Showing papers in "Developmental Psychobiology in 1975"
TL;DR: The results indicate that olfactory, tactual, and thermal nest cues influence infantile ultrasonic vocalization in the rat.
Abstract: The amount of time infant rats 3–13 days old spent emitting ultrasounds was greater when pups were placed in an empty dish than when placed in a dish containing clean bedding or soiled bedding from the nest. Pups from 5–13 days of age vocalized more when on the clean bedding than when on the soiled bedding. When placed on cloth covered dishes, pups vocalized most to the empty dish and least to the dish containing soiled bedding. The odor of clean bedding elicited less ultrasonic vocalization than the odor of the empty dish only when pups were less than 7 days old. Pups vocalized more to room temperature than to heated conditions. The results indicate that olfactory, tactual, and thermal nest cues influence infantile ultrasonic vocalization in the rat.
203 citations
TL;DR: Mechanically driven mobile artificial mothers effectively prevented the development of stereotyped body-rocking in rhesus monkeys and the data suggest that emotional responsiveness was reduced in monkeys raised with mobile mothers, compared to primates raised with stationary devices.
Abstract: Mechanically driven mobile artificial mothers effectively prevented the development of stereotyped body-rocking in rhesus monkeys. Monkeys were maternally separated at birth and assigned to 2 groups. Both groups were placed with surrogates, identical in construction except that for 1 group the surrogate was in motion 50% of the time from 0500 hours to 2400 hours each day, and for the other group the surrogate was stationary. All but 1 of the 10 monkeys raised with stationary artificial mothers developed rocking as an habitual pattern whereas none of the 9 monekys raised with mobile mothers did so. The data also suggest that emotional responsiveness was reduced in monkeys raised with mobile mothers, compared to monkeys raised with stationary devices.
144 citations
TL;DR: It is suggested that infant rhesus monkeys form stronger attachments to monkey mothers than to inanimate surrogate mothers, a phenomenon which has not been as clearly demonstrated using other indices of attachment strength.
Abstract: Mother-reared and surrogate-peer-reared rhesus monkeys were separated from their respective attachment objects at 6 months of age and tested for the following 9 weeks to determine their home-cage behavior and their pituitary-adrenocortical responses to stress. Both groups displayed a strong immediate behavioral response to separation which was characterized by increased vocalization, increased locomotion, and decreased self-play. However, the surrogate-peer-reared infants showed a subsequent recovery in their levels of self-play whereas the mother-reared infants instead developed stereotypic behavior patterns such as repetitive pacing. The 2 groups displayed similar plasma cortisol responses to weekly sessions in an apparatus equipped with animated toy "monsters". Mother-reared but not surrogate-peer-reared subjects, however, also manifested elevated cortisol levels when an animal in an adjacent cage was captured and removed for stress testing. Mother-reared infant monkeys thus responded in a stronger and more prolonged manner to the loss of their attachment object than surrogate-peer-reared infants. These results suggest that infant rhesus monkeys form stronger attachments to monkey mothers than to inanimate surrogate mothers, a phenomenon which has not been as clearly demonstrated using other indices of attachment strength.
95 citations
TL;DR: To determine whether bilateral olfactory bulbectomy led to a change in nursing behavior, and thus death, bilaterally bulbectomized pups from 14 litters at 2, 7, and 11 days of age were studied.
Abstract: The effect of olfactory bulbectomy in nursing behavior was studied in rat pups (Wistar DAB). Pups from letters letters were bilaterally bublectomized (BX) at 2 7 and 11 days of age. Those treated on Days 2 and 7 consisted of 3 BX pups 2 surgical control (SC) pups and 2 handling control (HC) pups. Those treated on Day 11 consisted of 1 or 2 BX 1 or 2 incompletely bulbectomized pups (IBX) 2 SC pups and 2 HC pups. These and 3 additional nontreated letters were observed on Days 4 7 10 13 and 16. Fewer BX pups survived through Day 10 than SC HC IBX and nontreated pups. Differential treatment of the BX pups by females or littermates was not seen. Mammary glands of the females were functional but the BX pup stomachs had little or no milk. BX pups spent less time nursing and more time away from the female and their littermates than did the other pups. The survival may be improved if the olfactory bulbectomy were performed at a later age because animals have more days of nursing experience in a normal situation and because they may be able to compensate for the lass of olfaction with the use of another sensory system.
