Deficiency of the paternally inherited gene Magel2 alters the development of separation-induced vocalization and maternal behavior in mice

The behavior of offspring results from the combined expression of maternal and paternal genes. Genomic imprinting silences some genes in a parent-of-origin specific manner, a process that, among all animals, occurs only in mammals. How genomic imprinting affects the behavior of mammalian offspring, however, remains poorly understood. Here, we studied how the loss of the paternally inherited gene Magel2 in mouse pups affects the emission of separation-induced ultrasonic vocalizations (USV). Using quantitative analysis of more than 1000 USVs, we characterized the rate of vocalizations as well as their spectral features from postnatal days 6–12 (P6–P12), a critical phase of mouse development that covers the peak of vocal behavior in pups. Our analyses show that Magel2 deficient offspring emit separation-induced vocalizations at lower rates and with altered spectral features mainly at P8. We also show that dams display altered behavior towards their own Magel2 deficient offspring at this age. In a test to compare the retrieval of two pups, dams retrieve wildtype control pups first and faster than Magel2 deficient offspring. These results suggest that the loss of Magel2 impairs the expression of separation-induced vocalization in pups as well as maternal behavior at a specific age of postnatal development, both of which support the pups' growth and development.     

Development of echolocation and communication vocalizations in the big brown bat, <Emphasis Type="Italic">Eptesicus fuscus</Emphasis>

Big brown bats form large maternity colonies of up to 200 mothers and their pups. If pups are separated from their mothers, they can locate each other using vocalizations. The goal of this study was to systematically characterize the development of echolocation and communication calls from birth through adulthood to determine whether they develop from a common precursor at the same or different rates, or whether both types are present initially. Three females and their six pups were isolated from our captive breeding colony. We recorded vocal activity from postnatal day 1 to 35, both when the pups were isolated and when they were reunited with their mothers. At birth, pups exclusively emitted isolation calls, with a fundamental frequency range <20 kHz, and duration >30 ms. By the middle of week 1, different types of vocalizations began to emerge. Starting in week 2, pups in the presence of their mothers emitted sounds that resembled adult communication vocalizations, with a lower frequency range and longer durations than isolation calls or echolocation signals. During weeks 2 and 3, these vocalizations were extremely heterogeneous, suggesting that the pups went through a babbling stage before establishing a repertoire of stereotyped adult vocalizations around week 4. By week 4, vocalizations emitted when pups were alone were identical to adult echolocation signals. Echolocation and communication signals both appear to develop from the isolation call, diverging during week 2 and continuing to develop at different rates for several weeks until the adult vocal repertoire is established.     

Juvenile vocalizations ofCtenomys talarum (Rodentia: Octodontidae)

We analysed the physical structure and functional interpretation of juvenile vocalizations ofCtenomys talarum Thomas, 1898. Two sounds, one true vocalization (care-elicitation call) and one mechanical sound (nursing sound) were recorded during the nestling period. Care-elicitation calls were emitted by isolated pups and caused the mother to respond by immediately approaching the pups to take care of them. This maternal response to the care-elicitation calls ofC. talarum pups, together with the production of these vocalizations during the first weeks after birth, when pups depend exclusively on their mother to obtain food and maintain their body temperature, give preliminary support for the recent theory that care-elicitation cries are honest advertisements of offspring need.     

Identification of different song types in the European Nightjar Caprimulgus europaeus

ABSTRACT

Capsule: Two distinct song types were identified for male European Nightjars Caprimulgus europaeus with their relative frequency of use changing through the breeding season, indicating a possible link to paired status.

Aims: To test whether two song types could be defined in audio recordings and whether use differed in relation to the paired status of males.

Methods: Unattended acoustic recording devices were placed at a Nightjar study site in Nottinghamshire, United Kingdom, and recordings of churring vocalizations were made during two periods of the breeding season. These recordings were then analyzed to identify the presence/absence of the song terminal phrase and associated audible features.

Results: Two distinct song types were identified in the recorded audio data that differed in their terminal phrasing and overall song duration. The number of Nightjar songs with a terminal phrase increased significantly between the two sampling periods, from lower levels during the site arrival period, to higher levels during the first clutch initiation period.

