strategy, remarkably little is known about how intrafamilial conflicts influence maternal effects. We argue that much of the considerable variation in the relationship between maternally derived hormones, nutrients and other compounds and the resulting offspring phenotype might be explained by the presence of conflicting selection pressures on different family members. In this review, we examine the existing literature on maternal hormone allocation as a case study for maternal effects more broadly, and explore new hypotheses that arise when we consider current findings within a framework that explicitly incorporates the different evolutionary interests of the mother, her offspring and other family members. Specifically, we hypothesise that the relationship between maternal hormone allocation and offspring phenotype depends on a mother's ability to manipulate the signals she sends to offspring, the ability of family members to be plastic in their response to those signals and the capacity for the phenotypes and strategies of various family members to interact and influence one another on both behavioural and evolutionary timescales. We also provide suggestions for experimental, comparative and theoretical work that may be instrumental in testing these hypotheses. In particular, we highlight that manipulating the level of information available to different family members may reveal important insights into when and to what extent maternal hormones influence offspring development. We conclude that the evolution of maternal hormone allocation is likely to be shaped by the conflicting fitness optima of mothers, fathers and offspring, and that the outcome of this conflict depends on the relative balance of power between family members. Extending our hypotheses to incorporate interactions between family members, as well as more complex social groups and a wider range of taxa, may provide exciting new developments in the fields of endocrinology and maternal effects.

Within-litter covariance of allele-specific MHC heterozygosity, coccidian endoparasite load and growth is modulated by sibling differences in starting mass

Although littermates in altricial mammals usually experience highly similar environmental conditions during early life, considerable differences in growth and health can emerge among them. In a study on subadults of a European rabbit (Oryctolagus cuniculus) population with low MHC polymorphism, we tested whether litter-sibling differences in endoparasitic coccidia load and body mass at the end of the vegetation period were associated with within-litter differences in starting body mass (measured around 2 weeks prior to weaning) and in immune-genetic (MHC class II DRB) constitution. We hypothesized that siblings with a lighter starting mass might be more susceptible to endoparasite infections and thus, negative effects of a more unfavourable MHC constitution might be particularly pronounced in such individuals. Within-litter comparisons revealed that animals with a lighter starting mass reached a relatively lower body mass in autumn. Furthermore, there were indications for an allele-specific heterozygote advantage, as animals with heterozygous combinations of the allele Orcu-DRB*4 had relatively lower hepatic coccidia loads than their littermates with certain homozygous allele combinations. Consistent with our hypothesis, significantly higher hepatic coccidia loads and tendentially lower autumn body masses in homozygous compared to heterozygous individuals for the allele Orcu-DRB*4 were evident in initially lighter but not in heavier siblings, suggesting synergistic effects between an unfavourable MHC constitution and a light starting mass. Taken together, these effects might lead to notable differences in fitness among litter siblings, as a low body mass and a high endoparasite burden are key factors limiting young rabbits' survival during winter.

Within-Family Environment and Cross-Fostering Stress Affect Behavior and Physiology in Wild Cavies (Cavia aperea)

Stability of personality traits is well-documented for a wide variety of animals. However, previous results also suggest that behavioral phenotypes are plastic during early ontogeny and can be adaptively shaped to the social environment. In cavies (Cavia aperea), it has already been documented that the size at birth relative to siblings (size rank) greatly influences various behavioral and physiological traits that last at least until independence. The aim of the current study was (1) to investigate if behavioral and physiological differences between pups of the same litter persist until after independence and influence development long-lasting, (2) to determine the potential plasticity in response to changes in the early within-family environment by cross-fostering pups either to the same, a lower, or a higher size rank in a foster-family. We measured three behavioral traits (number of interactions with a novel object, distance moved in an open field, struggle docility) and two physiological traits (resting metabolic rate and basal cortisol levels). We predicted that cross-fostering into a litter where pups occupy the same size rank would not change the expression of traits. Cross-fostering to a different size rank should not influence the expression of traits if repeatability measures indicate low plasticity. Alternatively, if the traits are plastic, animals should adjust trait expression to fit with the size rank occupied in the foster litter. Initial differences in struggle docility, distance moved in an open field and in baseline cortisol concentration between pups of different size-ranks did not remain stable beyond independence. In addition, we found remarkable plasticity of the measured traits in response to cross-fostering to the same, a smaller or larger size-rank, suggesting that differences between pups are more the result of social constraints leading to adaptive shaping of individual phenotypes within a family. We also found a significant influence of the cross-fostering procedure itself. Cross-fostered individuals were less bold, grew slower and showed elevated resting metabolic rates. This finding suggests a cautious interpretation of previous cross-fostering studies and stresses the need for proper control groups to reliably separate the effect of cross-fostering per se from those induced by an experimental treatment.