94 citations
TL;DR: Apparently, isolation does not result in an arresting of development, but rather in the formation of behavior patterns otherwise absent in normally reared rats.
Abstract: Male and female hooded rats were weaned at 17 days and then reared in isolation or in social groups. Intersession habituation of locomotor activity in the open field was tested at 15, 25, and 45 days. Contrary to some previous reports, the 15-day-old animals showed significant habituation. At 45 days, however, the isolates showed very little habituation compared to the social animals. The isolates showed a similar pattern of development in their emergence into the open field. Apparently, isolation does not result in an arresting of development, but rather in the formation of behavior patterns otherwise absent in normally reared rats.
70 citations
TL;DR: Eight litters of hamsters living in a large enclosure were observed from birth to 120 days, where females are normally dominant, and a gradual shift followed in the relative aggressiveness of the sexes.
Abstract: 8 litters of hamsters living in a large enclosure were observed from birth to 120 days in an experiment designed to explore the population size limiting behavior of these animals in response to pressures of crowding. For the first 65 days each litter was confined to a separate compartment. The litters were observed 4 at a time for 15 minutes 1-3 times daily for the first 50 days. After 50 days the observation period was increased to 30 minutes 1 immediately before the lights went off and 1 immediately after the period of maximum activity. After the mothers were removed the 8 litters were again observed twice daily from ages 60-65 days. At 66 and 67 days the doors between the 4 quadrants of each enclosure were uncovered and behavior was then observed for the following 1 hour. From 68 through 120 days during which the litters continued to be combined into 2 groups of 4 observations were made for 1 hour twice a day. Comparisons of absolute frequencies of fighting playing and marking were made. Basic motor coordination was reached in the 3rd week. Marking was seen during the 6th week when males began to mount their mothers sexually but lordosis was absent in female pups before the 7th week. Fighting appeared at this time with the female bei ng the more aggressive. After the litters were combined a gradual shift followed in the relative aggressiveness of the sexes. Females which had been initiating 90% of the attacks became less aggressive while the males increased their aggressiveness to the former female level. Maternal behavior was disturbed with a marked tendency to infanticide by other members of the group.
66 citations
TL;DR: Observation of rat litters over the entire preweaning period revealed long term changes to result from handling pups, including fewer good nests and less likely to be found in a single group than control litters.
Abstract: Observation of rat litters over the entire preweaning period revealed long term changes to result from handling pups Handled litters had fewer good nests and were less likely to be found in a single group than control litters These differences are not attributable to the disturbances per se, as litters where the mother was removed did not differ significantly from undisturbed controls The differences occurred predominantly in the daytime when maternal activity was at its lowest
60 citations
TL;DR: The head turning of 70 infants ranging in age from 0-72 hr was observed following a procedure known to reduce or eliminate lateral differences in the infant's response to stimulation, and infants older than 12 hr made significantly more right than left head turns whereas younger infants exhibited no such difference.
Abstract: Developmental changes in the relationship between lateral differences in responsiveness and the assumption of asymmetrical head postures were examined. The head turning of 70 infants ranging in age from 0–72 hr was observed following a procedure (maintenance of the head in a midline position) known to reduce or eliminate lateral differences in the infant's response to stimulation. Even following this procedure infants older than 12 hr made significantly more right than left head turns whereas younger infants exhibited no such difference. When lateral differences in sensitivity were not interfered with in 0 to 12 hr old infants (n = 20) they, kile the older infants, made significantly more right than left turns. These results are discussed in terms of possible developmental consequences of a reciprocity between asymmetric head position and lateral differences in sensitivity.
59 citations
TL;DR: The behavioral changes suggest that offspring of mice injected with phenobarbital during pregnancy are less responsive to the stimuli in their environment which maintain behavior.
Abstract: Mature offspring of C57BL/6J mice (Mus musculus) injected daily with phenobarbital (40 mg/kg) for the last third of pregnancy differed from saline and untreated control animals on 3 measures of behavior. Offspring of phenobarbital treated animals had higher locomotor scores than controls during an open field activity test at 75 days of age. Male offspring were also tested on a 1-trial passive avoidance task and treated animals were found to be deficient. Finally, female offspring responded less than controls on fixed ratio schedules of reinforcement. The behavioral changes suggest that offspring of mice injected with phenobarbital during pregnancy are less responsive to the stimuli in their environment which maintain behavior.