Conclusion: This study showed that the use of Nightjar song types appears to vary through the breeding season, with males being more likely to produce song with a terminal phrase during the first clutch initiation period, when they are more likely to be paired or in the presence of a female. The unattended acoustic recording method may provide a minimally intrusive means of assessing the number of Nightjar breeding pairs and not just singing males.     

Dog Pups’ Attractiveness to Humans Peaks at Weaning Age

The domestic dog (Canis lupus familiaris) is characterized by greatly reduced parenting investment compared with the wild type wolf (C. l. lupus) from which it is descended. Unlike wolf pups, which are reared by both parents into their second year of life, dog pups are abandoned by their mother at weaning around eight weeks of age. This relatively small parental involvement may contribute to the high pup mortality observed in dogs not living as pets. We hypothesized that people would find dog pups most attractive around weaning age when conspecific parental care is significantly reduced and pup mortality rate is high. Younger and older pups would benefit less from human intervention because in the former case the mother is providing care, and in the latter their survival is already compromised. To test this hypothesis, 51 participants rated the attractiveness of 39 black and white headshot photographs presented on a computer screen of dog pups from three breeds (Jack Russell Terrier, Cane Corso, and White Shepherd), from birth to 7 months old. In line with our hypothesis, attractiveness of Cane Corsos peaked at 6.3 weeks of age; Jack Russell Terriers’ attractiveness peaked at 7.7 weeks; and White Shepherds were most attractive at 8.3 weeks. There were also differences in attractiveness between the breeds, with Cane Corsos rated less attractive than the other two breeds. If this attractiveness motivates humans to care for the dog pups and thereby improves pup survival, this could confer significant advantages to dogs, and may contribute to our understanding of the process of domestication.     

The structure of innate vocalizations in Foxp2‐deficient mouse pups

Heterozygous mutations of the human FOXP2 gene are implicated in a severe speech and language disorder. Aetiological mutations of murine Foxp2 yield abnormal synaptic plasticity and impaired motor‐skill learning in mutant mice, while knockdown of the avian orthologue in songbirds interferes with auditory‐guided vocal learning. Here, we investigate influences of two distinct Foxp2 point mutations on vocalizations of 4‐day‐old mouse pups (Mus musculus). The R552H missense mutation is identical to that causing speech and language deficits in a large well‐studied human family, while the S321X nonsense mutation represents a null allele that does not produce Foxp2 protein. We ask whether vocalizations, based solely on innate mechanisms of production, are affected by these alternative Foxp2 mutations. Sound recordings were taken in two different situations: isolation and distress, eliciting a range of call types, including broadband vocalizations of varying noise content, ultrasonic whistles and clicks. Sound production rates and several acoustic parameters showed that, despite absence of functional Foxp2, homozygous mutants could vocalize all types of sounds in a normal temporal pattern, but only at comparably low intensities. We suggest that altered vocal output of these homozygotes may be secondary to developmental delays and somatic weakness. Heterozygous mutants did not differ from wild‐types in any of the measures that we studied (R552H ) or in only a few (S321X ), which were in the range of differences routinely observed for different mouse strains. Thus, Foxp2 is not essential for the innate production of emotional vocalizations with largely normal acoustic properties by mouse pups.     

The effects of repeated early deprivation on ultrasonic vocalizations and ontogenetic development in mandarin vole pups

