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.     

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. © 2009 Wiley Periodicals, Inc. Dev Psychobiol 52: 35–43, 2010     

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.     

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. © 2010 Wiley Periodicals, Inc. Dev Psychobiol 53: 37–46, 2011.     

Preobesity in the infant OLETF rat: the role of suckling

The Otsuka Long Evans Tokushima Fatty (OLETF) rat model of obesity, which lacks CCK(1) receptors, has been extensively investigated over the last decade. We have recently focused on the early developmental stages of this strain, finding that OLETF pups are heavier than controls from birth and hyperphagic already from postnatal (PN) Day 2. OLETF mothers present differential maternal behavior patterns and increased nursing time and frequency, which might contribute to the preobese characteristics of the pups. The present investigation examined the pups' ability to gain weight from the nursing episodes. First, we measured the pups' weight gain from one nursing bout from their own dam. Next we examined the pups' weight gain after a feeding competition test with control pups from foster dams of both strains. OLETF pups gained more weight than controls from their own dams on PN Week 1 due to a higher suckling rate (and/or efficacy) and on PN Week 3 due to increased nursing time. When competing with control pups, OLETF pups gained significantly more weight after the same nursing bout, regardless of the strain of the mother. Body mass index (BMI) was significantly higher in OLETF pups compared to controls. The maternal parameters assessed from the experiment were latencies to pup retrieval and to nursing, and nursing duration; differences were only observed in nursing time. OLETF dams increased their average nursing time over the PN weeks, while control dams decreased their nursing time toward weaning. The results suggest an important contribution of OLETF pups toward their own preobese development.     

Competition and cooperation among huddling infant rats

Huddling is expressed by infant rats and continues to be an important behavior throughout adulthood. As a form of behavioral thermoregulation, huddling is thought to play an essential role in compensating for inadequate physiological thermoregulation early in development. Infant rats, however, are capable of heat production shortly after birth using brown adipose tissue (BAT) and exhibit thermogenesis in the huddle, suggesting that huddling does not obviate the need for endothermy during cold exposure. In the present experiment, 4-pup huddles of infant rats (2- or 8-day-olds) were exposed to two subthermoneutral temperatures, and BAT thermogenesis was inhibited in 0, 2, or 4 of the rats in each huddle. Inhibition of BAT thermogenesis compromised the pups' ability to maintain huddle temperature, but surprisingly did not result in enhanced huddling at either age. These results suggest that effective huddling during cold exposure requires the thermal resources provided by endothermy. Furthermore, the heat provided by BAT appears to shape behavioral interactions in the huddle during development.     

Scent marking, dominance and serum testosterone levels in male domestic rabbits

The European rabbit, both in its wild and domesticated forms, has been a pioneer species in the study of mammalian chemical communication, and illustrates well the difficulty of understanding the functional significance of these often complex signals. Here we investigate the performance of one of the rabbit's most conspicuous chemical signaling behaviors, chin marking (chinning), and the hypothesis that this expresses social dominance. In tests of 21 chinchilla-strain sexually mature males we predicted 1) that animals would show marked and stable individual differences in the frequency of chinning, 2) that these differences would correlate with behaviors associated with dominance such as intrasexual mounting, and 3) that individual differences in the frequency of chinning and dominance-related behaviors would correlate with individual differences in a commonly used physiological indicator of dominance, concentration of serum testosterone. Supporting these predictions and consistent with previous reports, animals showed large and stable individual differences in the frequency of chinning which correlated with the behavioral indicators of dominance and less strongly, with serum testosterone. As our animals had been kept in single cages and without direct contact with other males since weaning, these findings raise the question as to how and when during development such differences among individuals arise. We are currently investigating the possible relation between pups' intrauterine position, postnatal competition among littermates for milk and thermally advantageous positions in the litter huddle, and later differences in indicators of dominance such as those reported here.     

The effects of litter size during gestation and lactation on rat development prior to weaning

Physiological and behavioral development of rats was affected by prenatal nutrition and postnatal litter size. Prenatal nutrition was manipulated by combining differences in maternal nutrition with variation of prenatal litter size produced by pre-mating isolation of 1 uterine horn. The pups were reared in postnatal litters of either 4 or 12. During the 1st postnatal week, development and free behavior were affected only by the prenatal treatments. During the 2nd postnatal week, both prenatal and postnatal effects were observed. Development and behavior during the 3rd postnatal week reflected only postnatal litter size. Brain weight at weaning was influenced by both the prenatal and postnatal treatments.     

