Neither testosterone levels nor aggression decrease when the male Mongolian gerbil (Meriones unguiculatus) displays paternal behavior

The first studies that correlated mammalian paternal behavior and testosterone levels indicated that the concentration of this steroid hormone decreases when males exhibit paternal care. However, recent studies have also shown that testosterone levels do not decrease when males display paternal behavior. In this study, we measured testosterone levels in plasma throughout the reproductive cycle of the Mongolian gerbil. Testosterone concentrations were correlated with paternal care as well as aggression. We also examined whether there is a trade-off between paternal behavior and aggression in this mammal. Our results show that Mongolian gerbil testosterone levels do not decrease when the males give paternal care. Likewise, male Mongolian gerbils exhibit high levels of aggression while displaying paternal behavior, indicating that there is no trade-off between aggression and paternal behavior. More studies are needed to determine whether testosterone is involved in the regulation of paternal behavior in this rodent.     

Steroid hormones and aggression in female Galápagos marine iguanas

We studied steroid hormone patterns and aggression during breeding in female Galápagos marine iguanas (Amblyrhynchus cristatus). Females display vigorously towards courting males after copulating (female-male aggression), as well as fight for and defend nest sites against other females (female-female aggression). To understand the neuroendocrine basis of this aggressive behavior, we examined changes in testosterone (T), estradiol (E₂), corticosterone (CORT), and progesterone (P₄) during the mating and nesting periods, and then measured levels in nesting females captured during aggressive interactions. Testosterone reached maximal levels during the mating stage when female-male aggression was most common, and increased slightly, but significantly, during the nesting stage when female-female aggression was most common. However, fighting females had significantly lower T, but higher E₂ and P₄, than non-fighting females. It remains unclear whether these changes in hormone levels during aggressive interactions are a cause or a consequence of a change in behavior. Our results support the “challenge hypothesis”, but suggest that E₂ and/or P₄ may increase in response to aggressive challenges in females just as T does in males. Females may be rapidly aromatizing T to elevate circulating levels of E₂ during aggressive interactions. This hypothesis could explain why non-fighting females had slightly elevated baseline T, but extremely low E₂, during stages when aggressive interactions were most common. Although P₄ increased rapidly during aggressive encounters, it is unclear whether it acts directly to affect behavior, or indirectly via conversion to E₂. The rapid production and conversion of E₂ and P₄ may be an important mechanism underlying female aggression in vertebrates.     

A single injection of 17 beta-estradiol facilitates sexual behavior in castrated male mice

Sexual behavior was assessed in castrated adult CD-1 male mice given exogenous steroids under various treatment regimens. Castrated mice maintained on 20 μg testosterone (T) daily for 1 week, but given 250 μg testosterone propionate (TP) on the day of testing showed higher levels of copulatory activity than intact mice or the males receiving an additional dose of 20 μg T on the test day, although plasma testosterone levels were not different at the time of behavioral testing. Castrated males given 50, 125, or 250 μg TP for 1 week including the day of testing showed higher levels of sexual behavior than males receiving the same doses of TP only once, on the test day. A single injection of 17β-estradiol (E₂) completely restored the male copulatory pattern, including ejaculation, in castrated mice under every condition examined. Testosterone and dihydrotestosterone (DHT) were less effective than E₂, as was the combination of E₂ and DHT. The relative efficacy of a single dose of T, DHT, and E₂ plus DHT was dependent upon factors such as the delay between steroid administration and testing, as well as whether or not the castrated mice received androgen replacement prior to testing. Estradiol benzoate (E₂B) was not capable of restoring sexual behavior in castrated mice in this study. The comparison of results obtained with TP, T, E₂, and E₂B suggests that an appreciable, but not necessarily sustained, elevation of E₂ levels in the brain may be critical in the facilitation of male copulatory behavior in mice.     

A single injection of 17β-estradiol facilitates sexual behavior in castrated male mice

Sexual behavior was assessed in castrated adult CD-1 male mice given exogenous steroids under various treatment regimens. Castrated mice maintained on 20 μg testosterone (T) daily for 1 week, but given 250 μg testosterone propionate (TP) on the day of testing showed higher levels of copulatory activity than intact mice or the males receiving an additional dose of 20 μg T on the test day, although plasma testosterone levels were not different at the time of behavioral testing. Castrated males given 50, 125, or 250 μg TP for 1 week including the day of testing showed higher levels of sexual behavior than males receiving the same doses of TP only once, on the test day. A single injection of 17β-estradiol (E₂) completely restored the male copulatory pattern, including ejaculation, in castrated mice under every condition examined. Testosterone and dihydrotestosterone (DHT) were less effective than E₂, as was the combination of E₂ and DHT. The relative efficacy of a single dose of T, DHT, and E₂ plus DHT was dependent upon factors such as the delay between steroid administration and testing, as well as whether or not the castrated mice received androgen replacement prior to testing. Estradiol benzoate (E₂B) was not capable of restoring sexual behavior in castrated mice in this study. The comparison of results obtained with TP, T, E₂, and E₂B suggests that an appreciable, but not necessarily sustained, elevation of E₂ levels in the brain may be critical in the facilitation of male copulatory behavior in mice.     

