Adult Intruders Research Articles

Ultrasonic Vocalizations in Adult C57BL/6J Mice: The Role of Sex Differences and Repeated Testing.

Ultrasonic vocalizations (USVs) are a major tool for assessing social communication in laboratory mice during their entire lifespan. At adulthood, male mice preferentially emit USVs toward a female conspecific, while females mostly produce ultrasonic calls when facing an adult intruder of the same sex. Recent studies have developed several sophisticated tools to analyze adult mouse USVs, especially in males, because of the increasing relevance of adult communication for behavioral phenotyping of mouse models of autism spectrum disorder (ASD). Little attention has been instead devoted to adult female USVs and impact of sex differences on the quantitative and qualitative characteristics of mouse USVs. Most of the studies have also focused on a single testing session, often without concomitant assessment of other social behaviors (e.g., sniffing), so little is still known about the link between USVs and other aspects of social interaction and their stability/variations across multiple encounters. Here, we evaluated the USVs emitted by adult male and female mice during 3 repeated encounters with an unfamiliar female, with equal or different pre-testing isolation periods between sexes. We demonstrated clear sex differences in several USVs' characteristics and other social behaviors, and these were mostly stable across the encounters and independent of pre-testing isolation. The estrous cycle of the tested females exerted quantitative effects on their vocal and non-vocal behaviors, although it did not affect the qualitative composition of ultrasonic calls. Our findings obtained in B6 mice, i.e., the strain most widely used for engineering of transgenic mouse lines, contribute to provide new guidelines for assessing ultrasonic communication in male and female adult mice.

Observation of Infanticide in the Particolored Flying Squirrel (Hylopetes alboniger) from a Forest in Northeastern Bangladesh

Infanticide, the intentional killing of conspecific infants, has been observed widely in mammals, including squirrels. Several ground squirrels revealed this behavioral anomaly, yet no report has been published on the flying squirrels. We report an observation of killing an infant of the particolored flying squirrel (Hylopetes alboniger) by a conspecific adult intruder in Satchari National Park of northeast Bangladesh. Among the several adaptive explanations for infanticide, our observations point to sexual selection in males that confers a competitive advantage by making females receptive. However, the resource competition hypothesis, where the intruder's offsprings gain access to resources as a result of the infanticide cannot be ruled out. Extensive studies are needed to document the extent of infanticide in flying squirrels and their adaptive significance.

Escalation of cocaine self-administration in adulthood after social defeat of adolescent rats: role of social experience and adaptive coping behavior.

The link between adolescent social stress and substance abuse is modeled in social defeat of adolescent male rats, at an age when social experiences are essential for neurobehavioral maturation. We investigated the role of social experience and social defeat stress during adolescence on social behavior and cocaine self-administration (CocSelfAd) in early adulthood. We manipulated social experience by housing male rats in pairs (PH) or singly (SH) on postnatal day (P) 21. In addition, rats were subjected to social defeat from P35-44. Social behavior was measured during the first and last social defeat in PH and SH adolescents and PH adults. After assessing the behavioral response to novelty and cocaine (P57-61), intrajugular catheters were implanted and CocSelfAd was analyzed. Residents were less aggressive toward PH adolescent intruders compared to PH adult intruders. Adults were submissive and defensive when attacked, whereas PH adolescents froze. In the course of repeated defeats, adolescent PH rats increased freezing, while SH rats decreased freezing. Longer attack-induced freezing after repeated defeats predicted escalated CocSelfAd in adulthood. PH controls acquired CocSelfAd more slowly than PH defeated and SH rats. Defeated PH rats increased CocSelfAd during progressive ratio schedules of reinforcement and during a 24-h continuous access binge compared to PH controls and SH defeated rats. Social defeat in adolescence of PH rats caused persistent increases in adult CocSelfAd. Adolescent PH rats coped with attacks adaptively by increasing freezing behavior after repeated social defeats, a measure that predicted CocSelfAd in adulthood.

Intraspecific Intrusion at Bald Eagle Nests

Competition for nesting territory has been shown to act as a density-dependent feedback mechanism influencing population growth rate. However, little is known about the nature of territorial interactions between established breeders and floaters. We examined territorial intrusion rates and associated behaviours at active Bald Eagle Haliaeetus leucocephalus nests in the lower Chesapeake Bay in 2012 and 2013. The average intrusion rate experienced at study nests during the reproductive period was 0.28 ± 0.32 intrusions/h. Variance in intrusion rate was high and there was no apparent predictive pattern to these events. Juvenile intrusions occurred closer to the nest than adult intrusions, and breeders showed higher response rates toward adults, with 78% of adult intruders eliciting a response compared to 47% of juveniles. Breeding adults responded to intruders significantly more often and more aggressively when in the presence of their mate. Further research is necessary to broadly describe the relationship ...

