Flexibility in thermal requirements: a comparative analysis of the wide-spread lizard genus Sceloporus.

Adaptation or acclimation of thermal requirements to environmental conditions can reduce thermoregulation costs and increase fitness, especially in ectotherms, which rely heavily on environmental temperatures for thermoregulation. Insight into how thermal niches have shaped thermal requirements across evolutionary history may help predict the survival of species during climate change. The lizard genus Sceloporus has a widespread distribution and inhabits an ample variety of habitats. We evaluated the effects of geographical gradients (i.e. elevation and latitude) and local environmental temperatures on thermal requirements (i.e. preferred body temperature, active body temperature in the field, and critical thermal limits) of Sceloporus species using published and field-collected data and performing phylogenetic comparative analyses. To contrast macro- and micro-evolutional patterns, we also performed intra-specific analyses when sufficient reports existed for a species. We found that preferred body temperature increased with elevation, whereas body temperature in the field decreased with elevation and increased with local environmental temperatures. Critical thermal limits were not related to the geographic gradient or environmental temperatures. The apparent lack of relation of thermal requirements to geographic gradient may increase vulnerability to extinction due to climate change. However, local and temporal variations in thermal landscape determine thermoregulation opportunities and may not be well represented by geographic gradient and mean environmental temperatures. Results showed that Sceloporus lizards are excellent thermoregulators, have wide thermal tolerance ranges, and the preferred temperature was labile. Our results suggest that Sceloporus lizards can adjust to different thermal landscapes, highlighting opportunities for continuous survival in changing thermal environments.

Patterns and predictors of inter-litter differences in rabbit pup locomotor activity, based on an automatized quantification method.

Individual-level sibling interactions in the litter huddle have been studied extensively, especially in the domestic rabbit (Oryctolagus cuniculus). However, little is known about inter-litter differences in pup activity patterns during early postnatal life, in particular regarding the drivers of such variation. In our study on 2-3-day-old rabbit pups, we predicted lower locomotor activity in litters with lower mean body masses on the day of birth (starting body mass) and with lower daily milk intake per pup, possibly constituting a behavioral strategy of pups to cope with associated energetic constraints. For an automatized assessment of pup locomotor activity in the litter huddle, we successfully developed and validated a method based on the quantification of dissimilarities between consecutive frames of video footage. Using this method, we could confirm a U-shaped time course of litter-level locomotor activity, with maximum values shortly before and after the once-daily nursing typical for the rabbit. As predicted, between-litter variation in mean starting body mass and in daily milk intake affected the degree of locomotor activity in the litter huddle, in an interactive way. That is, in litters with heavier starting body masses, pup locomotor activity was greater in pups with an initially higher milk intake, suggesting that only pups with better body condition and a higher energy intake could afford higher levels of activity. This interaction was exclusively apparent during the middle phase of the 24 h inter-nursing interval, when litter activity was low. Shortly before nursing, when pups show higher levels of locomotor behavior in anticipation of the mother's arrival, and shortly after nursing when the pups were more active possibly due to adjustments of their positions in the huddle, activity levels were decoupled from pups' starting body mass and previous milk intake. Our findings highlight the importance of pup body mass and daily energy intake, two parameters known to be related to maternal characteristics, in shaping inter-litter differences in pup locomotor activity.

Avoiding the effects of translocation on the estimates of the metabolic rates across an elevational gradient.

Body maintenance costs are often considered a proxy for performance in fitness traits. Maintenance energy requirements are measured as minimal metabolic rate of inactive, postabsorptive individuals in the laboratory. For mountain-dwelling species, translocation to the laboratory often means that they are also moved to another elevation. Due to physiological adaptations to local oxygen pressure, rapid elevational change can alter metabolic rate and translocation may result in erroneous estimates of body maintenance costs. In this study, we measured resting metabolic rate (RMR) of three populations of the Mesquite lizard (Sceloporus grammicus, Wiegmann 1828) at their native elevations (i.e., 2600, 3200 and 4100 m). Our results showed that at native elevations, mass specific RMR of lizards from the high elevation population (4100 m) did not differ from the RMR of the other populations (i.e., 2600 and 3200 m), whereas the lizards from the low elevation (2600 m) had lower RMR than those from the intermediate population. These results differ from a previous study in which the RMR of lizards from the same populations were reported to increase with native elevation when translocated and measured at an intermediate elevation. Hence, our results show that translocation in elevation can affect metabolic measures. We caution researchers that changes in elevation may preclude accurate measures of RMR in some animals and may therefore incorrectly predict performance of fitness-related traits.