58 citations
TL;DR: All measures of brain and behavior showed a similar pattern of results: hybrids were generally more advanced than either of their inbred parent strains; differences among inbred strains were large, but differences among hybrid crosses were quite small.
Abstract: Six inbred strains and 3 F2 hybrid crosses of mice were assessed for developmental status at 32 days after conception (about 13 days after birth). Phenotypes measured included body weight, brain weight, maturity of 14 reflexive behaviors, myelination of 80 fiber tracts, and thickness of the external granular layer of the cerebellum. All measures of brain and behavior showed a similar pattern of results: hybrids were generally more advanced than either of their inbred parent strains; differences among inbred strains were large, but differences among hybrid crosses were quite small. Acceleration of F2 mice compared to their homozygous relatives ranged from .5 to 2.4 days mean difference. Developmental ages of inbred litters ranged from 28.7 to 32.2 days, whereas hybrid litters ranged from 31.5 to 32.7 days.
48 citations
TL;DR: Intensive corticosterone treatment given to rats during the 1st postnatal week irreversibly decreases DNA accumulation in the cerebrum and cerebellum and after such hypercorticism in infancy rats were tested as adults in 2 conditioned active avoidance tasks and were impaired in the acquisition of 2-way active avoidance.
Abstract: Intensive corticosterone treatment given to rats during the 1st postnatal week irreversibly decreases DNA accumulation in the cerebrum and cerebellum. After such hypercorticism in infancy rats were tested as adults in 2 conditioned active avoidance tasks. In comparison with litter-mate controls, the treated rats were impaired in the acquisition of 2-way active avoidance but not in the acquisition of 1-way active avoidance. These data are consistent with other observations suggesting a hyperresponsiveness or hyperemotionality following corticosterone treatment in infancy.
TL;DR: Weight gain over gestation decreased as a function of increasing drug dose and eye opening was delayed in the drug groups; the 5.0 mg/kg offspring made more conditioned avoidance responses than did the 3.0mg/kg and saline offspring.
Abstract: Twenty-five Sprague-Dawley derived rats were administered 1.0, 3.0, or 5.0 mg/kg of methamphetamine HCl or saline twice daily throughout gestation beginning on Day 1 of pregnancy. Rats were allowed to deliver normally; offspring were culled to 8 and sexed on Day 7, and weaned on Day 21. All females had viable litters except at the 5.0 mg/kg dose where 4 of 7 failed to deliver. The rats given methamphetamine delivered earlier than did controls. Weight gian over gestation decreased as a function of increasing drug dose. No gross anomalies were visible in the offspring. Litter size decreased as a function of increased dose and eye opening was delayed in the drug groups; the 5.0 mg/kg offspring made more conditioned avoidance responses than did the 3.0 mg/kg and saline offspring.
TL;DR: Individual male mice of several groups were observed during daily encounters with a male intruder, and aggressive behavior was most intense in residents actually living with a female and least intense in sexually naïve residents living alone.
Abstract: Individual male mice of several groups were observed during daily encounters with a male intruder. The groups differed with regard to social and sexual experience of the resident animal and of the intruder. Aggressive behavior was most intense in residents actually living with a female and least intense in sexually naive residents living alone. Residents that had only once cohabited with a female for a short time obtained intermediate scores. Experienced intruders were attacked less than naive ones.
TL;DR: It is discussed that embryonic responsiveness to temperature changes in the environment during natural incubation may play some role in later behavioral capabilities.
Abstract: In 3 experiments we have attempted to determine the extent to which the chick embryo is behaviorally and physiologically responsive to short term changes (.5-2 hr) in the normal ambient incubation temperature. Embryos ranging in age from 6-20 days of incubation have been examined after exposure to temperatures ranging from 30.5 degrees to 44.4 degrees C (normal incubator and nesting temperatures are 37-38 degrees C). At 6,9 and 12 days of age the heart rate and the duration of amnion contractions were significantly altered by exposure to temperature either higher or lower than normal although overt neuromuscular activity (motality) remains unchanged. At 15 and 20 days, however, (hatching occurs on Day 21) the rate of neuromusclar activity is altered at both low and high temperatures. At 20 days, beak-clapping, vocalization, and respiration rates also change reliably upon short-term exposure to both high and low temperatures. The possibility is discussed that embryonic responsiveness to temperature changes in the environment during natural incubation may play some role in later behavioral capabilities.