In polytocous mammals, weakling neonates, but not their stronger littermates, benefit from specialized foraging

Adjusting foraging strategies is a common phenomenon within groups of animals competing for the same resource. In polytocous mammals, neonates concurrently compete for limited milk and alternate between two foraging (suckling) strategies: adaptable exploratory foraging with random sampling of teats, and ordered foraging with a tendency towards exploiting a particular suckling position. Some theoretical (game theory) models have shown that weaker siblings in particular benefit from foraging specialization (suckling order). Neonate piglets establish a well-defined suckling order that develops gradually and fluctuates throughout the lactation period, implying the existence of inter-individual differences in foraging strategies. We therefore analyzed suckling behavior in pigs to determine whether one foraging strategy was more beneficial to neonates in terms of their body weight and foraging environment. We found that intermediate and heavy littermates tended to adjust their suckling strategy according to the foraging environment; however, the selected foraging strategy did not affect their overall growth performance. Lighter individuals that consumed significantly less milk did not greatly alternate their foraging strategy according to the foraging environment, but their growth rate was significantly higher whenever they performed less-exploratory foraging behavior. Although suckling order appeared to be a relatively stable behavioral phenotype, it was beneficial exclusively for weaklings. These results confirm theoretical predictions and indicate that specializing in a suckling position is a beneficial strategy for weaker, light neonates. These findings suggest that physically weaker neonates might have driven the evolution of neonatal foraging specialization.

With a little help from her friends (and family) part I: the ecology and evolution of non-maternal care in mammals

In the class Mammalia, most young are cared for exclusively by their mothers. In species where mothers receive help, however, non-maternal caregivers may play a crucial role in development and life history trajectories. In turn, recipients of such care may have important impacts on caregivers of all types. In Part I of this overview, we briefly review the evolutionary barriers to widespread non-maternal care in mammals, and explain why the exceptions are of particular theoretical importance. We also summarize the current understanding of the selective forces leading to non-maternal care, and the taxa and types of caretakers amongst which it occurs. Finally, we argue for a fresh look at the categorization schemes that have traditionally been used to separate various types of mammalian non-maternal caregivers. This two-part introduction is aimed at scientists from multiple disciplines who study diverse organismal systems. It draws from the social and biological sciences literatures to provide an overview of this special issue of Physiology and Behavior's suite of methodological offerings and theoretical underpinnings.

Increased developmental density decreases the magnitude of indirect genetic effects expressed during agonistic interactions in an insect

The expression of aggression depends not only on the direct genetic effects (DGEs) of an individual's genes on its own behavior, but also on indirect genetic effects (IGEs) caused by heritable phenotypes expressed by social partners. IGEs can affect the amount of heritable variance on which selection can act. Despite the important roles of IGEs in the evolutionary process, it remains largely unknown whether the strength of IGEs varies across life stages or competitive regimes. Based on manipulations of nymphal densities and > 3000 pair-wise aggression tests across multiple life stages, we experimentally demonstrate that IGEs on aggression are stronger in field crickets (Gryllus bimaculatus) that develop at lower densities than in those that develop at higher densities, and that these effects persist with age. The existence of density-dependent IGEs implies that social interactions strongly determine the plastic expression of aggression when competition for resources is relaxed. A more competitive (higher density) rearing environment may fail to provide crickets with sufficient resources to develop social cognition required for strong IGEs. The contribution of IGEs to evolutionary responses was greater at lower densities. Our study thereby demonstrates the importance of considering IGEs in density-dependent ecological and evolutionary processes.

