Time and Sex in the Male Mouse: Temporal Regulation of Infanticide and Parental Behavior

Infanticide is a violent but successful reproductive strategy found in many mammals, particularly rodents. In male house mice (Mus domesticus and M. musculus) the act of ejaculation provides a reliable neural signal for timing the birth of their offspring. However, a unique chronobiological aspect of this phenomenon is the extraordinary temporal latency that can occur between the stimulus (coital ejaculation) and its adaptive neural response (male mice cease killing pups and behave parentally toward them instead). Specifically, the inhibition of infanticide is often time delayed for many days after a male ejaculates, but virtually always occurs before or around the time his own sired offspring would be born 18-20 days later. Furthermore, infanticide spontaneously reemerges 50-60 days after mating. In CF-1 stock male mice this entire behavioral sequence is synchronized with the female's reproductive cycle, and occurs even in the total absence of social cues or changes in pituitary or gonadal hormones after mating. When entrained and mated at 22 h (lightdark 11:11) or 27 h (lightdark 13.5:13.5) T-cycles, photoperiodic cues appeared to synchronize this dramatic shift in behavior, because a sudden transition from pup killing to parenting was matched with the number of light/dark cycles experienced after ejaculation rather than the amount of real time experienced, suggesting a circadian timing link. Housing in constant light accelerated the postmating transition to parenting, whereas constant dark significantly delayed the transition to parenting, but still occurred by 3 weeks after mating. Most males tend to oscillate between infanticide and parental behavior for several days before locking in to constant parenting, regardless of lighting conditions. Variation in the time delay between ejaculation and the inhibition of infanticide was consistent within young individuals (<10 months of age), but in older males (> 18 months of age) the time interval between ejaculation and parenting was significantly prolonged and attenuated. Another unique aspect of this phenomenon is that variation among individuals in their timing and response to light cues is correlated with phenotypic variation in sex steroid exposure during late fetal development. So far, no simple physiological explanation can account for the neural mechanism triggered by ejaculation that coordinates these time-delayed behavioral changes toward pups.     

Changes in incidence of infanticidal and parental responses during the reproductive cycle in male and female wild mice Mus musculus

Most studies of infanticide in rodents have been carried out on laboratory animals. In the present study, pairs of infanticidal male and female wild mice were observed for changes in their responsiveness to alien pups during the reproductive cycle. Females killed pups during pregnancy, adopted alien pups during the lactation period, and killed pups again a month after the weaning of their offspring. In males, the transition from infanticidal to parental responses occurred as early as the first half of their mates' pregnancy, and only half of them resumed infanticide a month after the weaning of their litters. Subsequent experiments showed that two independent factors, the experience of copulation and cues from the pregnant female, accounted for the cessation of infanticide and onset of parental care in the male. The adaptive value of the co-existence of multiple mechanisms underlying the cessation of infanticide in male and female house mice is discussed with respect to the social life of this species.     

Is Mating Alone Enough to Inhibit Infanticide in Male Bank Voles?

Infanticide, the killing of conspecific young, is commonly recognized as an adaptive behavioural strategy enhancing the fitness of the perpetrator. Infanticide is supposed to be inhibited in several male rodent species after mating with a time lag to the time when perpetrators own offspring would be born. This is because males with no parental care do not recognize their own offspring. It has been suggested that copulation alone is enough to inhibit infanticidal behaviour in male rodents. Infanticidal behaviour occurs in more than 50% of male bank voles (Myodes glareolus), and offspring loss because of infanticide may have a great effect on breeding success and population recruitment. In a laboratory experiment, we studied whether infanticidal male bank voles after successful mating stop the killing of pups. Infanticidal males were paired with a female until successful copulation. After the young were born, the males’ infanticidal behaviour was studied from the time of expected birth of own pups until their post‐weaning age. We predicted that mated infanticidal males are inhibited from committing infanticide especially during the time period when pups are less than 10 d old. Against our prediction, 67% of the infanticidal males continued the killing of pups in the age of 3 d. Infanticidal behaviour remained stable, and half of the males were infanticidal still at the age of weaning of pups. Our results are contradictory to previous studies, as we observed no inhibition of infanticide during early life of pups nor increase in infanticide again when their own offspring would have reached the ‘safe’ age and size after weaning. We suggest that mating alone is not sufficient to inhibit infanticide. Thus, we suggest that other cues of the female with whom the male mated with or on her territory are needed for inhibition to occur.     