Resting metabolic rates increase with elevation in a mountain-dwelling lizard.

Individuals that inhabit broad elevational ranges may experience unique environmental challenges. Because temperature decreases with increased elevation, the ectotherms living at high elevations have to manage limited activity time and high thermoregulatory effort. The resting metabolic rate (RMR) of a postabsorptive animal is related to its total energy requirements as well as many other fitness traits. Mesquite lizards (Sceloporus grammicus) living on La Malinche Volcano, Mexico, inhabit a wide elevational range with some populations apparently thriving above the tree line. We measured the RMR of lizards from different elevations (i.e., 2,600, 3,200, and 4,100 m) at four ecologically relevant temperatures (i.e., 15, 25, 30, and 35 °C) and found that RMR of mesquite lizards increased with temperature and body mass. More importantly, lizards from the high-elevation population had mass specific RMR that was higher at all temperatures. While the higher RMRs of high-elevation populations imply higher metabolic costs at a given temperature these lizards were also smaller. Both of these traits may allow these high elevation populations to thrive in the face of the thermal challenges imposed by their environment.

Individual differences in early body mass affect thermogenic performance and sibling interactions in litter huddles of the house mouse.

We asked whether within-litter differences in early body mass are associated with differences in house mouse pups' thermogenic performance and whether such variation predicts individual differences in competitive interactions for thermally more advantageous positions in the huddle. We explored pups' thermogenic performance in isolation by measuring changes in (maximal) peripheral body temperatures during a 5-min thermal challenge using infrared thermography. Changes in peripheral body temperature were significantly explained by individual differences in body mass within a litter; relatively lighter individuals showed an overall quicker temperature decrease leading to lower body temperatures toward the end of the thermal challenge compared to heavier littermates. Within the litter huddle, relatively lighter pups with a lower thermogenic performance showed consistently more rooting and climbing behavior, apparently to reach the thermally advantageous center of the huddle. This suggests that within-litter variation in starting body mass affects the pups' thermal and behavioral responses to environmental challenges.

Trophic specialization and morphological divergence between two sympatric species in Lake Catemaco, Mexico.

The association of morphological divergence with ecological segregation among closely related species could be considered as a signal of divergent selection in ecological speciation processes. Environmental signals such as diet can trigger phenotypic evolution, making polymorphic species valuable systems for studying the evolution of trophic-related traits. The main goal of this study was to analyze the association between morphological differences in trophic-related traits and ecological divergence in two sympatric species, Astyanax aeneus and A. caballeroi, inhabiting Lake Catemaco, Mexico. The trophic differences of a total of 70 individuals (35 A. aeneus and 35 A. caballeroi) were examined using stable isotopes and gut content analysis; a subset of the sample was used to characterize six trophic and six ecomorphological variables. In our results, we recovered significant differences between both species in the values of stable isotopes, with higher values of δ15N for A. caballeroi than for A. aeneus. Gut content results were consistent with the stable isotope data, with a higher proportion of invertebrates in A. caballeroi (a consumption of invertebrates ten times higher than that of A. aeneus, which in turn consumed three times more vegetal material than A. caballeroi). Finally, we found significant relationship between ecomorphology and stable isotopes (r = .24, p < .01), hence, head length, preorbital length, eye diameter, and δ15N were all positively correlated; these characteristics correspond to A. caballeroi. While longer gut and gill rakers, deeper bodies, and vegetal material consumption were positively correlated and corresponded to A. aeneus. Our results are consistent with the hypothesis that morphological divergence in trophic-related traits could be associated with niche partitioning, allowing the coexistence of closely related species and reducing interspecific competition.

Functional modularity in lake-dwelling characin fishes of Mexico.