Early deprivation is popularly used in rodent models as an early life social stress to investigate and determine the factors that affect the development of the brain and behavior. Ultrasonic calls made by pups play an important role in parental–pup interactions during the neonatal period. However, whether repeated early deprivation affects the properties of ultrasonic vocalizations (USVs) produced by mandarin vole (Microtus mandarinus) pups, and whether ontogenetic development is subsequently affected, remains unclear. Here we measured USVs and developmental parameters in mandarin vole pups deprived of their parents and littermates for 3 h per day (ED, which is significantly different from 5 min isolation used to induce USVs) and another pup group developed under normal nest conditions (PC). Repeated measures analysis indicated that the number of USVs from ED pups was significantly lower than those from PC pups during the postnatal period (p < 0.05). The pulse durations of ED pups were longer than those of PC pups at two (p < 0.001) and five days of age (p < 0.05), but shorter at 14 days of age (p < 0.001). Compared with PC pups, the frequency range of the ED pups was wider at 18–45 kHZ, variable during the first week, smaller and narrower at 18–30 kHZ at eight and 11 days of age, and became stable similar to PC pups at 25 kHZ after 14 days of age. ED also reduced pup body weight significantly and resulted in earlier eye opening compared with PC pups (p < 0.001). A positive relationship was also found between USV emissions and levels of parental care received by pups. It appears that pup USVs are an important age-dependent behavioral phenotype and an effective communicative method between parents and offspring. Prolonged parental and littermate deprivation (ED) may alter USVs emitted by pups and then ontogenetic development and parental care. Mandarin voles show USV properties similar to socially monogamous rodents and this add further support to the hypothesis that species with different social systems produce different patterns of ultrasonic vocalizations. USVs, ontogenetic development and parental care are closely associated.     

Responsiveness of a male Mueller's gibbon to his own species-song,that of a lar gibbon,and a synthetic song of similar frequency

The study compared the responsiveness of a captive male Mueller's gibbon to audio recordings of dawn songs of wild-living male Mueller's gibbons and lar gibbons, and to a synthetic song constructed from screech owl vocalizations. Though wild-caught, the subject had not been exposed to songs of his own species for at least 10 years, and in fact, only rarely performed the dawn song himself. Initially, the subject responded vigorously to all three recordings, with no preference being shown for the recording of his own species. However, a very strong preference for Mueller's vocalizations did develop following a period of concentrated and exclusive exposure to Mueller's songs.     

CONFERENCE ANNOUNCEMENTS

ABSTRACT

In most mammals, adults produce relatively low frequency vocalizations compared to those of juveniles. This rule is not maintained however at least in four species of ground squirrels, whose juveniles call at the adult's fundamental frequency. These findings have been obtained however with separate sets of juveniles and adults and no data is available concerning the ontogeny linked to these differences. Here we analyze the acoustic structure of alarm calls of the same Yellow Spermophilus fulvus and Speckled S. suslicus ground squirrel individuals, recorded as pups and as adults after hibernation. We found the fundamental frequencies of adults within the same frequency ranges as those of pups, in spite of the significant difference in body mass. In ground squirrels, severing the relationship between body size and call frequency removes some vocal cues to age. We discuss some functional hypotheses advanced to explain manipulations with fundamental frequencies in ground squirrels and other animals, and suggest the lack of data for discussing the mechanisms of such vocal tuning.     

Human Classification of Context-Related Vocalizations Emitted by Familiar And Unfamiliar Domestic Cats: An Exploratory Study

ABSTRACT

Previous research has shown that human classification of contextspecific domestic cat “meow” vocalizations is relatively poor, although improves with experience and/or general affinity to cats. To investigate whether such classification further improves when recipients (humans) of the vocalizations reside with the vocalizing animal (cat), cat owners (n=10) were asked to listen to eight audio recordings of a single meow (4 from their own cat and 4 from an unfamiliar cat) produced during one of four possible contexts, and identify the context in which each meow was emitted. Contexts comprised food preparation, food-withholding, negotiating a barrier, and attention solicitation. In addition, participants were asked to rate 20 meow vocalizations (produced by unfamiliar cats in the four contexts) on scales measuring pleasantness and urgency, in order to investigate whether participants reached consensus on the emotional content of the vocalizations. Successful identification of both the context and the hypothesized emotional content of the vocalization would comprise the first steps in our understanding of whether human-directed cat “meow” vocalizations are fully advantageous. Forty percent of the participants identified the correct contexts at a level greater than chance when the vocalizations belonged to their own cat. However, no participants performed above chance when vocalizations belonged to an unfamiliar cat. Participants’ urgency ratings were not significantly influenced by the context in which the vocalization was produced. Pleasantness ratings, however, were significantly higher for the context of negotiating a barrier in comparison with attention solicitation. These results suggest that the domestic cat, as a species, does not have a context-specific repertoire of human-directed vocalizations. Successful context classification of cat meows however, was possible for some owners. Whether such success was due to individual learning ability or recognition of hypothesized emotional content of the call requires further investigation.     