Adaptive switch from infanticide to parental care: how do beetles time their behaviour?

Abstract In species where parents may commit infanticide, temporal kin recognition can help ensure parents kill unrelated young but care for their own offspring. This is not true recognition, but rather depends on accurate timing of the arrival of young and a behavioural switch from killing to caring for offspring. Mistakes have clear fitness consequences; how do species that use temporal kin recognition ensure accurate timing? We manipulated photic cues and show that the switch from infanticide to parental care in the burying beetle Nicrophorus vespilloides depends on day‐length inputs. Extending the light period after carcass discovery influenced timing of both oviposition and the cessation of infanticide. Manipulation of the light : dark cycle after oviposition also influenced timing of the switch to parental care. The timing mechanism is therefore sensitive to photic cues and access to a carcass and is not triggered by oviposition. The behavioural switch is directly related to the timing mechanism rather than changes in reproductive physiology. Given the conserved nature and extensive homology of genetic influences on biological timing, we speculate that the molecular mechanisms regulating circadian behaviour may have been co‐opted to allow beetles to determine how much time has passed after carcass discovery even though this is over 50 h.     

Distinct preoptic‐BST nuclei dissociate paternal and infanticidal behavior in mice

Paternal behavior is not innate but arises through social experience. After mating and becoming fathers, male mice change their behavior toward pups from infanticide to paternal care. However, the precise brain areas and circuit mechanisms connecting these social behaviors are largely unknown. Here we demonstrated that the c‐Fos expression pattern in the four nuclei of the preoptic‐bed nuclei of stria terminalis (BST) region could robustly discriminate five kinds of previous social behavior of male mice (parenting, infanticide, mating, inter‐male aggression, solitary control). Specifically, neuronal activation in the central part of the medial preoptic area (cMPOA) and rhomboid nucleus of the BST (BSTrh) retroactively detected paternal and infanticidal motivation with more than 95% accuracy. Moreover, cMPOA lesions switched behavior in fathers from paternal to infanticidal, while BSTrh lesions inhibited infanticide in virgin males. The projections from cMPOA to BSTrh were largely GABAergic. Optogenetic or pharmacogenetic activation of cMPOA attenuated infanticide in virgin males. Taken together, this study identifies the preoptic‐BST nuclei underlying social motivations in male mice and reveals unexpected complexity in the circuit connecting these nuclei.     

Scheduled daily mating induces circadian anticipatory activity rhythms in the male rat

Daily schedules of limited access to food, palatable high calorie snacks, water and salt can induce circadian rhythms of anticipatory locomotor activity in rats and mice. All of these stimuli are rewarding, but whether anticipation can be induced by neural correlates of reward independent of metabolic perturbations associated with manipulations of food and hydration is unclear. Three experiments were conducted to determine whether mating, a non-ingestive behavior that is potently rewarding, can induce circadian anticipatory activity rhythms in male rats provided scheduled daily access to steroid-primed estrous female rats. In Experiment 1, rats anticipated access to estrous females in the mid-light period, but also exhibited post-coital eating and running. In Experiment 2, post-coital eating and running were prevented and only a minority of rats exhibited anticipation. Rats allowed to see and smell estrous females showed no anticipation. In both experiments, all rats exhibited sustained behavioral arousal and multiple mounts and intromissions during every session, but ejaculated only every 2-3 days. In Experiment 3, the rats were given more time with individual females, late at night for 28 days, and then in the midday for 28 days. Ejaculation rates increased and anticipation was robust to night sessions and significant although weaker to day sessions. The anticipation rhythm persisted during 3 days of constant dark without mating. During anticipation of nocturnal mating, the rats exhibited a significant preference for a tube to the mating cage over a tube to a locked cage with mating cage litter. This apparent place preference was absent during anticipation of midday mating, which may reflect a daily rhythm of sexual reward. The results establish mating as a reward stimulus capable of inducing circadian rhythms of anticipatory behavior in the male rat, and reveal a critical role for ejaculation, a modulatory role for time of day, and a potential confound role for uncontrolled food intake.     