Modular evolution promotes evolutionary change, allowing independent variation across morphological units. Recent studies have shown that under contrasting ecological pressures, patterns of modularity could be related to divergent evolution. The main goal of the present study was to evaluate the presence of modular evolution in two sister lacustrine species, Astyanax aeneus and A. caballeroi, which are differentiated by their trophic habits. Two different datasets were analyzed: (1) skull X-rays from 73 specimens (35 A. aeneus and 38 A. caballeroi) to characterize skull variation patterns, considering both species and sex effects. For this dataset, three different modularity hypotheses were tested, previously supported in other lacustrine divergent species; (2) a complete body shape dataset was also tested for four modularity hypotheses, which included a total of 196 individuals (110 Astyanax aeneus and 86 A. caballeroi). Skull shape showed significant differences among species and sex (P < 0.001), where Astyanax caballeroi species showed an upwardly projected mandible and larger preorbital region. For the skull dataset, the modularity hypothesis ranked first included three partitioning modules. While for the complete body dataset the best ranked hypothesis included two modules (head vs the rest of the body), being significant only for A. caballeroi.

Body mass modulates huddling dynamics and body temperature profiles in rabbit pups.

Altricial mammals typically lack the physiological capacity to thermoregulate independently during the early postnatal period, and in litter-bearing species the young benefit strongly from huddling together with their litter siblings. Such litter huddles are highly dynamic systems, often characterized by competition for energetically favorable, central positions. In the present study, carried out in domestic rabbits Oryctolagus cuniculus, we asked whether individual differences in body mass affect changes in body temperature during changes in the position within the huddle. We predicted that pups with relatively lower body mass should be more affected by such changes arising from huddle dynamics in comparison to heavier ones. Changes in pups' maximum body surface temperature (determined by infrared thermography) were significantly affected by changes in the number of their neighbors in the litter huddle, and indeed these temperature changes largely depended on the pups' body mass relative to their litter siblings. Lighter pups showed significant increases in their maximum body surface temperature when their number of huddling partners increased by one or two siblings whereas pups with intermediate or heavier body mass did not show such significant increases in maximum body temperature when experiencing such changes. A similar pattern was found with respect to average body surface temperature. This strong link between changes in the number of huddling partners and body surface temperature in lighter pups might, on the one hand, arise from a higher vulnerability of such pups due to their less favorable body surface area-to-volume ratio. On the other hand, as lighter pups generally had fewer neighbors than heavier ones and thus typically a comparatively smaller body surface in contact with siblings, they potentially had more to gain from increasing their number of neighbors. The present findings might help to understand how individual differences in body mass within a litter lead to the emergence of individual differences in sibling interactions during early postnatal life in different species of altricial and litter-bearing mammals.

Early development and the emergence of individual differences in behavior among littermates of wild rabbit pups.

The ontogeny of associated individual differences in behavior and physiology during early postnatal life, and in particular the emergence of such differences among litter siblings, has been hardly explored in mammals under natural conditions. We studied such within-litter differences in behavior in European rabbit pups Oryctolagus cuniculus prior to weaning, and whether and how these differences co-varied with other individual characteristics such as postnatal body temperature and early growth. The study was conducted under semi-natural conditions in a colony of rabbits of wild origin, where the young were born and developed in nursery burrows. We equipped two siblings per litter with interscapular skin temperature loggers on postnatal day 2 and recorded temperature profiles for 48h. Individual body (skin) temperatures of pups within litters were repeatable across time, indicating the existence of consistent individual differences. Such differences within litters were associated with relative differences in pre-weaning growth, revealing that relatively warmer pups showed a greater increase in body mass during the nest period. Between postnatal days 12 and 17, after the pups had reached a developmental stage of greater mobility, we carried out different behavioral tests: a handling-restraint test, an open field test and a jump-down test from a platform. Individual responses in the former two tests were associated, as those pups showing a quicker struggling response to restraint during handling also exhibited greater exploratory activity in the open field. This correlation across contexts suggests the existence of personality types in wild rabbits at an early developmental stage. Furthermore, pups' behavioral responses were strongly associated with their relative within-litter body mass at testing. Animals with a lower body mass compared to their siblings showed a relatively quicker struggle response to handling restraint and covered a relatively larger distance in the open field, suggesting greater reactivity and responsiveness of relatively lighter pups in these tests. In contrast, relatively heavier pups jumped sooner from the platform, which may have been due to their greater physical maturation. In conclusion, our study shows that individual differences in behavior and associated differences in body temperature and growth are already present during early postnatal life, although such relationships can be easily overlooked, as they predominantly emerge as relative differences among littermates.

Yawning reduces facial temperature in the high-yawning subline of Sprague-Dawley rats.