Investigation of body temperature as a possible feeding control in the suckling rat

Under natural conditions, rat pups are deprived of milk only when the dam leaves the nest. At this time the body temperatures of the pups will fall. In this study we have investigated the possibility that lowered body temperature of the pups stimulates behavioral changes in either the pups or the dam, leading to increased milk intake. To study behavioral changes of the pups, the milk intake of warm-fasted, cool-fasted, and fed littermates was compared to 10, 14 and 16 days of age. There was no effect of body temperature on milk intake at any age. To study behavioral changes in the dam, milk release by dams suckling entire cool litters was compared with that by dams suckling entire warm litters. Milk release was not significantly affected by the litter temperature. We conclude that lowered body temperature of suckling rat pups does not cause increased milk intake.     

Effects of diet and handling on initiation of independent ingestion in rats

The present study examined the effects of dietary manipulation on the age of onset of weaning in rat pups. In Experiment 1, female rats were placed on a standard chow (SC) or high-fat (HF) diet 1 week following mating. Pups were weighed daily from birth to Day 12, then animals were placed into specialized cages for separate recording of food intake of pups and dams. Pups were offered the same diet as their dam, and food intake and body weight were determined twice daily until Day 25. The results demonstrated that pups reared by dams fed the HF diet initiated independent ingestion on Day 16, approximately 24 hr before pups reared by dams fed the SC diet. There were no differences in body weight in pups across the two diets. While few differences were noted across diets in pups' or dams' behavior, HF pups appeared to demonstrate a delay in the establishment of circadian patterns of food intake. In Experiment 2, all dams were maintained on an SC diet until the day after parturition. At that time, dams and litters were placed into specialized cages and divided into four groups: HF/HF, HF/SC, SC/SC, and SC/HF (dam's diet/pup's diet, respectively). The results demonstrated that dams given the HF diet had pups that initiated food intake approximately 2 days before the pups of dams given the SC diet. In addition, pups offered the HF diet, independent of the dam's diet, initiated food intake approximately 0.8 days prior to pups offered the SC diet. Further, by Day 12, HF dams had pups that were heavier than SC dams. The results suggest that the onset of weaning in rats is affected by maternal diet and the weaning diet available to the pup.     

Thermal preference behavior in preweaning genetically obese (ob/ob) and lean (+/?, +/+) mice

Impaired nonshivering thermogenesis and lowered rectal temperatures (Tre) are hallmarks that appear early in the postnatal ontogeny of the genetically obese (ob/ob) mouse. Adult obese mice compensate behaviorally for these impairments and do not defend their low Tres. We predicted that, because young mice primarily rely on behavior to ensure thermal homeostasis during preweaning development, the appearance of the obese mouse's thermoregulatory impairment should promote their continued reliance on behavioral thermoregulation compared to lean pups. Accordingly, intact litters of pups from heterozygous lean (C57BL/6J, ob/+) and from homozygous lean (+/+) matings were tested at 6, 12, and 18 days postpartum on a thermal gradient (14-44 degrees C). Obese pups had lower pretest Tres than lean (+/?) littermates at 6 days and lower pretest Tres than both lean littermates and homozygous (+/+) lean control pups at 12 and 18 days. Exposure to the gradient ameliorated these differences (i.e., no posttest Tre differences among phenotypes). Correspondingly, obese pups preferred warmer gradient locations than +/+ pups but similar locations to their phenotypically lean (+/?) littermates until 18 days, when both lean groups preferred similar thermal locations compared to warmer-seeking obese pups. These data support our hypothesis and emphasize the age-dependent impact of the ob gene on altering mouse pups' thermal preferences.     

