Immediate postnatal sucking in the rabbit: its influence on pup survival and growth

This study was aimed at characterising the impact of immediate postnatal sucking on pup survival and development. The interactive effects of postnatal success with the day 0 weight of pups, the nest-access regimen (controlled or free) or parity of females was investigated. Pups (n = 900) were categorised according to their initial ingestion of colostrum. In primiparous does: (1) pup mortality between d0-d10 was higher for unsuccessful than for successful early suckers; (2) lighter d0-weight reduced survival for unsuccessful but not for successful pups; (3) free nest-access of females annihilated the survival advantage fostered by the initial sucking success. In secondiparous does, these impacts waned. Finally, whatever the does' parity, only d0-weight influenced pup weight-gain between d0-21. Thus, pup survival seemed to depend (at least in primiparae) on their ability to suck right after birth, and to display a pattern of energy saving without being disturbed by the females' nest entries.     

Primiparous females do not exhibit reduced maternal care in gray seals (Halichoerus grypus)

We compared the behaviors of primiparous and multiparous gray seal (Halichoerus grypus) females over the course of lactation to examine whether poorly developed maternal behaviors may play a role in the reduced lactation performance observed in primiparous females. Overall, primiparous females spent as much time interacting with their pups as multiparous females. The proportion of time spent nursing their pup increased significantly between early and peak lactation in both primiparous and multiparous females. Although there was no significant difference in the duration of nursing bouts as a function of reproductive status, primiparous females nursed significantly more frequently (bouts/hour) and, therefore, spent a significantly greater proportion of time nursing than multiparous females throughout lactation. Primiparous gray seal females were also significantly more active than multiparous females, however, the difference in activity represented only a small proportion of the overall time budget. We conclude that poorly developed maternal behaviors resulting from a lack of prior reproductive experience are unlikely to account for lower levels of milk energy transfer to pups in primiparous gray seals.     

Nursing Behavior in Mexican Free-tailed Bat Maternity Colonies

The nursing behavior of Mexican free-tailed bats (Tadarida brasiliensis mexicana) was examined in two large maternity cave roosts during six nursing seasons. After depositing pups in large creches, adults return to these creches two or more times daily to nurse. Densities, movements, and roosting associations of pups in creches were documented, and a night-vision device and infrared-sensitive video system were used to observe female-pup reunions and nursing affiliations. Densities of young (< 15 d) pups in creches averaged 4000 pups/m². Pups moved independently of one another between milk meals. Stable roosting associations among pups were absent, and females evidently could not predict where a given pup would be relocated. Upon returning to the creche, females searched for and relocated pups using vocal and olfactory cues. Experiments with marked individuals demonstrated that females nursed the same pups on subsequent nights. These same experiments indicated that female/pup recognition was mutual and that the pup participated in reunions by moving toward its putative mother. However, pups also attempted to suckle from other females and 35 instances of apparently successful milk “theft” were observed. Observed milk theft was insufficient to account for the frequency of alloparental nursing that was estimated independently using genotype tests. The high densities of bats within roosts and the lack of stable roosting associations among individuals argue against selection for shared nursing via kin selection and/or reciprocity. Errors in pup recognition, milk-dumping by females, and adoption by mothers whose own pup had died may occur, but high pup survival to weaning suggests that adoption is rare. The estimate of alloparental nursing obtained from genotype tests may be inflated as a result of increased opportunities for milk theft caused by our disturbance of adults. Current evidence suggests that energetic costs to females of alloparental nursing and searching for pups are compensated by mutualistic benefits obtained from aggregative roosting.     

Alloparenting in Steller sea lions (<Emphasis Type="Italic">Eumetopias jubatus</Emphasis>): correlations with misdirected care and other observations

Alloparental care is rarely observed in Steller sea lions (Eumetopias jubatus) where maternal care is extended to a single pup for up to 1 year or more. However, we observed 28 allonursing events and one case of adoption at a small breeding rookery in the western Gulf of Alaska between the years 2001 and 2005. Multiparous and primiparous females were observed nursing nonfilial individuals with equal frequency, but primiparous females spent significantly more time nursing nonfilial individuals. Multiparous females allowed allonursing only while sleeping and unaware while most primiparous females were aware that they were allonursing. These results are consistent with the misdirected-care hypothesis suggesting that primiparous (presumably younger) females nurse nonfilial pups due to inexperience, whereas multiparous (presumably older) females are victims of milk stealing during times of inattentiveness. Nonfilial pups were aggressively tossed most often during the pupping season and only by multiparous females, while allonursing events occurred more frequently after the pupping season. Starveling pups were not cared for by any female, but two were attended by a single bull during separate autumn seasons.     