Involvement of the GHSR in the developmental programming and metabolic disturbances induced by maternal undernutrition

The mismatch between maternal undernutrition and adequate nutrition after birth increases the risk of developing metabolic diseases. We aimed to investigate whether the hyperghrelinemia during maternal undernourishment rewires the hypothalamic development of the offspring and contributes to the conversion to an obese phenotype when fed a high-fat diet (HFD). Pregnant C57BL/6 J, wild type (WT) and ghrelin receptor (GHSR)^−/− mice were assigned to either a normal nourished (NN) group, or an undernutrition (UN) (30% food restricted) group. All pups were fostered by NN Swiss mice. After weaning, pups were fed a normal diet, followed by a HFD from week 9. Plasma ghrelin levels peaked at postnatal day 15 (P15) in both C57BL/6 J UN and NN pups. Hypothalamic Ghsr mRNA expression was upregulated at P15 in UN pups compared to NN pups and inhibited agouti-related peptide (AgRP) projections. Adequate lactation increased body weight of UN WT but not of GHSR^−/− pups compared to NN littermates. After weaning with a HFD, body weight and food intake was higher in WT UN pups but lower in GHSR^−/− UN pups than in NN controls. The GHSR prevented a decrease in ambulatory activity and oxygen consumption in UN offspring during ad libitum feeding. Maternal undernutrition triggers developmental changes in the hypothalamus in utero which were further affected by adequate feeding after birth during the postnatal period by affecting GHSR signaling. The GHSR contributes to the hyperphagia and the increase in body weight when maternal undernutrition is followed by an obesity prone life environment.     

Classification threshold and training data affect the quality and utility of focal species data processed with automated audio-recognition software

ABSTRACT

Automated recognition is increasingly used to extract information about species vocalizations from audio recordings. During processing, recognizers calculate the probability of correct classification (“score”) for each acoustic signal assessed. Our goal was to investigate the implications of recognizer score for ecological research and monitoring. We trained four recognizers with clips of Common Nighthawk (Chordeiles minor) calls recorded at different distances: near, midrange, far, and mixed distances. We found distance explained 49% and 41% of the variation in score for the near and mixed-distance recognizers, but only 3% and 6% of the variation for the midrange and far recognizers. We calculated detection functions for each of the recognizers at various score thresholds and found that the detection function for the near and mixed-distance recognizers satisfied the assumptions of density estimation for most score thresholds, while the detection function for the midrange and far recognizers did not. The detection functions also showed that score threshold choice is a decision about sampling area, not just about the balance between recall and precision. Overall, we showed that training recognizers with ‘high-quality’ clips that were recorded at close range will improve the utility of the data without affecting how many true positives the recognizer detects.     

Characterizing maternal isolation-induced ultrasonic vocalizations in a gene–environment interaction rat model for autism

Deficits in social communication and language development belong to the earliest diagnostic criteria of autism spectrum disorders. Of the many risk factors for autism spectrum disorder, the contactin-associated protein-like 2 gene, CNTNAP2, is thought to be important for language development. The present study used a rat model to investigate the potential compounding effects of autism spectrum disorder risk gene mutation and environmental challenges, including breeding conditions or maternal immune activation during pregnancy, on early vocal communication in the offspring. Maternal isolation-induced ultrasonic vocalizations from Cntnap2 wildtype and knockout rats at selected postnatal days were analyzed for their acoustic, temporal and syntax characteristics. Cntnap2 knockout pups from heterozygous breeding showed normal numbers and largely similar temporal structures of ultrasonic vocalizations to wildtype controls, whereas both parameters were affected in homozygously bred knockouts. Homozygous breeding further exacerbated altered pitch and transitioning between call types found in Cntnap2 knockout pups from heterozygous breeding. In contrast, the effect of maternal immune activation on the offspring's vocal communication was confined to call type syntax, but left ultrasonic vocalization acoustic and temporal organization intact. Our results support the “double-hit hypothesis” of autism spectrum disorder risk gene–environment interactions and emphasize that complex features of vocal communication are a useful tool for identifying early autistic-like features in rodent models.     