Maternal licking by virgin and lactating rats: water transfer from pups

Rat dams provide water to their young via milk. Dams reclaim much of this water by licking the pups' anogenital areas, stimulating reflexive urination and consuming the pups' urine. Sensitized virgin rats, induced to act maternally do not provide water to pups, but they nevertheless lick them. To determine whether bidirectional transfer of water between the rat mother and her litter mediates maternal licking, water transfer from pups to sensitized virgins was compared with that to lactating dams. We used time-lapse video recordings to measure anogenital licking of pups. Sensitized virgins and lactating dams spent equivalent amounts of time licking the anogenital regions of test litters. We quantified the amount of water transferred from offspring to both virgins and dams by injecting pups with tritiated water and measuring the radioactive label in maternal plasma after interaction with a litter of 5-day-olds. Dams obtained more than twice as much urine from the litter in 4 hr than did the maternal virgins. Differences in the amount of water obtained from pups were due to differences in urine availability caused by the receipt of milk from the dams. When the dams' nipples were ligated, so that their pups received no milk, ligated dams and virgins consumed equivalent amounts of pup urine. Maternal licking and urine consumption are not dependent solely upon the bidirectional exchange of water between the dam and her offspring.     

Isolation distress in two-week-old rats: influence of home cage, social companions, and prior experience with littermates

Behavioral and autonomic manifestations of acute isolation distress were studied in 2-week-old rats under novel and home-cage conditions. Levels of ultrasonic vocalization, heart rate, self-grooming, and rises were higher in novel surroundings, while locomotion and digging were more frequent in home cages. Next, we found that a group of 8 unanesthetized agemate pups was just as effective as an anesthetized dam in reducing (by 80-90%) all behaviors of isolated pups in novel surroundings. Finally, singletons reared without littermates during postnatal days 7-14 showed normal isolation responses, and these were as effectively alleviated by littermates as those of normally reared pups. Thus, isolation distress in rats this age is a complex response, modulated over time by aspects of the pups' environment and virtually prevented by the presence of cues common to both mother and littermates.     

Maternal stress and high-fat diet effect on maternal behavior, milk composition, and pup ingestive behavior

Chronic variable prenatal stress or maternal high-fat diet results in offspring that are significantly heavier by the end of the first postnatal week with increased adiposity by weaning. It is unclear, however, what role maternal care and diet play in the ontogenesis of this phenotype and what contributions come from differences already established in the rat pups. In the present studies, we examined maternal behavior and milk composition as well as offspring ingestive behavior. Our aim was to better understand the development of the obese phenotype in offspring from dams subjected to prenatal stress and/or fed a high-fat (HF) diet during gestation and lactation. We found that dams maintained on a HF diet through gestation and lactation spent significantly more time nursing their pups during the first postnatal week. In addition, offspring of prenatal stress dams consumed more milk at postnatal day (PND) 3 and offspring of HF dams consume more milk on PND 7 in an independent ingestion test. Milk from HF dams showed a significant increase in fat content from PND 10-21. Together these results suggest that gestational dietary or stress manipulations can alter the rat offspring's developmental environment, evidence of which is apparent by PND 3. Alterations in maternal care, milk composition, and pup consumption during the early postnatal period may contribute to long-term changes in body weight and adiposity induced by maternal prenatal stress or high-fat diet.     

Olfactory influences on the ingestive behavior of infant rats

Twenty-four-hour deprived infant rats from 1 to 12 days of age were exposed to the odor of milk, either by itself or accompanied by oral infusions of water, sucrose, quinine, or milk (Experiment 1). Pups from 3 to 9 days of age became behaviorally active, and mouthed and probed in response to the odor of milk, even in the absence of oral infusions. Six- and 9-day-old pups also increased their intake of various test solutions in the presence of milk odor. This responsiveness to milk odor was not seen in 1-day-old pups, and waned by 12 days of age. Other stimulus odors that were presented in the same context (Experiment 2) failed to elicit such intense behavioral activation, indicating that there was some degree of specificity in the responsiveness to milk odor. Finally, 2 manipulations that influence pups' ingestion of milk, deprivation and ambient temperature, were found to influence pups' responses to milk odor (Experiment 3).     

Maternal behavior variations and adaptations, and pup development within litters of various sizes in Wistar rat

Variations of two maternal behavior components (time spent with litter and rapidity of pup retrieving) as well as certain physical and developmental characteristics of pups (weight, relative weight gain, and neuromotor maturation) in rats were simultaneously studied in 29 various-sized litters in which interindividual variations were not experimentally amplified. Results showed mothers' behavioral adaptations to litters' characteristics (size and weight). Time spent with young was linked to litter size, whereas rapidity of pup retrieving was related to the pups' physical characteristic. Beyond these adaptive variabilities, residual variations subsisted between mothers. These variations were determinant for differences in pup development for only one component: the time that mothers spent with their litters, while pup retrieving component variations did not have any effect.     