BACKGROUND: Yawning is a stereotyped behavior that enhances blood flow to the skull, and the resulting counterflow has been hypothesized as a mechanism for brain cooling. Studies have shown that yawns are strongly associated with physiological and pathological conditions that increase brain temperature, and that they are followed by equivalent decreases in brain temperature. However, measured reductions in cranial or facial temperatures following yawning have yet to be reported, to our knowledge. To accomplish this, we used a subline of Sprague-Dawley rats that yawn at a much greater rate (20 yawns/h) than do outbred Sprague-Dawley rats (2 yawns/h). RESULTS: Using an infrared camera, we effectively evaluated thermal changes in the cornea and concha of these rats before, during, and after yawns. The maximum temperature in both regions significantly decreased 10 s following yawns (concha: -0.3 °C, cornea: -0.4 °C), with a return to basal temperatures after 20 s. CONCLUSIONS: This study is the first clear demonstration of yawning-induced thermal cooling on the surface of the face, providing convergent evidence that this behavior plays a functional role in thermoregulation. As other studies have demonstrated that yawning is capable of reducing cortical brain temperature, our current data support the idea that yawning functions as a thermoregulator, affecting all structures within the head.

Differential metabolism of brown adipose tissue in newborn rabbits in relation to position in the litter huddle.

Competition for resources can contribute importantly to the early development of individual differences in behavioral and physiological phenotypes. In newborn rabbits, littermates compete for thermally favorable positions within the litter huddle. As brown adipose tissue (BAT) is the principal site of thermogenesis in such altricial young, we investigated differences in rabbit pups' growth and morphological differences in BAT associated with position within the huddle. We formed three treatment groups (7 litters/group): GI-birth (pups killed at birth); GII-chronic thermal challenge (pups killed after exposure to a moderately cold environmental during postnatal days 1-3); GIII-acute thermal challenge (as for GII but pups killed after an additional 30min exposure to a very cold environment on postnatal day 3). Interscapular BAT was removed at death for histological analysis, and triglyceride concentrations measured in serum. Pups occupying central positions in the huddle had higher skin temperatures, obtained more milk, and were more efficient at converting this into body mass, than pups occupying peripheral positions. There was no significant difference in BAT morphology or triglyceride concentrations between pups at birth, nor between central and peripheral pups chronically exposed to moderate cold until postnatal day 3. However, during acute cold exposure at this age, peripheral pups were less able to maintain their body temperature, they depleted BAT fat reserves almost completely, and they had lower serum concentrations of triglycerides than central pups. These findings confirm the contribution of early sibling relations to individual differences in growth and metabolic processes associated with thermoregulation in newborn rabbits.

Intrauterine position as a predictor of postnatal growth and survival in the rabbit.

In mammals, body mass at birth is an important predictor of early postnatal growth and survival. Within litters, heavier young are more successful in competing for limited resources and show higher rates of growth and survival than their lighter sibs. In the present study, we investigated the contribution of two aspects of the intrauterine environment to within-litter differences in birth mass, growth and survival in the rabbit (Oryctolagus cuniculus): implantation site along the uterine horns and number of adjacent male fetuses. We used unilaterally ovariectomized mothers in order to infer relative sites of implantation from the birth order of pups from the single functional uterine horn. Pups from the extremities of the horn were significantly heavier at birth and weaning than their siblings from more central positions and had a higher probability of survival. The effect on body mass was still apparent 3 weeks after weaning in pups that had occupied positions at the ovarian end of the horn. The number of adjacent male fetuses did not affect individuals' growth or survival, and there were no differences between females and males. There were also no significant interactions between the different variables considered, indicating that the effects of implantation site on individuals' birth mass, growth and survival relative to littermates were independent of number of male neighbors, sex or litter size. Our study clearly demonstrates that in the rabbit, the site of implantation along the uterine horns is a major contributor to individual differences among littermates in early postnatal growth and survival.

Competition in newborn rabbits for thermally advantageous positions in the litter huddle is associated with individual differences in brown fat metabolism.