Endogenous clock gene expression in the suprachiasmatic nuclei of previsual newborn rabbits is entrained by nursing

The rabbit is particularly suitable for investigating the development of mammalian circadian function. Blind at birth, the pups are only visited by the mother to be nursed once every 24 h for about 3 min and so can be studied largely without maternal interference. They anticipate the mother's visit with increased behavioral arousal and with a rise in body temperature, both of which represent endogenous circadian rhythms. We now report that in newborn pups the suprachiasmatic nuclei of the hypothalamus (SCN; the main circadian pacemaker in mammals) show endogenous 24‐h rhythmicity in the expression of the clock genes Per1, Per2, and Bmal1. Pups nursed from postnatal days 1 to 7 and fasted to day 9 showed the same rhythms of clock gene expression as normally nursed controls. We also report that these rhythms are entrained by nursing. Pups killed on postnatal days 3–4 showed the same rhythms in gene expression as pups in the previous experiment, whereas littermates subsequently nursed from postnatal days 4 to 7 with nursing delayed 6 h showed a corresponding shift in the diurnal pattern of clock gene expression. Consistent with this, two groups of pups implanted with telemetric thermal sensors and nursed 6 h apart had daily patterns in body temperature synchronized with the two different nursing times. We conclude that the expression of clock genes associated with the newborn rabbit's circadian system is entrained by nonphotic cues accompanying nursing, the exact nature of which now needs to be clarified. © 2008 Wiley Periodicals, Inc. Develop Neurobiol, 2009     

Expression of autotaxin and lysophosphatidic acid receptors 1 and 3 in the developing rat lung and in response to hyperoxia

Abstract

We sought to evaluate lysophosphatidic acid (LPA) signaling improvement in lung development by assessing the expression of autotaxin and LPA receptor 1 and 3 (LPAR1 and LPAR3) in the neonatal rat lung during normal perinatal development and in response to hyperoxia. In the developmental study, rats were sacrificed on days 17, 19, and 21 of gestation; on postnatal days 1, 4, and 7; and at adulthood (postnatal 9 weeks). In the hyperoxia study, 42 postnatal 4-day-old rat pups were divided into seven groups and exposed to either 85% O₂ for 24, 72, or 120 h or room air for 0, 24, 72, or 120 h. The rats were then euthanized after 0, 24, 72, and 120 h of exposure. Immunofluorescence demonstrated that autotaxin, LPAR1, and LPAR3 proteins were broadly colocalized in airway epithelial cells, but mainly distributed in vascular endothelial and mesenchymal cells during the first postnatal week. The expression of autotaxin, LPAR1, and LPAR3 were increased during late gestation and then decreased after birth. Autotaxin expression and enzymatic activity were significantly increased at 72 and 120 h after exposure to hyperoxia. LPAR1 and LPAR3 expression was also increased after 120 h of hyperoxic exposure. These findings suggest that LPA-associated molecules were upregulated at birth and induced by hyperoxia in the developing rat lung. Therefore, the LPA pathway may be involved in normal lung development, including vascular development, as well as wound-healing processes of injured neonatal lung tissue, which is at risk of neonatal hyperoxic acute lung injury.     