A method for artificial feeding of motherless 2-week-old rat pups

When the rat mother is permanently removed from her litter at 14-15 days postnatally, weight loss in the pups routinely occurs. We now describe a method by which normal weight gain can be achieved at this age in the absence of the mother. Liquid flowing in a continuous stream down a glass surface in the home cage elicits spontaneous licking from the rat pups. If the liquid is a high fat milk formula, the rat pups maintain normal body weight. They do not maintain normal weight if low fat (bovine) milk is presented in this way or if the high fat milk formula is merely available in a reservoir on the cage floor.     

Behavioral interactions rather than milk availability determine decline in milk intake of weanling rats

We examined the relation between milk availability and milk intake during the period in which rat pups gradually abandon milk as a food source. The amount of milk produced by rat dams does not change from postpartum Day 15 to Day 20, but decreases thereafter and completely disappears around Day 30. In contrast, the amount of milk actually obtained by pups does begin to decline between Days 15 and 20. This decline in milk intake can be attenuated by integrating 20-day-old pups into 15-day-old litters. We concluded that pups do not begin to ingest less milk because of diminishing milk supplies. Rather, the decreased tendency of mothers to nurse older pups and the diminished tendency of older pups to extract available milk, together appear to underlie the decline in pups' milk consumption. Milk supplies decline after changes in behavioral interactions and may play an instrumental role in the eventual abandonment of suckling.     

Rat behavioral thermoregulation integrates with nonshivering thermogenesis during postnatal development

Rat pups are capable of behavioral thermoregulation, both in the nest and on a thermocline, as early as the 1st week of postnatal life, and these pups can also produce heat metabolically without shivering. The rat pup's primary source of nonshivering thermogenesis is the sympathetically mediated metabolism of brown adipose tissue (BAT). BAT is well formed in newborns and functions shortly after birth. While infant behavioral thermoregulation and BAT thermogenesis have been extensively studied, little is known about the extent to which thermoregulatory behavior can be influenced by BAT thermogenesis. In the present study, 2-, 7-, and 14-day-old pups were observed on a thermal gradient following pharmacological stimulation or inhibition of BAT thermogenesis, and their thermal preferences were quantified. The authors found that 7- and 14-day-old pups treated with norepinephrine (NE), which increases BAT thermogenesis, preferred cooler portions of the gradient than saline-treated controls, whereas 2-day-olds failed to show a similar NE-induced behavioral adjustment. These findings indicate that the ability to adjust thermoregulatory behavior to compensate for enhanced metabolic thermogenesis develops during the 1st week of postnatal life.     

Huddling by rat pups: ontogeny of individual and group behavior

A full account of behavioral development in rats must include the ontogeny of both individual and group behavior. Most of our accumulated knowledge, however, pertains to individual ontogenesis. Group behavior and its development are readily seen in the huddling behavior of rat pups. A rat huddle is an entity with characteristics and capabilities distinct from those of the individuals that comprise it. The huddle is a natural context for acquiring olfactory preferences for species odors. Olfactory learning in a huddle involves thermal and tactile stimulation from the mother's body but, surprisingly, not the rewards of suckling or of milk transfer. Although there is complete developmental continuity of huddling behavior, the sensory controls of huddling change dramatically during the first 2 weeks of postnatal life. Huddling behavior is initially controlled by thermal cues ("physiological huddling") and then becomes dominated by olfactory stimuli ("filial huddling"). The complex group behavior of huddling was modeled successfully with computational methods. Group behavior emerges from individual interactions, guided entirely by rules of individual behavior (no rules for group behavior). Three simple rules of autonomous activity/inactivity can spawn the patterns of aggregon formation displayed by groups of 7-day-old pups, but not by 10-day-olds. The developmental change evident by Day 10 requires adding a rule by which each individual is affected by the activity state of adjacent pups. Group behavior responded to manipulations of central oxytocin on Day 10, but not on Day 7.