Medial amygdala lesions modify aggressive behavior and immediate early gene expression in oxytocin and vasopressin neurons during intermale exposure

The medial amygdala and neuropeptides oxytocin (OXT) and vasopressin (VSP) have been associated aggressive behavior regulation. However, the specific mechanism involved in OXT and VSP modulation in distinct brain regions during hostile intermale aggressive behavior is undetermined. A retrograde tracer mouse model was employed using male C57BL/6 mice injected with rhodamine-conjugated latex microsphere suspensions in the right hypothalamic paraventricular nucleus. Adult male C57BL/6 mice (aged 14–16 weeks) were subjected to resident–intruder testing using juvenile intruder mice (aged 3 weeks) or adult intruder mice (aged 8 weeks). Following exposure, Fos protein expression was increased in the medial amygdala neurons of resident mice receiving the retrograde tracer. Thus, medial amygdala neurons projecting to or localized in the vicinity of the hypothalamic paraventricular nucleus showed immediate early gene (IEG) expression following resident–intruder testing that was considered an indirect marker of activation. Additionally, intermale aggression-related behaviors were inhibited or modified by exposure to juvenile or adult intruders, respectively, in mice that underwent medial amygdala lesioning. Furthermore, Fos protein expression in OXT-positive neurons was attenuated. Thus, ablation of medial amygdala neurons prevented immediate early gene expression in OXT- and VSP-positive neurons in the hypothalamus, bed nucleus of stria terminalis, and medial preoptic area during intermale exposure. These findings indicate that the medial amygdala likely modulates hostile aggressive behavior associated with immediate early gene expression in OXT and VSP neurons in specific brain areas, which may actually be instrumental in beneficial social interaction-related aggressive responses associated with mating, territorial defense, and offspring protection.

Territorial song sparrows tolerate juveniles during the early song-learning phase

Avian song learning is an important model system for understanding vocal learning in humans and other animals. Laboratory studies indicate that social interactions are critical for song learning, but field observations show that territorial males are aggressive to intruders, raising the question of whether young birds are tolerated, much less tutored in the wild. We examined how adult song sparrows (Melospiza melodia) treat juvenile and adult intruders during different seasons important for song learning—summer, autumn, and spring—using taxidermic mounts and song playback. Territorial adults responded aggressively to adult intruders throughout the year. Adults were tolerant of juvenile intruders in the summer, displayed somewhat reduced aggression in the autumn, but treated juveniles like adult intruders by the spring. In the summer and autumn trials, wild juveniles approached our simulated ‘‘interactions’’ between subjects and adult mounts; these wild juveniles were also tolerated, even at close distances. That juveniles can and do closely approach adults during the early sensitive phase of song learning suggests that direct interactions with adults are possible and might be important for learning. In contrast, since young birds are treated aggressively in early spring, most late song learning likely happens through eavesdropping or long-distance singing interactions.

Transport of animals between rooms: A little-noted aspect of laboratory procedure that may interfere with memory

This study investigated the effects of transporting animals from the experimental room to the animal facility in between experimental sessions, a procedure routinely employed in experimental research, on long-term social recognition memory. By using the intruder-resident paradigm, independent groups of Wistar rats exposed to a 2-h encounter with an adult intruder were transported from the experimental room to the animal facility either 0.5 or 6h after the encounter. The following day, residents were exposed to a second encounter with either the same or a different (unfamiliar) intruder. Resident's social and non-social behaviors were carefully scored and subjected to Principal Component Analysis, thus allowing to parcel out variance and relatedness among these behaviors. Resident rats transported 6h after the first encounter exhibited reduced amount of social investigation towards familiar intruders, but an increase of social investigation when exposed to a different intruder as compared to the first encounter. These effects revealed a consistent long-lasting (24h) social recognition memory in rats. In contrast, resident rats transported 0.5h after the first encounter did not exhibit social recognition memory. These results indicate that this common, little-noted, laboratory procedure disturbs long-term social recognition memory.