Twelve fundamental life histories evolving through allocation-dependent fecundity and survival

An organism's life history is closely interlinked with its allocation of energy between growth and reproduction at different life stages. Theoretical models have established that diminishing returns from reproductive investment promote strategies with simultaneous investment into growth and reproduction (indeterminate growth) over strategies with distinct phases of growth and reproduction (determinate growth). We extend this traditional, binary classification by showing that allocation‐dependent fecundity and mortality rates allow for a large diversity of optimal allocation schedules. By analyzing a model of organisms that allocate energy between growth and reproduction, we find twelve types of optimal allocation schedules, differing qualitatively in how reproductive allocation increases with body mass. These twelve optimal allocation schedules include types with different combinations of continuous and discontinuous increase in reproduction allocation, in which phases of continuous increase can be decelerating or accelerating. We furthermore investigate how this variation influences growth curves and the expected maximum life span and body size. Our study thus reveals new links between eco‐physiological constraints and life‐history evolution and underscores how allocation‐dependent fitness components may underlie biological diversity.

The early-life environment of a pig shapes the phenotypes of its social partners in adulthood

Social interactions among individuals are abundant, both in natural and domestic populations, and may affect phenotypes of individuals. Recent research has demonstrated that the social effect of an individual on the phenotype of its social partners may have a genetic component, known as an indirect genetic effect (IGE). Little is known, however, of nongenetic factors underlying such social effects. Early-life environments often have large effects on phenotypes of the individuals themselves later in life. Offspring development in many mammalian species, for example, depends on interactions with the mother and siblings. In domestic pigs, individuals sharing the same juvenile environment develop similar body weight later in life. We, therefore, hypothesized that offspring originating from the same early-life environment also develop common social skills that generate early-life social effects (ELSEs) that affect the phenotypes of their social partners later in life. We, therefore, quantified IGEs and ELSEs on growth in domestic pigs. Results show that individuals from the same early-life environment express similar social effects on the growth of their social partners, and that such ELSEs shape the growth rate of social partners more than IGEs. Thus, the social skills that individuals develop in early life have a long-lasting impact on the phenotypes of social partners. Early-life and genetic social effects were independent of the corresponding direct effects of offspring on their own growth, indicating that individuals may enhance the growth of their social partners without a personal cost. Our findings also illustrate how research devoted to quantifying IGEs may miss nongenetic and potentially confounded social mechanisms which may bias the estimates of IGEs.

Pre-breeding food restriction promotes the optimization of parental investment in house mice, Mus musculus

Litter size is one of the most reliable state-dependent life-history traits that indicate parental investment in polytocous (litter-bearing) mammals. The tendency to optimize litter size typically increases with decreasing availability of resources during the period of parental investment. To determine whether this tactic is also influenced by resource limitations prior to reproduction, we examined the effect of experimental, pre-breeding food restriction on the optimization of parental investment in lactating mice. First, we investigated the optimization of litter size in 65 experimental and 72 control families (mothers and their dependent offspring). Further, we evaluated pre-weaning offspring mortality, and the relationships between maternal and offspring condition (body weight), as well as offspring mortality, in 24 experimental and 19 control families with litter reduction (the death of one or more offspring). Assuming that pre-breeding food restriction would signal unpredictable food availability, we hypothesized that the optimization of parental investment would be more effective in the experimental rather than in the control mice. In comparison to the controls, the experimental mice produced larger litters and had a more selective (size-dependent) offspring mortality and thus lower litter reduction (the proportion of offspring deaths). Selective litter reduction helped the experimental mothers to maintain their own optimum condition, thereby improving the condition and, indirectly, the survival of their remaining offspring. Hence, pre-breeding resource limitations may have facilitated the mice to optimize their inclusive fitness. On the other hand, in the control females, the absence of environmental cues indicating a risky environment led to “maternal optimism” (overemphasizing good conditions at the time of breeding), which resulted in the production of litters of super-optimal size and consequently higher reproductive costs during lactation, including higher offspring mortality. Our study therefore provides the first evidence that pre-breeding food restriction promotes the optimization of parental investment, including offspring number and developmental success.

Mgat5 modulates the effect of early life stress on adult behavior and physical health in mice

Psychosocial adversity in early life increases the likelihood of mental and physical illness, but the underlying mechanisms are poorly understood. Mgat5 is an N-acetylglucosaminyltransferase in the Golgi pathway that remodels the N-glycans of glycoproteins at the cell surface. Mice lacking Mgat5 display conditional phenotypes in behaviour, immunity, metabolism, aging and cancer susceptibility. Here we investigated potential gene-environment interactions between Mgat5 and early life adversity on behaviour and physiological measures of physical health. Mgat5(-/-) mutant and Mgat5(+/+) wild-type C57Bl/6 littermates were subject to maternal separation or foster rearing as an early life stressor, in comparison to control mice reared normally. We found an interaction between Mgat5 genotype and maternal rearing condition in which Mgat5(-/-) mice subjected to early life stress had lower glucose levels and higher bone density. Mgat5(-/-) genotype was also associated with less immobility in the forced swim test and greater sucrose consumption, consistent with a less depression-like phenotype. Cortical neuron dendrite spine density and branching was altered by Mgat5 deletion as well. In general, Mgat5 genotype affects both behaviour and physical outcomes in response to early life stress, suggesting some shared pathways for both in this model. These results provide a starting point for studying the mechanisms by which protein N-glycosylation mediates the effects of early life adversity.