Synchronization of Mating Reactivity Rhythms in Populations of Paramecium bursaria

Cell populations of Paramecium bursaria show arhythmic mating reactivity after exposure to constant light (LL) for more than 2 wk. After this arhythmic population is exposed to darkness for 9 h, the mating reactivity rhythm of the cell population reappears. The phases of rhythms in individual cells are synchronized to each other. When the arhythmic population in constant light is exposed to dark pulses of various durations, the first peak of the recovered mating reactivity rhythm appears 6 h after the end of the dark pulse. Thus, in the case of dark pulses to cells in LL, the transition from dark to light sets the phase of the subsequent mating reactivity rhythm. When an arhythmic population in LL is transferred to constant darkness (DD), a rhythm of mating reactivity also appears and, in this case, the first peak of the rhythm occurs 18 h after the LL to DD transition. Therefore, arhythmic populations of cells in LL can be synchronized by either a dark pulse or by transition to continuous darkness. When the arhythmic populations in LL were transferred to various light/dark (LD) cycles, the mating reactivity rhythms entrained to LD cycles of 18 to 30 h in duration. Finally, mating rhythms can also be synchronized by treatment with puromycin (400 μg/ml for 6–18 h).     

Neurological effects of aromatase deficiency in the mouse

In the brain, the conversion from androgen into estrogen is an important process for the differentiation of the brain function in male rodents. The aromatase is expressed in some nucleus of the brain. To assess the functional significance of the aromatase gene in development and activation of sex-specific behavior, we analyzed behavioral phenotypes of the aromatase knockout (ArKO) male mice. ArKO males obviously decreased their fertility and showed deficits in male sexual behavior including mount, intromission and ejaculation. Noncontact penile erection was not significantly affected by defect of the aromatase gene. A reduction of aggressive behavior against male intruders was also observed in ArKO males, while they tend to exhibit aggression toward estrous females during male copulatory tests. Moreover, the infanticide toward the pups was observed in the ArKO males, whereas characteristic parental behavior, but not infanticide was observed in wild-type males. These results indicate that aromatase gene expression is a critical step not only for motivational and consummatory aspects of male sexual behavior, but also for aggressive and parental behaviors in male mice.     

Behavioral and endocrine dynamics associated with infanticide in a black and white colobus monkey (Colobus guereza)

Infanticide has been observed in several colobines, but only one infanticide has previously been documented for black and white colobus monkeys (Colobus guereza). This report describes an infanticide observed in Kibale National Park, Uganda, in the summer of 2001. An adult male from a neighboring group attacked and killed a young infant whose mother subsequently remained in her own group and was observed mating 10 days later. She engaged in two mating periods of several days each, separated by approximately 1 month, during which she copulated frequently with the dominant male within her group. Urinary estrogen (EC) and progesterone metabolite (PdG) excretion patterns demonstrated that the female resumed ovarian cycling soon after the infanticide. Because the infanticidal male did not attempt to mate with the victim's mother, his actions were not consistent with the predictions of the sexual-selection hypothesis. We speculate that infanticide associated with intergroup aggression may help secure access to high-quality resources for the infanticidal male's group.     