The influence of reproductive experience on milk energy output and lactation performance in the grey seal (Halichoerus grypus)

Although evidence from domestic and laboratory species suggests that reproductive experience plays a critical role in the development of aspects of lactation performance, whether reproductive experience may have a significant influence on milk energy transfer to neonates in wild populations has not been directly investigated. We compared maternal energy expenditures and pup growth and energy deposition over the course of lactation between primiparous and fully-grown, multiparous grey seal (Halichoerus grypus) females to test whether reproductive experience has a significant influence on lactation performance. Although there was no difference between primiparous females in milk composition and, thus, milk energy content at either early or peak lactation primiparous females had a significantly lower daily milk energy output than multiparous females indicating a reduced physiological capacity for milk secretion. Primiparous females appeared to effectively compensate for lower rates of milk production through an increased nursing effort and, thus, achieved the same relative rate of milk energy transfer to pups as multiparous females. There was no difference between primiparous and multiparous females in the proportion of initial body energy stores mobilised to support the costs of lactation. Although primiparous females allocated a greater proportion of energy stores to maternal maintenance versus milk production than multiparous females, the difference was not sufficient to result in significant differences in the efficiency of energy transfer to pups. Thus, despite a lower physiological capacity for milk production, primiparous females weaned pups of the same relative size and condition as multiparous females without expending proportionally more energy. Although reproductive experience does not significantly affect the overall lactation performance of grey seals, our results suggest that increases in mammary gland capacity with reproductive experience may play a significant role in the age-related increases in neonatal growth rates and weaning masses observed in other free-ranging mammals.     

Litter Size Determines Circadian Periodicity of Nursing in Rabbits

Nursing in rabbits occurs inside the nest with circadian periodicity. To determine the contribution of suckling stimulation in regulating such periodicity, we varied the size of the litters provided (1, 2, 4, or 6–8 pups). Nursing does, kept under a 14:10 (L:D) photoperiod, were continuously videotaped from parturition into lactation day 15. Although parturitions occurred throughout the day, a significant negative linear correlation (p?<?0.0001; r?=??0.68) was evident between time of delivery and time of nursing on lactation day 1, regardless of newborn number: longer intervals between these two events were seen in does delivering in the early morning than in those that gave birth late in the day. In rabbits suckling 6–8 pups, a Rayleigh analysis revealed that the population vector best describing their nursing pattern (across lactation days 1–15) had a phase angle?=?58° (corresponding to solar time 0352?h and rho?=?0.78; p?<?0.001). In contrast, the nursing pattern of does nursing litters smaller than 6 pups did not show circadian periodicity; rather, mothers showed multiple entrances into the nest box throughout the day. Cluster analysis revealed that the main equilibrium point of intervals between suckling bouts shifted from 24?h (6–8 pups) to 6?h (4 and 2 pups) and to as low as 4?h with 1 pup. In the groups nursing 2, 4, or 6–8 pups, most nursing episodes were followed by food and water intake. Those mothers also showed self-grooming of the ventrum and nipples after nursing. The incidence of these behaviors was lower in does nursing 1 pup. In conclusion, nursing in rabbits spontaneously occurs with circadian periodicity, but it is largely modulated by a threshold of suckling stimulation. (Author correspondence: gabygmm@gmail.com)     

Causes and predictors of nest mortality in a European rabbit population

We conducted a study on nest mortality of an individually marked population of European rabbits (Oryctolagus cuniculus L., 1758) living in a field enclosure. Over 4 years, we determined maternities and quantified pup mortality during the nest period of 703 pups born in subterranean breeding burrows. Overall, pup mortality occurred in 42.7% of the litters, whereas 32.4% of all born pups were affected. Mortality was highest during the first few postnatal days. In about 50% of the cases, we managed to quantify different causes of mortality such as malnutrition, flooding, cooling of the pups, infanticide, or predation. The pups’ body mass on postnatal day 1, the thermal environment and the number of litter mates were the most important predictors of nest mortality. Litter mortality risk decreased with increasing soil temperature around the subterranean nests. A comparatively higher average pup body mass lowered the nest mortality risk of a litter, whereas this effect was more pronounced when soil temperatures were low. Furthermore, mortality was lowest in medium-sized litters, most probably due to the balance between the thermal benefits of huddling with litter siblings and the costs of having them due to the lower share of milk obtained by the individual pups in larger litters. In addition, nest mortality depended on characteristics of the mother; mortality was increased in litters of low-ranking females and of mothers with lower body mass. In conclusion, our study highlights multiple causes and the effects of different environmental and social factors on nest mortality of this small mammal.     