The altricial young of the European rabbit (Oryctolagus cuniculus) are not brooded by the mother, and although they are born into an underground nest, depend importantly on the warmth and insulation provided by littermates for their early growth and survival. Consistent with previous studies, heavier pups occupied more central, thermally advantageous positions in the litter huddle, maintained higher body temperatures, obtained more milk, were more efficient at converting it to body mass, and consequently grew faster than their lighter sibs occupying the periphery of the huddle. In the present study we measured the expression of uncoupling protein 1 (UCP-1), which is essential for the metabolism of brown adipose tissue to generate body heat in response to cold. In nine litters of domestic rabbits maintained for the first four postnatal days at temperatures below their critical thermoneutral temperature, peripheral pups showed greater expression of UCP-1 than intermediate pups, and these greater expression than central pups. This suggests that during early development littermates of the rabbit experience differing degrees of activation of the sympathetic nervous system as a consequence of exposure to different thermal environments associated with different positions in the litter huddle. Whether this is associated with long term differences in the physiological response to cold and perhaps in the manner of responding to other environmental challenges is currently under investigation.

Possible contribution of position in the litter huddle to long-term differences in behavioral style in the domestic rabbit.

Many aspects of an animal's early development might potentially contribute to long-term individual differences in physiology and behavior. Here we asked whether differences among littermates of the domestic rabbit in the position in the litter huddle that they occupy during the early nest period might contribute to the development of distinct behavioral and physiological phenotypes. In each of 12 litters we determined the pup occupying the most peripheral, the most central, and an intermediate position in the huddle during the first postnatal week. We then tested the responses of these same individuals as nestlings, juveniles and young adults when confronted by a range of age-appropriate environmental challenges. Two behavioral tests appeared particularly discriminatory in identifying differences associated with early position in the huddle; latency of pre-weaning pups to jump down from a shelf, and the response of young adults to the fearful screams of a conspecific. In both cases animals that had occupied the periphery of the huddle showed behavioral responses indicative of a more proactive behavioral style than their "intermediate" or "central" littermates. We conclude that while consistent long-term differences in behavioral style associated with early position in the litter huddle exist in rabbits, future work is needed to confirm the causal nature of this association, to identify underlying mechanisms, and to refine methods of behavioral and physiological testing across the life span.

The effect of siblings on early development: a potential contributor to personality differences in mammals.

Although most mammals grow up in the company of same or different age sibs (or half sibs), surprisingly little attention has been given to how relations among them might influence the development of individual differences in morphology, physiology, and behavior. Here we review evidence from our work on domestic and wild European rabbits, and more recently on laboratory rats, mice, and domestic cats, supporting the proposition that in mammals early sibling relations contribute to the development of individual differences in these three domains and thereby to long-term behavioral differences of the kind we might consider part of an animal's behavioral style or personality. First we report a consistent and marked negative relation between litter size and individuals' body mass at birth and weaning, as well as marked within-litter differences in prenatal body mass and placental efficiency. We then report individual differences in preweaning behaviors associated with these morphological variables such as position occupied in the litter huddle and development of motor ability, as well as physiological differences in thermoregulation, immune parameters, and endocrine indicators of stress. Finally, we report first evidence from wild rabbits that early relations among littermates may have long-term consequences for individual differences in behavioral style. We conclude that in mammals, individual differences in early growth, physiology and behavior potentially important for the development of animal personality, are shaped to an appreciable extent by early sibling relations and that this little-researched field deserves closer attention.

Littermate presence enhances motor development, weight gain and competitive ability in newborn and juvenile domestic rabbits.

Interest has been growing in the influence siblings may have on individual development. While mammalian research has tended to emphasize competition among siblings for essential but often limited resources such as the mother's milk, there is also evidence of mutual benefits to be had from sibling presence, most notably for altricial young in enhanced thermoregulatory efficiency. In the present study we asked whether littermates of an altricial mammal, the domestic rabbit, might gain other developmental benefits from sibling presence. From postnatal days 1 to 25 we raised rabbit pups either together with their littermates or alone except for the brief, once daily nursing characteristic of this species, while controlling for litter size and ambient nest box temperature. At weaning on Day 25 the young were then transferred to individual cages. Before weaning, we found that pups raised separately from their littermates obtained less milk, and showed lower weight gain and slower development of the ability to maintain body equilibrium than their litter-raised sibs. This was the case even though the two groups did not differ in birth weight or in the ratio of converting milk into body mass in their temperature-controlled nest boxes. Postweaning, the isolation-raised animals were also less successful in competing for food and water when tested after deprivation than their litter-raised sibs. The present study adds to the growing evidence of the influence, in this case positive, that sibs (or half sibs) may have in shaping one another's development.

Development of behavior in the litter huddle in rat pups: within- and between-litter differences.