TL;DR: Effects of pinching the superior laryngeal nerves of young bats with a cold forcepts on their vocalizations suggest that the development of this nerve and its associated musculature occurs early in the bat's postnatal life.
Abstract: The ontogeny of FM pulses emitted by bats is a process of maturation that is not easily altered by experimental manipulation during early development. Infants that were hand-raised and infants that were raised by mothers with altered voices (the superior laryngeal nerves were cauterized pre- and postpartum) emitted normal adult calls. Effects of pinching the superior laryngeal nerves of young bats with a cold forcepts on their vocalizations suggest that the development of this nerve and its associated musculature occurs early in the bat's postnatal life.
TL;DR: Differences in locomotor activity, rather than in learning ability, account for early behavioral effects and that changes other than endocrine disturbances account for the adult deficits.
Abstract: Previous reports have suggested that neonatal thyroxine administration may enhance learning ability in rats as infants but the rats become deficient in learning as they grow older. This experiment compared neonatally thyroxine-injected rats with injected and nonhanded controls on passive and active avoidance and a geotactic cold-escape discrimination. Thyroxine-injected infant rats were superior to controls on active avoidance, inferior on passive avoidance, and equal on discriminated escape. Adolescent rats which had received postnatal thyroxine were inferior to controls in Lashley III maze acquisition whether or not they had received supplementary thryoxine at the time of training. These data suggest that differences in locomotor activity, rather than in learning ability, account for early behavioral effects and that changes other than endocrine disturbances account for the adult deficits. The results are compatible with previous contentions that excess neonatal thyroxine disrupts the timing of brain development.
TL;DR: RNA from brain of environmentally enriched rats hybridized to more unique DNA than did brain RNA from nonenriched controls, providing preliminary evidence for an increased transcription of the unique sequences of DNA in the enriched animals.
Abstract: Hybridization to unique DNA by RNA from brain or liver of rats given varying degrees of experience was investigated. RNA from brain of environmentally enriched rats hybridized to more unique DNA than did brain RNA from nonenriched controls. No significant differences were observed with liver RNA. This provides preliminary evidence for an increased transcription of the unique sequences of DNA in the enriched animals. The technique used has great promise for the investigation of experientially-induced alterations in gene expression.
TL;DR: Two experiments were conducted in which inbred C57BL/6J and DBA/2J mice were trained in an active avoidance jump-up task under distributed practice conditions at a variety of ages comprising a large part of their life-span, with increased footshock intensity enhancing escape responding without increasing avoidance responding.
Abstract: Two experiments were conducted in which inbred C57BL/6J and DBA/2J mice were trained in an active avoidance jump-up task under distributed practice conditions at a variety of ages comprising a large part of their life-span. DBA/2J mice were found to be basically an escaping strain and exhibited little change in behavior as a function of age. C57BL/6J mice developed consistent avoidance behavior when training was initiated at 31-36 days of age. Avoidance behavior appeared to reach a peak in the 45-day-old group and progressively decreased in 145- and 560-day-old groups. Increased footshock intensity in old DBA/2J mice enhanced escape responding without increasing avoidance responding. Increased footshock intensity in old C57BL/6J mice enhanced avoidance responding.
TL;DR: Locomotor activity in the neonatal rat was found to increase from birth until the beginning of the 3rd week of life, at which time it peaked and then subsequently declined, which reflected maturational changes in response to it.
Abstract: Locomotor activity in the neonatal rat was found to increase from birth until the beginning of the 3rd week of life, at which time it peaked and then subsequently declined. Subcutaneous injections of both .25 and 2.0 mg/kg of d-amphetamine increased activity in rats 1-21 days of age, with the maximum effect observed at 4 days of age. No tolerance to d-amphetamine was observed after 6 daily injections beginning at 1, 7, or 14 days of age. The decline in the excitatory effects of the drug thus reflected maturational changes in response to it. Consistent with other observations from this laboratory, no persisting effects of the neonatal drug treatment was observed on adult 2-way avoidance learning.
TL;DR: Early undernutrition was shown to be correlated with increased aggression in adult mice after isolation and this was significant in numbers of fights and shorter latency to fight in the DBA/2J strain.