Long-term social recognition memory in adult male rats: factor analysis of the social and non-social behaviors

A modified version of the intruder-resident paradigm was used to investigate if social recognition memory lasts at least 24 h. One hundred and forty-six adult male Wistar rats were used. Independent groups of rats were exposed to an intruder for 0.083, 0.5, 2, 24, or 168 h and tested 24 h after the first encounter with the familiar or a different conspecific. Factor analysis was employed to identify associations between behaviors and treatments. Resident rats exhibited a 24-h social recognition memory, as indicated by a 3- to 5-fold decrease in social behaviors in the second encounter with the same conspecific compared to those observed for a different conspecific, when the duration of the first encounter was 2 h or longer. It was possible to distinguish between two different categories of social behaviors and their expression depended on the duration of the first encounter. Sniffing the anogenital area (49.9% of the social behaviors), sniffing the body (17.9%), sniffing the head (3%), and following the conspecific (3.1%), exhibited mostly by resident rats, characterized social investigation and revealed long-term social recognition memory. However, dominance (23.8%) and mild aggression (2.3%), exhibited by both resident and intruders, characterized social agonistic behaviors and were not affected by memory. Differently, sniffing the environment (76.8% of the non-social behaviors) and rearing (14.3%), both exhibited mostly by adult intruder rats, characterized non-social behaviors. Together, these results show that social recognition memory in rats may last at least 24 h after a 2-h or longer exposure to the conspecific.

Territorial Behavior and Ecology of the Anemonefish Amphiprion melanopus on Guam1)

Field observations and experiments on the Indo-Pacific anemonefish Amphiprion melanopus at Guam show that it colonizes the aggregating sea anemone Physobrachia douglasi extensively and nearly exclusively. There are an average of four fish per colony. The total standard length of anemonefish at each aggregation is highly correlated with the total area covered by resident anemones, suggesting both a carrying capacity and some form of social control of growth of anemonefish2). Inter-colony migration of juveniles, as well as larval recruitment, may contribute to maintenance of the optimum colony size. Adults do not migrate. Both juveniles and adults defend territory. Juvenile territories are mutually exclusive areas within the confines of the anemone aggregation, while adult territories are considerably larger than the area covered by the anemone aggregation. Though territories of mated ♀♀ and ♂♂ overlap completely, female emphasis is peripheral relative to the ♂. Adult conspecific intruders are attacked more heavily and at greater distances than are juveniles. Intraspecific territoriality in juveniles probably reflects the limited availability of critical habitat. In adults it may function to protect the pair bond. Interspecific aggression is less intense and appears to protect both the spawn and host anemones from various predators.

Territorial Aggression of a Tropical Passerine, Zonotrichia Capensis, in Response to a Variety of Conspecific Intruders

[The expression of territorial aggression by reproductively active, resident birds varies between the sexes and in response to different intruder types. Previous studies have predicted that individuals should be more aggressive towards conspecific intruders of the same sex and tolerate intruders of the opposite sex and immature individuals. In this study, we investigated the behavioural responses of a tropical population of rufous-collared sparrows (Zonotrichia capensis) to a variety of caged intruder types: Singing Males, Silent Males, Females, and Juveniles. In this species, territories are used by the resident male and female and their young, and are also used by floaters — mature individuals that do not hold territories. Resident males responded similarly and aggressively to all adult intruders in terms of song number, closest approach to the intruder, time within 5 m of the intruder, and a composite aggression score. There was no significant variation in the response of resident females to the different intruder types, although the strongest responses of the resident female were to female intruders. Neither resident males nor females behaved aggressively towards juvenile intruders. These results fail to support the observational predictions for males and females that individuals should be most aggressive towards members of the same sex, who pose the greatest threat in terms of cuckoldry and territorial takeover., The expression of territorial aggression by reproductively active, resident birds varies between the sexes and in response to different intruder types. Previous studies have predicted that individuals should be more aggressive towards conspecific intruders of the same sex and tolerate intruders of the opposite sex and immature individuals. In this study, we investigated the behavioural responses of a tropical population of rufous-collared sparrows (Zonotrichia capensis) to a variety of caged intruder types: Singing Males, Silent Males, Females, and Juveniles. In this species, territories are used by the resident male and female and their young, and are also used by floaters — mature individuals that do not hold territories. Resident males responded similarly and aggressively to all adult intruders in terms of song number, closest approach to the intruder, time within 5 m of the intruder, and a composite aggression score. There was no significant variation in the response of resident females to the different intruder types, although the strongest responses of the resident female were to female intruders. Neither resident males nor females behaved aggressively towards juvenile intruders. These results fail to support the observational predictions for males and females that individuals should be most aggressive towards members of the same sex, who pose the greatest threat in terms of cuckoldry and territorial takeover.]