Within-female plasticity in sex allocation is associated with a behavioural polyphenism in house wrens

Sex allocation theory assumes individual plasticity in maternal strategies, but few studies have investigated within-individual changes across environments. In house wrens, differences between nests in the degree of hatching synchrony of eggs represent a behavioural polyphenism in females, and its expression varies with seasonal changes in the environment. Between-nest differences in hatching asynchrony also create different environments for offspring, and sons are more strongly affected than daughters by sibling competition when hatching occurs asynchronously over several days. Here, we examined variation in hatching asynchrony and sex allocation, and its consequences for offspring fitness. The number and condition of fledglings declined seasonally, and the frequency of asynchronous hatching increased. In broods hatched asynchronously, sons, which are over-represented in the earlier-laid eggs, were in better condition than daughters, which are over-represented in the later-laid eggs. Nonetheless, asynchronous broods were more productive later within seasons. The proportion of sons in asynchronous broods increased seasonally, whereas there was a seasonal increase in the production of daughters by mothers hatching their eggs synchronously, which was characterized by within-female changes in offspring sex and not by sex-biased mortality. As adults, sons from asynchronous broods were in better condition and produced more broods of their own than males from synchronous broods, and both males and females from asynchronous broods had higher lifetime reproductive success than those from synchronous broods. In conclusion, hatching patterns are under maternal control, representing distinct strategies for allocating offspring within broods, and are associated with offspring sex ratios and differences in offspring reproductive success.

Reproductive mode and the shifting arenas of evolutionary conflict

In sexually reproducing organisms, the genetic interests of individuals are not perfectly aligned. Conflicts among family members are prevalent since interactions involve the transfer of limited resources between interdependent players. Intrafamilial conflict has traditionally been considered along three major axes: between the sexes, between parents and offspring, and between siblings. In these interactions, conflict is expected over traits in which the resulting phenotypic value is determined by multiple family members who have only partially overlapping fitness optima. We focus on four major categories of animal reproductive mode (broadcast spawning, egg laying, live bearing, and live bearing with matrotrophy) and identify the shared phenotypes or traits over which conflict is expected, and then review the empirical literature for evidence of their occurrence. Major transitions among reproductive mode, such as a shift from external to internal fertilization, an increase in egg-retention time, modifications of embryos and mothers for nutrient transfer, the evolution of postnatal parental care, and increased interaction with the kin network, mark key shifts that both change and expand the arenas in which conflict is played out.

Within-litter differences in personality and physiology relate to size differences among siblings in cavies

Many aspects of an animal's early life potentially contribute to long-term individual differences in physiology and behaviour. From several studies on birds and mammals it is known that the early family environment is one of the most prominent factors influencing early development. Most of these studies were conducted on highly altricial species. Here we asked whether in the highly precocial cavy (Cavia aperea) the size rank within a litter, i.e. whether an individual is born as the heaviest, the lightest or an intermediate sibling, affects personality traits directly after birth and after independence. Furthermore, we investigated whether individual states (early growth, baseline cortisol and resting metabolic rate) differ between siblings of different size ranks and assessed their relation to personality traits. Siblings of the same litter differed in personality traits as early as three days after birth. Pups born heaviest in the litter were more explorative and in general more risk-prone than their smaller siblings. Physiological state variables were tightly correlated with personality traits and also influenced by the size rank within litter, suggesting that the size relative to littermates constitutes an important factor in shaping an individual's developmental trajectory. Our data add valuable information on how personalities are shaped during early phases of life and indicate the stability of developmentally influenced behavioural and physiological traits.