Abstract: Early undernutrition was shown to be correlated with increased aggression in adult mice after isolation. This was significant in numbers of fights and shorter latency to fight in the DBA/2J strain. Lower 5HIAA in the brain after pretreatment with a monoamine oxidase inhibitor was found in the undernourished group.
TL;DR: It is observed that a 40-dB hearing loss, beginning at 17 days postpartum, requires 2 days before it induces susceptibility to audiogenic seizures, and seizures were maximally severe 1-2 days after earplug removal.
Abstract: C57BL/6J mice are not typically susceptible to audiogenic seizures, but temporary conductive hearing loss (via earplugging) can induce susceptibility in these subjects. The present study observed that a 40-dB hearing loss, beginning at 17 days postpartum, requires 2 days before it induces susceptibility to audiogenic seizures. When this hearing loss occurred only between 17 and 21 days postpartum, seizures were maximally severe 1–2 days after earplug removal. The peak-to-peak amplitudes of the auditory evoked potential (AEP) were also maximal at this same time. But mice deprived of acoustic input between ages 42–46 days exhibited equally large or even larger AEP amplitudes, while not exhibiting susceptibility to audiogenic seizures. Animals at this older age were shown to be capable of convulsing, in that chronic conductive loss from 17–42 days maintained susceptibility to audiogenic seizures. The acoustic deprivation produced both behavioral (audiogenic seizures) and neural (augmented AEP amplitudes) supersensitivity to sound, but the former requires a critical period, whereas the latter does not. Some degree of residual hearing loss persisted at all ages for approximately 2 days after removal of the earplugs.
TL;DR: Animals from large litters were lighter at weaning and in adulthood and were more emotional in the open field than subjects from small litters, and high correlations were found between the 4 measures of brief aggression.
Abstract: Growth, emotionality, food competition, and aggression were examined in mice nursed in litters of 3 or 9 and reared in isolation until testing. Animals from large litters were lighter at weaning and in adulthood and were more emotional in the open field than subjects from small litters. They did not win more food competition tests than subjects from small litters although their consummatory behavior during food competition tests was greater. Subjects from large litters were more aggressive in initial encounters, but over repeated encounters became more submissive. In a 2nd open-field test, emotionality of large-litter subjects was reduced more than that of subjects from small litters. When later placed in group-living cages, subjects from small litters sustained less long term physical assault than subjects from large litters. High correlations were found between the 4 measures of brief aggression.
TL;DR: Results are discrepant with those obtained with a sample of full term infants because prematures are subject to developmental disparities in the regulation of arousal, neuromuscular, and visual systems due to differential impact of preterm extrauterine life.
Abstract: Neurological organization and visual fixation were found to be unrelated in a group of premature infants tested at their term date. These results are discrepant with those obtained with a sample of full term infants. Separate comparisons made of the neurological and fixation data revealed one significant group difference from the fixation data; all other analyses were nonsignificant. The discrepant findings may have occurred because prematures are subject to developmental disparities in the regulation of arousal, neuromuscular, and visual systems due to differential impact of preterm extrauterine life.
TL;DR: By avoiding the use of extrinsic sensory stimulation, the present study demonstrates a dietary effect upon ontogeny of cortical evoked potentials independent of any possible effect on sensory receptor mechanisms.
Abstract: Studies were carried out on the trans-callosal evoked response in rats born of dams fed either a low (8%) or normal (25%) protein diet beginning 5 weeks prior to mating and throughout gestation and lactation. After weaning, pups were fed the same diets as their mothers. Bipolar (surface vs depth) stainless steel stimulating and recording electrodes were positioned at corresponding loci in the right and left sensorimotor cortices. Trans-callosal evoked responses were measured under urethane anesthesia using twice-threshold bipolar pulses of .1 msec duration at ages 13, 21, and 60-66 days. Evoked response latency was significantly greater in malnourished animals at 13 days of age, whereas at adulthood no latency differences were seen. Poststimulation excitability (15-100 msec range) was not significantly affected by the dietary treatment. These results are interpreted as corroborating previous reports on rats undernourished during development using sensory evoked potentials. By avoiding the use of extrinsic sensory stimulation, the present study demonstrates a dietary effect upon ontogeny of cortical evoked potentials independent of any possible effect on sensory receptor mechanisms.
TL;DR: Rhesus monkeys raised with mechanically driven mobile artificial mothers for the first 10 months of life looked more at other animals and were more sensitive to stimulus differences than monkeys raised on stationary surrogates.