Lactating Females Do Not Discriminate Between Their Own Young and Unrelated Pups in the Communally Breeding Rodent, Octodon degus

Females in numerous rodent species engage in communal nesting and breeding, in which they share one or more nests to rear their young. A potential cost of communal nesting and breeding is that mothers divert resources to unrelated offspring. One way mothers could avoid this cost is to recognize and favour their own young over unrelated offspring when allocating maternal effort. We assessed whether female degus (Octodon degus), a communally nesting and breeding caviomorph rodent, discriminate between their own and unrelated offspring during lactation. Female degus previously have been shown to distinguish between their own and unrelated pups when exposed to odours from both. We measured pup discrimination based on differences in the retrieval behaviour of females that were in early or intermediate lactation directed towards their own and unrelated offspring; offspring presented were of similar or different age. Before any event of pup retrieval, lactating females spent similar amounts of time and interacted to a similar extent with their own and unrelated pups. During pup retrieval, all lactating females transported both pups to the nest. Neither relatedness to pups, nor pup‐age differences, influenced the order in which pups were retrieved to the nest. Dams waited similar amounts of time before retrieving the first pup when the first transported young was their own or unrelated. Likewise, females waited similar amounts of time before retrieving the second pup when the pup transported first was their own or unrelated. The time between first and second pup transport events was longer when dams were in early when compared with intermediate lactation, but only when pups were of similar age. All experimental subjects nursed unrelated pups after they were retrieved. Collectively, our results do not support the hypothesis that communally breeding female degus use their recognition ability to discriminate against unrelated offspring in favour of their own young.     

Absentee Mothering – Not So Absent? Responses of European Rabbit (Oryctolagus cuniculus) Mothers to Pup Distress Calls

Longer‐range acoustic parent‐offspring communication is widespread, but might be absent in species in which young are hidden in burrows during the mother's absence. The European rabbit (Oryctolagus cuniculus) is such a species, with unusually limited maternal care largely restricted to a short daily nursing bout. Based, however, on evidence of frequent infanticide in this species, we hypothesize that rabbits possess a mechanism promoting a maternal response to pup distress calls. We conducted playback experiments with distress calls of pre‐weaning pups played next to the breeding burrows of mothers in a field enclosure (i.e. next to the burrows where mothers give birth and raise their young). Calls were played shortly after pups were born (T1) when infanticide risk is maximal, and shortly before the pups start dispersing from the breeding burrow (T2). A high proportion of mothers (60.6%) responded to pup calls by rapidly returning to their breeding burrow and 40% of them investigated the area around the entrance. Return responses to the playback of pup calls did not differ between mothers during T1 and T2. Thus, our results confirm that rabbit mothers respond rapidly to pup distress calls and that this responsiveness may adaptively serve to repel potentially infanticidal females.     

Neonatal anesthesia for studies of hamster parental behavior when infanticidal aggression is a possibility

Experiments involving investigation of the neuroendocrine basis for paternal care in rodents risk activation of aggressive behavior toward pups. To minimize pain and suffering during tests of parental responsiveness requiring retrieval of a displaced pup to its nest, a method of anesthetizing the pup was developed in Djungarian hamsters, Phodopus campbelli. A surgical plane of anesthesia, as measured by criteria, such as respiratory depression, loss of the pedal reflex, and failure to increase respiratory rate or to vocalize in response to handling, was achieved by use of intraperitoneal administration of a combination of ketamine and xylazine. Both parents (tested separately) expressed normal behavior toward anesthetized pups. In random order, a saline-injected or anesthetized pup was displaced from its nest in the home cage. There were no differences in pick-up or retrieval rates between saline and anesthetized pups for either parent. A third test using an unmanipulated pup confirmed that parental behavior was not reduced toward an anesthetized pup. However, if anesthetized pups were tested first among littermates, retrieval by males was less likely. This method will, therefore, underestimate retrieval behavior in males, but not females. Adult male hamsters that had never been parents also expressed expected behavior by attacking the pup in 45% of cases. This method provides an efficient and effective means of protecting pups while allowing adults to express a wide range of parental and infanticidal behaviors. It also has application in behavioral screening of transgenic strains toward unrelated young.     