Fluctuating food resources influence developmental plasticity in wild boar

To maximize long-term average reproductive success, individuals can diversify the phenotypes of offspring produced within a reproductive event by displaying the 'coin-flipping' tactic. Wild boar (Sus scrofa scrofa) females have been reported to adopt this tactic. However, whether the magnitude of developmental plasticity within a litter depends on stochasticity in food resources has not been yet investigated. From long-term monitoring, we found that juvenile females produced similar-sized fetuses within a litter independent of food availability. By contrast, adult females adjusted their relative allocation to littermates to the amount of food resources, by providing a similar allocation to all littermates in years of poor food resources but producing highly diversified offspring phenotypes within a litter in years of abundant food resources. By minimizing sibling rivalry, such a plastic reproductive tactic allows adult wild boar females to maximize the number of littermates for a given breeding event.

The evolutionary significance of placental interdigitation in mammalian reproduction: contributions from comparative studies

The placenta is fundamental to mammalian reproduction and is surprisingly diverse in gross morphology among species. Whether and how this diversity affects maternal investment and fetal growth is still poorly understood. Contrary to suggestions that highly invasive hemochorial placentation is beneficial to fetal development, recent comparative studies have revealed that interdigitation - the degree of contact between maternal and fetal tissues at the area of exchange - strongly influences fetal growth rates. Species with labyrinthine placentae give birth to neonates of similar size to those of species with villous or trabecular placentae but in less than half the time. These findings suggest that there might be tradeoffs between fetal growth rates (higher with greater interdigitation) and gestation time (shorter with greater interdigitation), in association with type of interdigitation. Such tradeoffs might be the results of maternal-offspring conflict over the allocation of maternal resources, with paternal genes favouring greater interdigitation and so higher fetal growth, and maternal genes responding by reducing gestation time. These results emphasize the role of interdigitation as a means to increase the surface area for exchange, and are consistent with within species studies demonstrating that a higher surface area for exchange is associated with heavier neonates. Further studies could investigate the role of other traits in the evolution of placental diversity and their impact on fetal development.

Genotype-dependent responses to levels of sibling competition over maternal resources in mice

Research on phenotypic plasticity has often focused on how a given genotype responds to the changing physical environments such as temperature or diet. However, for many species the social environment has an equally important role because of competition for resources. During early development, the level of competition for limited (maternally provided) resources will often depend critically on the number of siblings. Therefore, competition among siblings should drive the evolution of genes that allow flexible responses to realized levels of competition and maternal resource availability. However, it is unknown whether genetically based differences between individuals exist in their response to the social environment that affect their future development. Using a quantitative trait locus approach in an experimental population of mice we demonstrate that effects of sibling number on body weight depend on individual genotype at seven loci, over and above the general negative litter size effect. Overall, these litter size-by-genotype interactions considerably modified the degree to which increasing litter size caused reduced weight. For example at one locus this effect leads to a 7% difference in body weight at week 7 between individuals experiencing the extremes of the normal range of litter sizes in our population (five to nine litter mates). The observed interaction between genotype and the competitive environment can produce differences in body weight that are similar in magnitude to the main effect of litter size on weight. Our results show that different genotypes respond to the social environment differentially and that interaction effects of genotype with litter size can be as important as genotype-independent effects of litter size.

Litter size variation in hypothalamic gene expression determines adult metabolic phenotype in Brandt's voles (Lasiopodomys brandtii)

Background

Early postnatal environments may have long-term and potentially irreversible consequences on hypothalamic neurons involved in energy homeostasis. Litter size is an important life history trait and negatively correlated with milk intake in small mammals, and thus has been regarded as a naturally varying feature of the early developmental environment. Here we investigated the long-term effects of litter size on metabolic phenotype and hypothalamic neuropeptide mRNA expression involved in the regulation of energy homeostasis, using the offspring reared from large (10–12) and small (3–4) litter sizes, of Brandt's voles (Lasiopodomys brandtii), a rodent species from Inner Mongolia grassland in China.

Methodology/Principal Findings

Hypothalamic leptin signaling and neuropeptides were measured by Real-Time PCR. We showed that offspring reared from small litters were heavier at weaning and also in adulthood than offspring from large litters, accompanied by increased food intake during development. There were no significant differences in serum leptin levels or leptin receptor (OB-Rb) mRNA in the hypothalamus at weaning or in adulthood, however, hypothalamic suppressor of cytokine signaling 3 (SOCS3) mRNA in adulthood increased in small litters compared to that in large litters. As a result, the agouti-related peptide (AgRP) mRNA increased in the offspring from small litters.