Abstract: Rhesus monkeys raised with mechanically driven mobile artificial mothers for the first 10 months of life looked more at other animals and were more sensitive to stimulus differences than monkeys raised on stationary surrogates. Both groups looked less and were not as discriminating as wild-born monkeys. The results are consistent with other evidence suggesting that one of the long-range consequences of maternal mobility is to reduce emotional responsiveness to novel situations.
TL;DR: The hypothesis of a cholinergic inhibitory mechanism controlling behavioral arousal and becoming efficient after the 25th day of life is supported.
Abstract: A tropine-induced changes in behavioral arousal were examined in the developing rat between 21 and 40 days of age using a single exploratory trial on an elevated T-maze. Under atropine (10 mg/kg, i.p.) spontaneous activity as measured by numbers of errors, rearing responses, and stereotyped responses was significantly decreased in animals less than 25 days of age. It was significantly increased, however, between 26 and 30 days, with maximal action on the 28th day. After 30 days the effect of atropine was negligible. These results support the hypothesis of a cholinergic inhibitory mechanism controlling behavioral arousal and becoming efficient after the 25th day of life.
TL;DR: Handling during the first and second 10-day intervals appears to be additive and to approximate the effects of 20 days of handling.
Abstract: In Experiment 1 rabbits were handled for the first 10 days of life while controls were not disturbed. In Experiment 2 rabbits were handled on Days 11-20. In Experiment 3 rabbits were handled on Days 1-10, 11-20, 1-20, or were not handled. Starting at 31 days the animals were given an open-field test, a test for exploratory behavior toward novel stimuli, and a test of social behavior. The data were not consistent with the critical period concept. Instead, handling during the first and second 10-day intervals appears to be additive and to approximate the effects of 20 days of handling.
TL;DR: Parents of cichlid fish caring for their own young and nonreproductive adult males induced to exhibit parental behavior by daily presentation of free-swimming conspecific young appear to be species-specific.
Abstract: Three parameters of parental care were recorded for pairs of cichlid fish (Cichlasoma nigrofasciatum) caring for their own young to determine the normal parental role of the male. Nonreproductive adult males were induced to exhibit parental behavior (e.g., herding, fin-digging, and defense of young) by daily presentation of free-swimming conspecific young. This process of sensitization to young appears to be species-specific in that males did not exhibit parental care toward alien (Hemichromis bimaculatus) young. Parental behavior in reproductive and nonreproductive males, as well as the implications and function of sensitization in these fish, is discussed.
TL;DR: Some pups from the large litters were heavier and some exhibited developmental landmarks earlier than did most pups reared in small litters, and the effects of the large-litter treatment were not uniform; pups in this group exhibited markedly higher variability with regard to all measures.
Abstract: The variability in the effects of rearing rats in large litters was examined. Subjects reared in litters of 16 were stunted and delayed in the appearance of eye-opening and fur-eruption as compared with pups reared in litters of 8. However, the effects of the large-litter treatment were not uniform; pups in this group exhibited markedly higher variability with regard to all measures than did pups from small litters. In fact, some pups from the large litters were heavier and some exhibited developmental landmarks earlier than did most pups reared in small litters.
TL;DR: Three-way analyses of variance for Age, Strain, and Illumination effects on parameters of bar pressing, stimulus change, and light duration patterns per circadian period (24 hr) indicate that old mice are less active than young mice and the A/J mice are more active than C57BL/6J mice.
Abstract: Young (5 month) and old mice (23 and 26 months) of the inbred A/J and C57BL/6J strains (N = 80) were isolated in ventilation chambers with ad lib food and water for 1 week at a time. Each animal was allowed to select its own lighting schedule by pressing one lever to turn on a light of either 10.76 or 166.40 lux illumination intensity and pressing another lever to turn the light off. Three-way analyses of variance for Age, Strain, and Illumination effects on parameters of bar pressing, stimulus change, and light duration patterns per circadian period (24 hr) indicate that old mice are less active than young mice and the A/J mice are less active than C57BL/6J mice. Young mice spent more time in the dark than old mice and the A/J albino mice spent more time in the dark than did the C57BL/6J pigmented mice. Other age and strain differences were noted as well and are discussed with respect to some theories of light exposure by nocturnal animals.