Observed infanticides following a male immigration event in black howler monkeys, Alouatta pigra, at Palenque National Park, Mexico

This study describes two cases of directly observed and one case of nearly observed infanticide after an adult male immigrated into a multimale–multifemale group of black howler monkeys (Alouatta pigra) at Palenque National Park, Mexico. The immigrant male entered the group alone, injured the central adult male, presumably evicted the noncentral adult male, and killed all three young infants present over the course of three consecutive days in February 2010. Three weeks after the infanticide events, the three adult females who lost their infants were observed to sexually solicit and copulate with both the infanticidal male and the injured central male. Multimale mating is an effective reproductive strategy that females employ to confuse paternity and reduce the risk of infanticide, but the extent to which promiscuous mating after infanticide events is part of a counterstrategy in this species is still unknown. More cases of infanticide will need to be observed to assess the degree to which infanticide avoidance shapes the social system of the black howler monkey.     

Circadian mating activity and effect of pheromone pre-exposure on pheromone response rhythms in the moth Spodoptera littoralis

Mating in moths is generally mediated by female-produced sex pheromones. Mating activity, female pheromone production/release and male pheromone responsiveness all show diurnal variations in many species. We found that the response of the male Egyptian cotton leafworm, Spodoptera littoralis, to sex pheromone gland extracts showed a diel rhythm in olfactometer tests, and the variation was persistent for at least 1 day in constant darkness. High male response to sex pheromone was correlated in time with high mating and locomotor activity. Male S. littoralis, maintained in constant darkness and exposed to pheromone gland extracts on a daily basis, showed an induced temporal variation in response after several days, in contrast to unexposed males. This suggests that in the absence of other external zeitgebers, exposure to sex pheromone may function to synchronise circadian behavioural rhythms in male moths. The daily rhythm in mating activity in S. littoralis is also shown to be persistent for at least 2 days in constant darkness. Pairs mated significantly less when either the male or female had been raised in a light:dark cycle 10 h out of phase, indicating that the proposed circadian rhythm in mating activity is composed of rhythmic mating preference/ability in both sexes.     

Courtship and copulation in Tarsius bancanus

The reproductive behavior of 6 paired, captive Bornean tarsiers was studied over an 8-month period. Seven copulations were observed. Females signalled males by visual displays and olfactory cues from vulval rubbing. Males signalled females with courtship calls heard before matings. After a courtship lasting from 1 to 2 h, copulation occurred with the male thrusting 61-190 times for 60-90 s, ending in ejaculation. The female regulated timing of mating by rejection or avoidance of the male. Multiple matings were not observed, and mating occurred once or twice a night during each night of estrus. This copulatory pattern of infrequent matings of short duration and active female solicitation and regulation of copulating timing suggests a harem or monogamous system.     

Maternal Aggression and Litter Size in the Female House Mouse

It has been commonly argued that, in house mice, female post-partum fighting against a male intruder functions to protect the offspring from infanticide. The aim of this study was to test the hypothesis that maternal aggression is actually related to pup defence and, specifically, according to parental investment theory, that its intensity should increase with litter size. 60 nulliparous albino female mice were mated and randomly assigned to four experimental groups in which litters were culled at birth to 0, 4, 8, or 12 pups, respectively. On day 8 after delivery all females were tested for maternal aggression against a stranger adult male conspecific (5-min exposure). No aggression occurred in the group in which all pups had been removed. In the other groups, the proportion of females displaying overt aggression increased with litter size. Several scores of female agonistic behaviour (proportion of females displaying overt aggression, total attacking time, frequency of tail rattling) were significantly higher for the females rearing 8 and 12 pups than for the females rearing 4 pups. Aggressive behaviour of females rearing 12 pups was not significantly higher than that of females rearing 8 pups. No male committed infanticide. These results support the hypothesis that rodent maternal aggression is strictly related to offspring defence and are consistent with the theoretical prediction that, the costs of the defence being equal and the gain in fitness increasing with litter size, the intensity of maternal defence of the young should increase with their number.     