Territorial behavior of the Korean wood-feeding cockroach, Cryptocercus kyebangensis

We investigated the territorial behavior of Cryptocercus kyebangensis from the viewpoint of offspring protection. Aggressive behavior against adult intruders occurred more frequently in resident adults with their young than those without their young. Resident adults without offspring more frequently attacked intruders of the same sex than those of the opposite sex, whereas those with offspring attacked regardless of the sex of adult intruders. In paired residents, cooperative territorial behavior occurred in a higher proportion in pairs with offspring than those without offspring. Responses of residents to nymphal intruders depended on the head capsule size of the intruders. Resident adults rarely attacked intruders smaller than their own offspring but frequently attacked larger intruders. Resident nymphs frequently attacked intruders smaller than themselves, whereas they escaped or showed jerking behavior when they encountered larger intruders.

Hypothalamic paraventricular nucleus, oxytocin, and maternal aggression in rats.

Both the lesion of the parvocellular region of the PVN (FIG. 1) and the acute reduction of OT synthesis in that nucleus (FIG. 2) increase maternal aggression in rats. Previous work showed that ibotenic acid as well as the OT antisense in the PVN reduced the level of OT in the brainstem, but not in the pituitary. Therefore, the oxytocinergic parvocellular neurons of the PVN appear to exert an inhibitory effect on the aggressive behavior of the lactating female rat against an adult intruder. In a relationship of a different nature, mother-infant, a facilitatory effect of OT has been shown. Previous work showed a significant decrease of OT mRNA levels in the PVN of female rats during the first 10 days after delivery compared to late pregnancy, which is the inverse ratio of the natural temporal evolution of maternal aggressive behavior. Furthermore, in the present work, a functional decrease of OT mRNA was probably the effect of the antisense in the PVN. In conclusion, OT cells in the PVN appear to play different roles on maternal care and maternal aggression.

Rules of engagement for adult salamanders in territorial conflicts with heterospecific juveniles

Coexisting species within a guild have the potential for resource overlap and consequently for interspecific competition (e.g., interspecific territoriality). When the adults are of different sizes, which frequently is the case in terrestrial salamander communities in eastern North America, competition may occur between juveniles of the larger species and adults of the smaller species. Adults of the relatively small redbacked salamander (Plethodon einereus: up to 13 cm total length) defend intra- and intersexual territories on the forest floor, and they are broadly sympatric with the larger P. glutinosus (up to 21 cm total length). Although individuals of the two species share the same forest floor habitat, we found significantly fewer juveniles of P. glutinosus sharing territories with 336 same-size adults of P. cinereus than would be expected by random chance alone. In laboratory experiments, residential adults of P. cinereus were as aggressive toward juvenile intruders of P. glutinosus as they were toward adult conspecific intruders. Therefore, adults of P. cinereus appear to defend territories against juveniles of P. glutinosus, illustrating how interference competition may depend on the symmetry of sizes between the species.

Pregrouping aggression and defense scores influences alcohol consumption for dominant and subordinate rats in visible burrow systems

Five male/two female rat colonies were established in visible burrow systems, with males selected for pregrouping attack scores and also evaluated in open field and cat odor tests. Dominant-subordinate pregrouping attack differences suggested that the males becoming dominant are those showing more persistent and higher level attack. For six colonies showing dominant-subordinate behavioral differences, pregrouping defense tests failed to predict subordinate status. However, pregrouping defense scores were reliably correlated with subordinate pre-postgrouping change scores for voluntary ethanol consumption. Subordinates showed higher ranked ethanol consumption than dominants, but these groups were not different on pregrouping ethanol consumption. Subordinate postgrouping ethanol consumption was positively correlated with pregrouping attack toward an adult intruder, consonant with previous findings that highly aggressive subordinates are the targets of more intense attack by dominants. These results provide further support for a view that subordination stress increases voluntary ethanol consumption in male rats and suggest some additional individual differences factors that may be involved in increased ethanol consumption for male subordinates. © 1992 Wiley-Liss, Inc.