Conclusions/Significance

These findings support our hypothesis that natural litter size has a permanent effect on offspring metabolic phenotype and hypothalamic neuropeptide expression, and suggest central leptin resistance and the resultant increase in AgRP expression may be a fundamental mechanism underlying hyperphagia and the increased risk of overweight in pups of small litters. Thus, we conclude that litter size may be an important and central determinant of metabolic fitness in adulthood.

Placentation and maternal investment in mammals

The mammalian placenta exhibits striking interspecific morphological variation, yet the implications of such diversity for reproductive strategies and fetal development remain obscure. More invasive hemochorial placentas, in which fetal tissues directly contact the maternal blood supply, are believed to facilitate nutrient transfer, resulting in higher fetal growth rates, and to be a state of relative fetal advantage in the evolution of maternal-offspring conflict. The extent of interdigitation between maternal and fetal tissues has received less attention than invasiveness but is also potentially important because it influences the surface area for exchange. We show that although increased placental invasiveness and interdigitation are both associated with shorter gestations, interdigitation is the key variable. Gestation times associated with highly interdigitated labyrinthine placentas are 44% of those associated with less interdigitated villous and trabecular placentas. There is, however, no relationship between placental traits and neonatal body and brain size. Hence, species with more interdigitated placentas produce neonates of similar body and brain size but in less than half the time. We suggest that the effects of placental interdigitation on growth rates and the way that these are traded off against gestation length may be promising avenues for understanding the evolutionary dynamics of parent-offspring conflict.

Ultrasonographic characterisation of prenatal development in European brown hares (Lepus europaeus PALLAS, 1778): an evolutionary approach

The European brown hare is one species in which reproduction appears to be particularly complex. Therefore, we studied the reproductive tract and prenatal development using high-resolution ultrasound of 159 pregnancies in 45 captive female hares. Consecutive examination of live hares was particularly useful in evaluating the very early stages of gestation. As such, it was possible to detect corpora lutea by Day 3 and embryonic vesicles, representing the earliest uterine embryonic stages, by Day 6. On Day 11, the heart beat of the embryo was detectable for the first time. We defined ultrasonographic milestones that were characteristic of the different stages of pregnancy. We also calculated growth models using eight different morphological parameters, including development of the corpus luteum. On the basis of these results, it is now possible to estimate the gestational age of a pregnant doe without knowing the date of conception. In contrast with the European rabbit, a distant relative, European hares give birth to precocial young. A comparison of the prenatal growth rate of both species suggests that the precocial state of the hare neonate is a more recently derived evolutionary characteristic, whereas the altricial condition of rabbits represents the ancestral mode.

Selection, inheritance, and the evolution of parent-offspring interactions

Very few studies have examined parent-offspring interactions from a quantitative genetic perspective. We used a cross-fostering design and measured genetic correlations and components of social selection arising from two parental and two offspring behaviors in the burying beetle Nicrophorus vespilloides. Genetic correlations were assessed by examining behavior of relatives independent of common social influences. We found positive genetic correlations between all pairs of behaviors, including between parent and offspring behaviors. Patterns of selection were assessed by standardized performance and selection gradients. Parental provisioning had positive effects on offspring performance and fitness, while remaining near the larvae without feeding them had negative effects. Begging had positive effects on offspring performance and fitness, while increased competition among siblings had negative effects. Coadaptations between parenting and offspring behavior appear to be maintained by genetic correlations and functional trade-offs; parents that feed their offspring more also spend more time in the area where they can forage for themselves. Families with high levels of begging have high levels of sibling competition. Integrating information from genetics and selection thus provides a general explanation for why variation persists in seemingly beneficial traits expressed in parent-offspring interactions and illustrates why it is important to measure functionally related suites of behaviors.

Evolution and comparative genomics of odorant- and pheromone-associated genes in rodents

Chemical cues influence a range of behavioral responses in rodents. The involvement of protein odorants and odorant receptors in mediating reproductive behavior, foraging, and predator avoidance suggests that their genes may have been subject to adaptive evolution. We have estimated the consequences of selection on rodent pheromones, their receptors, and olfactory receptors. These families were chosen on the basis of multiple gene duplications since the common ancestor of rat and mouse. For each family, codons were identified that are likely to have been subject to adaptive evolution. The majority of such sites are situated on the solvent-accessible surfaces of putative pheromones and the lumenal portions of their likely receptors. We predict that these contribute to physicochemical and functional diversity within pheromone-receptor interaction sites.