Early postnatal growth in mammals can be considerably influenced by litter size and often differs among littermates in relation to birth mass. In a study of Long Evans laboratory rats we asked whether within- and between-litter differences in body mass and growth are related to behavioral development during early postnatal life. For this, we analyzed the amount of general motor activity and the display of directed, seemingly goal-oriented interactions within the litter huddle in previsual pups. During the study period from postnatal days 2 to 11, we found significant changes in pup behavior, showing a nonlinear, quadratic shape. General motor activity and, more specifically, the display of behaviors apparently directed to reaching central positions in the litter huddle increased during the first postnatal days and then decreased again. However, pups from small litters that grow more rapidly than pups from large litters, showed a faster increase in both behaviors, whereas the young from large litters reached a higher maximum. We also found striking within-litter differences in the amount of directed behavior performed by light and heavy pups, with higher levels in the former group, most probably because light pups that have a less favorable body mass-to-volume ratio and more often occupy peripheral positions in the litter huddle, make a greater effort to reach thermally favorable central positions. In conclusion, our study shows there to be consistent between-litter as well as within-litter differences in behavioral patterns during early life. These differences might have important implications for an individual's long-term behavioral and physiological performance.

Nipple preference and contests in suckling kittens of the domestic cat are unrelated to presumed nipple quality.

We studied the development of suckling behavior and weight gain in 11 litters (52 kittens) of free-ranging domestic cats until postnatal day 28 just before the start of weaning. In six of these litters, we also recorded milk intake and contests for access to nipples. Already within 12 hr of birth kittens showed a preference for posterior nipples, and by postnatal day 3 each had developed a preference for particular nipples. In fact, 86% of kittens used one particular nipple most often, and even when the mother changed the side she lay on to nurse. Contests for access to nipples occurred throughout the study period at an average rate of one to two contests per kitten per hour of nursing. Contrary to suggestions in the literature that kittens compete for more productive nipples, we found no relation between kittens' use of particular nipples and their weight gain, milk intake, or involvement in contests during suckling. We suggest that kittens' preference for posterior nipples as well as their establishment of an individual "teat order" might function to optimize the number of nipples remaining productive across lactation, and to reduce energetically costly scrambles and potentially injurious contests among littermates.

Differential development of body equilibrium among littermates in the newborn rabbit.

Interest is growing among psychobiologists and behavioral ecologists in the role of sibling relations in shaping individual development and life histories. In litters of domestic rabbits Oryctolagus cuniculus the heaviest pups at birth are more likely to survive the critical first postnatal week, they compete more effectively with littermates for milk and well-insulated positions in the litter huddle, and are the heaviest at weaning. Here we report that high birth weight pups are also better able to maintain body equilibrium. Testing pups' ability to maintain equilibrium when placed on a 15 degrees ramp for 2 min each day during the first postnatal week, we found that pups showed a continual daily improvement in their ability to maintain balance while moving on the ramp, rarely lost balance by postnatal day 8, and that heavier pups could maintain balance better and earlier than their lighter littermates. Better ability to maintain body equilibrium, however achieved, may help explain heavier pups' advantage in competing for vital resources such as milk and in gaining access to better-insulated positions in the litter huddle. It also provides further support for the usefulness of birth weight, not only as an absolute measure but also relative to the weight of other littermates, as a predictor of different developmental trajectories, behavioral and physiological, among same-age siblings in this mammal.

Why do heavy littermates grow better than lighter ones? A study in wild and domestic European rabbits.

Birth mass can vary considerably among mammalian littermates. Heavier pups often show higher growth rates than their lighter siblings, which might positively affect fitness-relevant parameters during later life. Such a correlation between birth mass and pre-weaning growth within litters was confirmed by our study of wild-type and domestic European rabbits (Oryctolagus cuniculus) living in a semi-natural environment and under laboratory housing conditions, respectively. Our study indicates that at least two main mechanisms account for this relationship in our study species: heavier pups had a higher milk intake and also showed a more efficient conversion of milk into body mass. Furthermore, our study suggests that the better milk conversion by heavy pups was driven by three synergistic mechanisms: heavier pups had comparatively more huddling partners in the nest, they did not need to perform large amounts of proactive behavior in order to reach and remain in a central position within the litter huddle, and they could maintain a comparatively higher body temperature most probably due to their more favorable surface area to volume ratio. In conclusion, our study of European rabbits provides strong evidence that both under natural conditions and in the laboratory, within-litter differences in birth mass are maintained and may even increase during pups' early postnatal development.