On the Ontogeny of Display Behaviour in the Black-Headed Gull

In a study of the form development of the species-specific displays in the black-headed gull, a gradual change of these motor patterns was found, as well as a retardation of this change, in birds raised in a situation lacking the normal social context (GROOTHUIS, 1989). In this experimental context, low frequencies of aggressive and fear behaviour were also found. These results have led to the hypothesis that the ontogenetic change of display behaviour is influenced by the development of internal factors for agonistic behaviour. To put this idea to a test, a study was started to investigate the relation between the development of aggressive and fear behaviour and of display behaviour during ontogeny. Standard stimulus experiments, simulating an adult intruder on the territory, were carried out at least once a week during the first 9 weeks of age in groups of territorial, hand-raised birds. The frequencies of two forms of fear behaviour, hiding and escape, and of aggressive pecking gradually increased in this order. The proportion of aggressive behaviour that was immediately followed by withdrawal was high in 2-week old chicks, but decreased considerably with age. The frequency of sudden stops after approach and of incomplete aggressive pecks increased with age, suggesting that overt aggression became more inhibited by fear, resulting in longer lasting interactions with more display behaviour. This interpretation is supported by the finding that stopping as well as sitting occurred in the behavioral chain at the switch between overt aggression and overt escape. The displays gradually changed in frequency and in form, via incomplete ones to the adult complete form. Oblique-, forward- and choking-like postures were almost exclusively performed in an agonistic context, in contrast to the begging display. It is concluded that each of these three agonistic displays develop from postures, dominated by a tendency to escape or to hide, to complete display by an addition of a simultaneously activated tendency to behave aggressively. This is based on: 1. similarities in frequency changes during ontogeny of overt aggressive and fear behaviour on the one hand, and the displays on the other hand; 2. the occurrence of the different displays during sudden stops after aggressive approach, during withdrawal, and in sitting which followed escape and was often followed by aggression; 3. the temporal relationship of the displays with overt aggression and with each other, and differences in this respect between incomplete and complete display; 4. similarity in form of the displays with overt aggressive and fear behaviour. Functional aspects of this development are discussed. The motivation of display in older gulls is studied separately, mainly by analysing normal social interactions. The increase in display frequency and decrease in overt aggression and fear during early ontogeny was maintained in later ontogeny. As a consequence, adults hardly showed overt aggressive behaviour. This finding could be explained by assuming an increasing mutual inhibition of each other's overt expression by internal mechanisms controling aggression and fear. However, this explanation could not account for other findings: 1. in contrast to young birds, adults frequently perform display outside agonistic contexts; 2. in contrast to young, adult birds no longer perform their display during those categories of locomotion for which evidence was obtained that they were caused by a simultaneous activation of aggression and fear; 3. adult gulls perform the oblique-forward sequence simultaneously with those of the opponent. The increase in synchronously performed display during late ontogeny suggests an increasing independence of display of the precise motivational state of the animal. It is concluded that the displays, once developed under the influence of internal factors controlling aggressive and fear behaviour, become fixed in form and independent of these factors. It seems as if adult birds may use these motor patterns as a kind of subroutines, applicable in different behavioural programs. If aggression is performed by adults, display is almost always performed also, suggesting that internal factors for agonistic behaviour, when aroused, still control display behaviour. It is suggested therefore that the ontogenetic change in causal organisation of display behaviour in gulls is more adequately described by the term extension of motivational factors than by the term emancipation of the original ones.

Signaling Subordinate and Female Status: Two Hypotheses for the Adaptive Significance of Subadult Plumage in Female Tree Swallows

Abstract Delayed plumage maturation in males is relatively common among North American passerines, but the Tree Swallow (Tachycineta bicolor) is one of few species in which 1-yr-old females have a distinct subadult plumage. Although they are reproductively mature, most subadult females do not breed in their first year because of intense intrasexual competition for nesting sites. Early in the season, subadult female floaters explore for recently vacated nest sites. The subadult plumage of young females could be adaptive by communicating their low threat to residents, thereby decreasing the cost of this exploration. To determine whether resident aggression depends on intruder color, we observed live intrusions and conducted model presentations. When the resident female was out of sight or did not respond to intruders, resident males were significantly less aggressive toward subadult females than toward adult intruders in both the nest-building/egg-laying and incubation stages. Early in the season, resident females were equally aggressive toward subadult female and adult intruders. When presented simultaneously with adult and subadult female models, resident males were always more aggressive toward the adult model, whereas females were aggressive toward either model. We propose two hypotheses for the adaptive significance of subadult plumage in female Tree Swallows: subordinance signaling and sex signaling. Our results suggest that subadult females may reduce resident aggression by signaling their female status to resident males, rather than by signaling their subordinate status to resident females.