Communal nesting and communal nursing in house mice,Mus musculus domesticus

The functional significance of communal nesting and nursing is poorly understood. Female house mice often communally nest, and within these communal nests females appear to indiscriminately nurse all pups, a rare trait for any mammal. In this study, the hypothesis that communal nesting provides protection from conspecific infanticide was tested and supported in semi-natural populations of house mice. Conspecific infanticide in single-mother nests (69%, N = 412) was twice that in communal nests (33%, N = 508). Because this major benefit of communal nesting does not require communal nursing, direct benefits to communal nursing itself were tested. Most proposed benefits should result in heavier weaning weights, but no differences were found between communal and single nests in the semi-natural populations. If communal nursing is to be avoided in communal nests, dams must recognize their own pups. Retrieval tests conducted in the laboratory produced equivocal results. Dams discriminated between pups that differed in age, but not between their own and other age-matched pups. The major survival advantage of communal nesting, coupled with the failure to find nutritional advantages for communally nursed pups, supports a recent suggestion that communal nursing is an unavoidable consequence of communal nesting. This hypothesis is further strengthened by data indicating that communal nesting partners tend to be kin, thereby providing inclusive fitness benefits to communal nursing. Although costs of communal nursing were proposed and tested, no such costs were found. We also show from 15 observations of infanticide that all classes of adults (territorial and non-territorial males, pregnant and non-pregnant females) are infanticidal. These observations are in conflict with previous laboratory studies.     

Do Laboratory Mouse Females that Lose Their Litters Behave Differently around Parturition?

Efficiency in laboratory mouse breeding is hampered by poor reproductive performance, including the loss of entire litters shortly after birth. However, the underlying mechanisms are not yet fully understood and establishing the cause of death in laboratory mouse pups can be complicated. Newborn mouse pups are generally hidden in nests, dead pups are often eaten by the female, and the widespread practice of leaving periparturient females undisturbed complicates inspection, which may delay the discovery of pup loss. In order to efficiently prevent problems with litter loss, it is important to find key factors for survival. We investigated differences in periparturient behavior between female laboratory mice whose pups survived until weaning and females whose entire litters were lost. Video recordings of 82 primiparous females of the C57BL/6 strain or knockouts with C57BL/6 background were used. The mice were observed from 24 h before until 24 h after parturition and female behaviors coded using a pre-established ethogram. The relationship between behavior and survival was analyzed using logistic models, where litter survival was regressed on the proportion of 30-s observations with at least one occurrence of the behavior. We found that females with surviving litters performed more nest building behavior during the last 24 h before parturition (p = 0.004) and spent less time outside the nest during the entire observation period (p = 0.001). Increased litter survival was also associated with more passive maternal behaviors and the female ignoring still pups less. Females that lost their litters performed more parturition-related behaviors, suggesting prolonged labor. The results indicate that maternal behavior plays a significant role in laboratory mouse pup survival. Complications at parturition also contribute to litter mortality.     

The importance of paternal care on pup survival and pup growth in Peromyscus californicus when required to work for food

We examined paternal care in the monogamous California mouse when wheel running was necessary to obtain food. Paternal and maternal care, pup survival and feeding were compared in litters with single females (Male Absent) and paired females (Male Present) under three different foraging requirements. When wheel running was required for obtaining food (Wheel Contingent), the male's presence significantly increased pup survival. There were no significant effects of the male's presence on pup survival in the No Wheel (standard laboratory housing with ad lib food) or Wheel Noncontingent (running wheel with ad lib food) groups. Males may have indirectly increased pup survival by helping generate food through wheel running and then subsequently spending less time eating than females. While females reduced the time spent in parental care as pups aged, males increased the time spent in parental care as pups aged. Thus as pups became larger and more expensive to rear, the male increased parental care. These results indicate that paternal care benefits pup survival more under conditions where the parents must forage to obtain food than when food is provided ad lib in the standard laboratory environment.     

How Do Infanticidal Male Bank Voles (Myodes glareolus) Find the Nest with Pups?