Development of social vocalizations in mice

Adult mice are highly vocal animals, with both males and females vocalizing in same sex and cross sex social encounters. Mouse pups are also highly vocal, producing isolation vocalizations when they are cold or removed from the nest. This study examined patterns in the development of pup isolation vocalizations, and compared these to adult vocalizations. In three litters of CBA/CaJ mice, we recorded isolation vocalizations at ages postnatal day 5 (p5), p7, p9, p11, and p13. Adult vocalizations were obtained in a variety of social situations. Altogether, 28,384 discrete vocal signals were recorded using high-frequency-sensitive equipment and analyzed for syllable type, spectral and temporal features, and the temporal sequencing within bouts. We found that pups produced all but one of the 11 syllable types recorded from adults. The proportions of syllable types changed developmentally, but even the youngest pups produced complex syllables with frequency-time variations. When all syllable types were pooled together for analysis, changes in the peak frequency or the duration of syllables were small, although significant, from p5 through p13. However, individual syllable types showed different, large patterns of change over development, requiring analysis of each syllable type separately. Most adult syllables were substantially lower in frequency and shorter in duration. As pups aged, the complexity of vocal bouts increased, with a greater tendency to switch between syllable types. Vocal bouts from older animals, p13 and adult, had significantly more sequential structure than those from younger mice. Overall, these results demonstrate substantial changes in social vocalizations with age. Future studies are required to identify whether these changes result from developmental processes affecting the vocal tract or control of vocalization, or from vocal learning. To provide a tool for further research, we developed a MATLAB program that generates bouts of vocalizations that correspond to mice of different ages.     

Response of Wolves to Experimental Disturbance at Homesites

ABSTRACT Events during the denning period (parturition to first autumn) often determine the reproductive success of wolves (Canis lupus). Consequently, there is concern about the potential adverse effects of human-caused disturbance at wolf den and rendezvous sites (homesites), but relatively little information on this subject is available. We conducted standardized experimental disturbance treatments at 12 unique wolf homesites in the Northwest Territories, Canada, during summers 2002 and 2003. The treatment consisted of an intruder approaching a homesite once per day for 3 consecutive days and recording behavioral responses, response distance, and response intensity of wolves. We counted pups and estimated their ages prior to the initial treatment at each site. Adult wolves moved pups at 3 of 6 treated homesites in each year. The amount and type of known human activity within a pack's home range did not influence whether adults moved pups in response to the treatment. The response intensity of wolves to the treatment was inversely related to the amount of human activity near a homesite. There was no relationship between the distance at which wolves responded to the intruder and the amount or type of human activity. There was a positive relationship between increasing age of pups and their relocation in response to the treatment. Reproductive success was not influenced by the treatment or by the amount and type of human activity. Treated sites were used by wolves the following year in the same proportion as untreated sites. It appears that pups are most vulnerable early in the year when less mobile; therefore, managers should consider age of pups before human activity at or near wolf homesites occurs.     

Vocal Ontogeny in Neotropical Singing Mice (Scotinomys)

Isolation calls produced by dependent young are a fundamental form of communication. For species in which vocal signals remain important to adult communication, the function and social context of vocal behavior changes dramatically with the onset of sexual maturity. The ontogenetic relationship between these distinct forms of acoustic communication is surprisingly under-studied. We conducted a detailed analysis of vocal development in sister species of Neotropical singing mice, Scotinomys teguina and S. xerampelinus. Adult singing mice are remarkable for their advertisement songs, rapidly articulated trills used in long-distance communication; the vocal behavior of pups was previously undescribed. We recorded 30 S. teguina and 15 S. xerampelinus pups daily, from birth to weaning; 23 S. teguina and 11 S. xerampelinus were recorded until sexual maturity. Like other rodent species with poikilothermic young, singing mice were highly vocal during the first weeks of life and stopped vocalizing before weaning. Production of first advertisement songs coincided with the onset of sexual maturity after a silent period of ≧2 weeks. Species differences in vocal behavior emerged early in ontogeny and notes that comprise adult song were produced from birth. However, the organization and relative abundance of distinct note types was very different between pups and adults. Notably, the structure, note repetition rate, and intra-individual repeatability of pup vocalizations did not become more adult-like with age; the highly stereotyped structure of adult song appeared de novo in the first songs of young adults. We conclude that, while the basic elements of adult song are available from birth, distinct selection pressures during maternal dependency, dispersal, and territorial establishment favor major shifts in the structure and prevalence of acoustic signals. This study provides insight into how an evolutionarily conserved form of acoustic signaling provides the raw material for adult vocalizations that are highly species specific.     

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 30 min 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.