Infanticide and effectiveness of pup protection in bank voles: does the mother recognise a killer?

Infanticide, the killing of conspecific young, has been documented in numerous species of mammals, especially rodents. In that infanticide is costly to the victim mother, natural selection should favour counter-strategies by females to protect their pups. We studied the frequency of infanticide by male and female bank voles (Clethrionomys glareolus) and the effectiveness of dams in recognising and deterring infanticide. In trials in which unprotected pups were exposed to voles of both sexes, one third of male and female intruders killed pups. When mothers were present at the nest site, not a single female and only 2 of 25 males were able to commit infanticide. Females acted aggressively towards all intruders and hence did not discern between infanticidal and non-infanticidal males and females. Aggression of dams against any intruder indicates that all strange individuals near the nest site of a territorial species form an equal threat to pups. However, the presence of the aggressive dam is sufficient to deter most intruders from harming offspring by keeping them at a distance from the nest site. Heat run before mating, in a species with postpartum oestrus and vulnerable pups in the nest, also might lure potential infanticidal males away from the nest site and obscure the nest location. Received in revised form: 17 October 2001 Electronic Publication     

Running wheel size influences circadian rhythm period and its phase shift in mice

Running wheels are widely used in studies on biological rhythms. In mice wheel diameters have ranged from 11 cm to 23 cm. We provided mice with running wheels of two different sizes: 15 cm diameter and 11 cm diameter. The amount of running in the 12-h light:12-h dark condition and the endogenous period of wheel running in constant darkness was determined over 40 days. On the 1st day in constant darkness all animals were exposed to a 15-min light pulse at circadian time 13. The animals in the small wheel ran significantly less both in 12 h light: 12 h dark and constant darkness, and showed a longer endogenous period in constant darkness compared to animals in the large wheel. Moreover, after the light pulse at circadian time 13, mice in the small wheel showed a significantly smaller phase delay in running wheel activity than mice in the larger wheels. The data suggest that the magnitude of a photic phase shift depends on the amount and timing of activity the animals display in relation to this stimulus. It can be concluded that technical features of the running wheel can influence the circadian period of wheel running.     

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.     

The induction of pseudopregnancy and pregnancy by mating in albino rats exposed to constant light

Adult female albino rats anovulatory as a result of exposure to constant light (CL rats) were shown to ovulate in response to limited coital stimulation without necessarily becoming either pseudopregnant or pregnant. A higher level of coital stimulation would induce pseudopregnancy provided that intromission was not prevented. The occurrence of an ejaculation was not essential for the induction of pseudopregnancy but the incidence of pseudopregnancy was higher when a series of intromissions included one or more intromissions associated with ejaculation than when it did not. Similar findings regarding the induction of pseudopregnancy were obtained in experiments on female rats maintained under normal light-dark conditions (LD rats). Increasing the interval between intromissions from less than 1 min to as much as 20 min did not reduce the incidence of pseudopregnancy in CL rats. Splitting a set number of intromissions into two groups separated by up to 180 min did not reduce the incidence of pseudopregnancy in CL or LD rats. Conditions for the induction of pregnancy were more critical than for the induction of pseudopregnancy. CL rats showed only a low incidence of pregnancy and, if pregnant, either had small litters or did not deliver living pups even when a high level of coital stimulation from several males, including multiple ejaculations, occurred over a limited period of time. A high success rate was only observed when CL rats were mated with a single male overnight. LD rats showed a higher incidence of pregnancy than CL rats in response to a restricted amount of coital stimulation over a short time period, especially when an ejaculation was the terminal event in the mating sequence. This dependence on the occurrence of an ejaculation as the terminal event was not observed in CL rats, probably because, unlike LD rats, their uteri were not distended with fluid at the time of mating.     