Infanticide, the killing of conspecific young, occurs in most mammal species, like in our study species, the bank vole (Myodes glareolus). Infanticide by adult males is regarded as a strong factor affecting recruitment of young into population. It is considered as an adaptive behaviour, which may increase male fitness via resource gain or an increased access to mates. When an intruder is approaching the nest, the mother should not be present, as her nest guarding is very aggressive and successful. Pups use ultrasonic vocalisation to call their mother when mother leaves nest for foraging but it is not know which cues do infanticidal males use to find the nest with vulnerable pups to commit infanticide? We studied whether the pups' sounds or the olfactory cues of the nest guide the males of known infanticidal behavioural trait towards the nest with vulnerable pups. Four nest boxes in a large indoor arena offered different nesting cues: nest odour, pup vocalisation, both odour and sound or control with no cue. The result showed that infanticidal males were more active in their searching behaviour than non‐infanticidal males and seemed to target the nest providing only acoustic cues. Four of the males, all infanticidal, intruded the nest box. Infanticidal males seem to actively search for nests with vulnerable pups by eavesdropping pup begging calls for absent mother. However, under natural conditions, mother presence and aggressive nest protection may be an effective counter strategy against strange male infanticide. When trapping study voles from the wild, we monitored occurrence of male infanticide across the breeding season from early to late summer. Proportion of infanticidal males was between 25 and 29% of all males tested along the breeding season. Our results suggest that male infanticide seems to cause a stable threat for pup mortality in increasing breeding season density.     

Parental care and sexual interactions in Mongolian gerbils (Meriones unguiculatus) during the postpartum estrus

The behavior of male and female Mongolian gerbils was continuously video-recorded from 24 h before the parturition to the end of postpartum estrus. We noted that the mean interval between the delivery of the last pup and the first mounting was 13 h and 32 min. Over the whole duration of the postpartum estrus (7 h and 41+/-57 min), females spent significantly more time in crouching over, pup licking, nest building and "digging" activities, and were more recipient of allogrooming than males. Maternal pup retrievals were not very frequent (14.0+/-5.6 episodes), but males never retrieved pups nor exhibited a full sequence of nest building activity. Males spent a longer mean time in bodily interactions with females, as compared to the time they spent with pups, and engaged in intense copulatory activity (592.8+/-40.5 mounts or 1 episode per 46.7s); even during the delivery process males attempted to force copulation, but females rejected mounting in all cases. We conclude that females exhibited higher levels of parental care than males, and our findings suggest that males compete with pups in attracting the female's attention, since they actively disrupt maternal care or persistently persecute the female in order to copulate.     

The role of the young in the control of the hormonal events during lactation and behavioral weaning in the golden hamster

This study was designed to determine the relationship between the behavioral and physiological changes occurring in the hamster over the course of lactation. Experimental (E) females were given litters of six 3-day-old pups on Day 13 of lactation and allowed to raise them. Control (C) females raised their own litters of six pups. Groups of E and C females were decapitated at 1000 and 1700 hr when their pups were either 8, 18, or 28 days old. Nursing was observed for 1 hr on the 3 days prior to autopsy. Nursing behavior disappeared by Day 28 in C females. E females exhibited nursing behavior at levels equal to those observed in C females until E pups were 28 days old and 38 days had elapsed since parturition. Despite the fact that E mothers continued to nurse pups on day 18 postpartum when pups were 8 days old, E pups showed lower growth rates than did C pups. Prolactin (PRL) levels remained elevated when E pups were 8 days old even though E pups did not grow normally. PRL levels decreased over time in both C and E females and reached baseline by Day 28 although nursing behavior was still elevated in E females. Thus, nursing behavior did not stimulate PRL release during late lactation. Estradiol (E₂) levels in C females remained at baseline until Day 28 when levels increased. LH, FSH, and Progesterone (P) levels in C females showed a dramatic diurnal pattern which disappeared by Day 28, when levels dropped. Levels of E₂, P, LH, and FSH in E females closely paralleled those of C females only when groups were compared with respect to pup age. Thus, behavioral weaning and the return to estrous cyclicity appear to be dependent upon the age of the pups rather than the time elapsed since parturition. However, milk production and PRL release appear to be more closely tied to the number of days postpartum and can be dissociated from the amount of nursing behavior observed.     