Intra- and lnterspecific aggression in two species of field crickets,Gryllus integer andG. alogus

Adult, laboratory-reared, and singly caged field crickets, Gryllus integer, of known ages and representing F1 to F4 generations, were exposed to 15–20-minute bouts of social interaction with introduced adult intruder males on alternate days from approximately 1 week after adult molt to time of death. A total of 3,523 paired-male bouts for 116 male G. integer were recorded over a 2.5-year study period. Adult males used as intruders were wild-caught or laboratory reared G. integer, but for nine of the 116 experimental males, wild-caught intruders of a closely related sympathetic species, G. alogus, were used for all or many of the bouts. Bouts between male G. integer and conspecific intruders were characterized by significantly elevated levels of aggression (t=6.04, P<0.001) compared with males exposed to heterospecific G. alogus intruders in interspecific interactions. Significant differences (p<0.005, chi-squared test of independence) existed at four out of five levels in a hierarchy of agonistic behaviors. Most G. integer males exposed to both G. integer and G. alogus intruders showed significantly reduced aggression levels during the interspecific interactions (p<0.001, Mann-Whitney U test). Conspecific G. integer dyads in the intraspecific interactions showed significantly stable dominant-subordinate relationships over time. Confusion surrounding the use of the term aggression is described with reference to Arthropods in general and Orthopteran insects in particular, and some likely proximate and ultimate bases for reduced interspecific aggression in field crickets are discussed.

A test of individual recognition in the stomatopod Gonodactylus festate

The mantis shrimp Gonodactylus festae lives in rock cavities which it must frequently defend against conspecifics. To determine if individual recognition is used to mediate these aggressive contests, adult intruders were matched sequentially against residents that were larger and smaller than themselves. Intruders were repulsed by large residents, but were able to evict small ones from their cavities. Later, each intruder was exposed once to a cavity containing the odour of the animal that deeated it and once to a cavity containing the odour of the animal it defeated. Intruders rapidly entered cavities containing the odour of the resident they defeated, but were hesitant to enter cavities containing the odour of the animal that had successfully defended against them. This demonstrates that G. festae can distinguish between the odours of two previously encountered opponents and suggests that this species is capable of individual recognition.

Some situational and experiential determinants of maternal aggression in mice

Five experiments were conducted in order to examine the influence of some simple situational and experiential factors on the maternal aggression displayed by Rockland-Swiss (R-S) Albino mice toward adult male R-S intruders. The results of Experiment 1 showed that aggression is highest during the early lactation period (Postpartum Days 4 and 8) and is absent by the end of the lactation period (Postpartum Days 16 and 20). Experiment 2 showed that the aggression displayed by recently parturient (i.e., early lactating) females rapidly declines during a 10 minute test session with most attacks exhibited during the first 3 minutes of the encounter and few thereafter. The results of Experiment 3 showed that the behavior is significantly reduced when aggression testing takes place in a neutral cage as compared to testing in the homecage environment. The findings of Experiment 4 showed that young male and female intruders (25 and 45 days of age) are attacked as vigorously as adult (65 days of age) intruders of both sexes. The results of Experiment 5 showed that postweaning social deprivation (i.e., isolation) does not influence the development of aggression by lactating females. The significance of these findings for our understanding of the psychobiological basis of maternal aggression and other forms of agonistic behavior is discussed.

Maternal behaviour in virgin female golden hamsters ( Mesocricetus auratus waterhouse): the role of the age of the test pup

One hundred and forty-four naive virgin female golden hamsters were each given a single 15 min test with three pups aged from day 1 (<24 hr) to day 18. A group of eight females was tested with each age of pup. Pups aged from day 1 to day 6 were generally attacked like prey, killed and eaten. Pups of intermediate age (day 6 to day 10) were usually initially attacked but this was often followed by maternal responses. The females', behaviour with the oldest pups suggested that they were being treated as strnge adult intruders. This result differs from that of a similar experiment with mice in which the youngest pups were found to be the most effective for eliciting materal responses. An explanation for this difference in terms of the evolutionary history of the golden hamster species is proposed.