Male dominance rank and reproductive success in an enclosed group of Japanese macaques: with special reference to post-conception mating

The mating behaviour and reproductive success of male Japanese macaques (Macaca fuscata) were studied in relation to the female sexual cycles, which were monitored from the plasma profiles of gonadotropins and ovarian hormones. Based on observations of the mating behaviour during four successive mating seasons and paternity identification by DNA fingerprinting in 35 out of 37 offspring born in the subsequent birth seasons, the correlations between (1) male dominance rank and timing of mating, and (2) male dominance rank and reproductive success were examined. The results may be summarized as follows. (1) The number of copulations with ejaculation by any male was positively correlated with the male dominance rank, but not with the identified numbers of offspring fathered by each male. (2) Males could not choose ovulatory females as mating partners: the number of copulations with ejaculation with females during ovulatory weeks was not related to the male's rank. Monopolized copulations in consortship were mostly observed between high-ranking males and non-lactating parous females after conception. (3) Paternity testing showed that the male copulating most frequently with a female was not the identified father in 11 out of 15 cases. Prediction of the fathers of offspring was difficult even from the number of copulations occurring at around the estimated time of ovulation. An adaptive explanation of these correlations is discussed.     

Concealed fertility and extended female sexuality in a non-human primate (Macaca assamensis)

In numerous primates living in mixed-sex groups, females display probabilistic cues of fertility to simultaneously concentrate paternity to dominant males while diluting it amongst others as a means to reduce the risk of infanticide and to increase male care for offspring. A few species, however, lack these cues and potentially conceal fertility from males; yet, to date, little is known about mating patterns and their underlying proximate mechanisms in such species. Here, we investigated mating activity and sexual consortships relative to female reproductive state in wild Assamese macaques (Macaca assamensis), a species where females lack prominent anogenital swellings and copulation calls. During two mating seasons (2837 contact hours) we recorded sexual and social behaviors, sexual consortships, and collected 1178 fecal samples (n = 15 females) which were analyzed for progestogen concentrations to assess female reproductive state and to determine the timing of ovulation and conception. Although mostly conceiving in their first ovarian cycle, females were sexually receptive throughout the entire 4-month mating season, and within-cycle mating frequencies were not increased during fertile phases. Dominant males did not monopolize fertile matings, and consortships by high-ranking males lasted for long periods, which were not exclusively linked to female fertile phases. Furthermore, females copulated promiscuously but not randomly, i.e. for almost every female, matings were concentrated to a certain male, irrespective of male rank. Collectively, we demonstrate that fertility is undisclosed to males. The extreme extended female sexuality facilitated by concealed fertility may allow females to create differentiated mating relationships within a promiscuous mating system. Our study provides important new insight into the plasticity of female sexuality in non-human primates.     

Influence of experimental illumination and seasonal variation on crossbreeding mating in the snail Biomphalaria glabrata

The crossbreeding activities of the Schistosoma mansoni vector snail Biomphalaria glabrata were counted in a laboratory aquarium throughout the year under two regimes of 12h light: 12h dark from 7 A. M. to 10 P. M. Mating increased significantly in Autumn and Winter and just missed a significant inverse correlation with temperature and a direct one with locomotion. Other similar experiments were carried out to compare mating under various illumination conditions in complete daily cycle measurements. Mating counts decreased under the regimes which submitted snail to a total exposure of 12h light and 12h dark during a daily cycle in the following sequence: 12h light:12h dark alternating hourly with light gradient, 12h light:12h dark, 1h light:1h dark and 12h dark:12h light. Under two constant illuminations, the mating scored less than under the previous conditions, except under 12h dark:12h light. Under darkness the mating count was lower than under light conditions. There was no way to differentiate the night and day rhythms of mating on different days in each regime, except for mating under 12h light:12h dark alternating with light gradient, constant dark and 12h dark:12h light conditions. Mating increased in certain light and temperature conditions, in which the intensities should have an optimum value.