ATYPICAL PUP REARING STRATEGIES BY SEA OTTERS

Eight tagged sea otter (Enhydra lutris) pups in central Prince William Sound, Alaska, weighed 6–15 kg at the time of separation from their mother. Four pups weighing 15 kg were able to forage successfully on their own. Three pups weighing ≤9 kg had negligible chances of survival and apparently were abandoned by sick females. Abandonment of a pup may reduce the burden on a sick female, enabling recovery and subsequent reproduction. One of the three sick females that abandoned a pup in this study recovered and pupped again. Abandonment of pups should occur most often in populations where females are stressed by poor food resources. Reassociation with a previous offspring, as observed once in this study, also may occur most frequently in food-limited populations where reproductive failures are most common and pup survivorship is significantly increased by additional maternal assistance.     

Pup Shoving by Adult Naked Mole-Rats

Adult naked mole‐rats (Heterocephalus glaber) characteristically perform an unusual behavior toward young: they shove small pups frequently and vigorously around the nest. We studied 15 litters in five captive colonies to quantify which adults shove pups, changes in shoving frequencies as pups develop, how external disturbances affect pup‐shoving frequencies, and behavior of juveniles that were not shoved as pups. In all litters and colonies the breeding female shoved pups significantly more often than any other individual. Breeding females also shoved adult colony mates, but at far lower rates than they shoved pups. Breeding males shoved pups about half as often as did breeding females. Together, the parents shoved pups ten times more often than did nonbreeders. Frequencies of pup shoving peaked when pups were 3–4 wk old, roughly coincident with weaning. When colonies were disturbed experimentally, frequencies of pup shoving increased dramatically, whereas rates at which nonbreeding adults were shoved decreased sharply. We separated four newly‐weaned litters and raised half the pups apart from their colony. When these litters were reunited 4–9 wk later, the unshoved (experimental) pups were the same size as the frequently shoved (control) pups, but the unshoved pups were significantly less likely to flee from a disturbance. Shoving of small pups encourages them to flee from danger, and also may enforce weaning.     

Female Intrasexual Territoriality and its Potential Adaptive Significance: The Pampean Grassland Mouse as an Ecological Model Species

Territorial behaviour in female small mammals has been proposed as a mechanism to defend limited ecological resources or their pups against conspecific infanticidal or predators. Female territorial behaviour very often is associated with reproductive activity due to the fact that frequency and intensity of aggression are exhibited mainly when females are pregnant or lactating. In vole and mice species, female territoriality would be a counterstrategy to prevent the killing of their pups by conspecific breeding females. To study whether female territoriality is a strategy for pups or nest defence against infanticidal breeding females, and whether time invested in nursing young affects aggressive response of mothers, we used the Pampean grassland mouse (Akodon azarae) as an ecological model species. We conducted resident–intruder tests between lactating females. Differences in residency time (48 vs. 72 h) of focal females in their home territory were also included in the analysis. In all cases, the pups of both resident and intruder mothers were placed with the nesting material from their reproductive cages. Resident mothers were always more aggressive than intruders and they were even more aggressive when they spent more time nursing their pups. Contrarily, intruder females exhibited the greatest values of submissive behaviours. Our results show that female territoriality of A. azarae would represent a strategy to protect pups from potentially infanticidal females. We discuss the extent of female intrasexual territoriality and its potential adaptive significance in relation to strategies which lead to increase their reproductive success.     

Paternal behavior increases testosterone levels in offspring of the California mouse

Paternal care during early development influences pup survivorship in the monogamous and biparental California mouse, Peromyscus californicus. Moreover, paternal pup retrievals impact development of adult offspring aggression and the neuropeptide vasopressin, yet little is known about the underlying mechanisms of these developmental changes. Because testosterone can increase arginine vasopressin and aggression, we hypothesized that paternal pup retrievals increase testosterone levels in prepubertal male P. californicus pups. Male pups were assigned to one of three groups: hormonal baseline, nonretrieval control, or retrieval. On postnatal days 18-21, all pups and the mother were removed from the cage, and the focal male pup was placed either outside of the nest to elicit paternal retrievals (retrieval group) or in the nest to discourage paternal retrievals (nonretrieval group). Testosterone was elevated at 45-min, but not 90-min, post-manipulation in retrieved compared to nonretrieved pups. Moreover, there was a significant positive correlation between pup retrievals and testosterone in the 45-min group. This rapid testosterone rise in response to paternal retrievals may facilitate an increase in aggression and vasopressin in adult offspring. Therefore, this period of development previously viewed as hormonally quiescent may be more active in response to paternal behavior than previously thought.