Non-invasive administration of 17β-estradiol rapidly increases aggressive behavior in non-breeding,but not breeding,male song sparrows

17β-Estradiol (E₂) acts in the brain via genomic and non-genomic mechanisms to influence physiology and behavior. There is seasonal plasticity in the mechanisms by which E₂ activates aggression, and non-genomic mechanisms appear to predominate during the non-breeding season. Male song sparrows (Melospiza melodia) display E₂-dependent territorial aggression throughout the year. Field studies show that song sparrow aggression during a territorial intrusion is similar in the non-breeding and breeding seasons, but aggression after an intrusion ends differs seasonally. Non-breeding males stop behaving aggressively within minutes whereas breeding males remain aggressive for hours. We hypothesize that this seasonal plasticity in the persistence of aggression relates to seasonal plasticity in E₂ signaling. We used a non-invasive route of E₂ administration to compare the non-genomic (within 20 min) effects of E₂ on aggressive behavior in captive non-breeding and breeding season males. E₂ rapidly increased barrier contacts (attacks) during an intrusion by 173% in non-breeding season males only. Given that these effects were observed within 20 min of E₂ administration, they likely occurred via a non-genomic mechanism of action. The present data, taken together with past work, suggest that environmental cues associated with the non-breeding season influence the molecular mechanisms through which E₂ influences behavior. In song sparrows, transient expression of aggressive behavior during the non-breeding season is highly adaptive: it minimizes energy expenditure and maximizes the amount of time available for foraging. In all, these data suggest the intriguing possibility that aggression in the non-breeding season may be activated by a non-genomic E₂ mechanism due to the fitness benefits associated with rapid and transient expression of aggression.     

Female marmosets' behavioral and hormonal responses to unfamiliar intruders

The endocrine control mechanisms for female mammalian aggression have been largely unstudied. Although it has been proposed that androgens may modulate female aggressive behavior in a similar manner to males, very little conclusive evidence exists. Previous work in male marmosets found that post‐encounter increases in testosterone (T) were dependent on the intensity of aggression displayed during the aggressive encounter. We exposed female marmosets (Callithrix kuhlii), a monogamous and biparental primate, to aggressive interactions with unfamiliar intruders. Individual female marmosets exhibited changes in T and estradiol (E₂) that are associated with aggressiveness dependent on the intensity of aggression displayed as well as their role during the encounter. Resident females exhibited increased E₂ immediately following an encounter in which they displayed high rates of aggression. If resident females received high rates of aggression from the intruder, the resident displayed increased T 24 hr following the encounter. Interestingly, if the female was an intruder in the encounter, the intensity of her aggression was associated with increased cortisol immediately following the trials, whereas received aggression was associated with increased T and E₂ immediately following the trial. Female primates do exhibit situation‐dependent changes in gonadal steroids in association with aggression that may serve to prime them for future aggressive interactions. Am. J. Primatol. 73:1072–1081, 2011. © 2011 Wiley‐Liss, Inc.     

Androgen‐metabolizing enzymes show region‐specific changes across the breeding season in the brain of a wild songbird

The Lapland longspur (Calcarius lapponicus) is an arctic‐breeding songbird that shows rapid behavioral changes during a short breeding season. Changes in plasma testosterone (T) in the spring are correlated with singing but not territorial aggression in males. Also, T treatment increases song but not aggression in this species. In contrast, in temperate‐zone breeders, song and aggression are highly correlated, and both increase after T treatment. We asked whether regional or temporal differences in androgen‐metabolizing enzymes in the longspur brain explain hormone‐behavior patterns in this species. We measured the activities of aromatase, 5α‐reductase and 5β‐reductase in free‐living longspur males. Aromatase and 5α‐reductase convert T into the active steroids 17β‐estradiol (E₂) and 5α‐dihydrotestosterone (5α‐DHT), respectively. 5β‐Reductase deactivates T via conversion to 5β‐DHT, an inactive steroid. We examined seven brain regions at three stages in the breeding season. Overall, aromatase activity was high in the hypothalamus, hippocampus, and ventromedial telencephalon (containing nucleus taeniae, the avian homologue to the amygdala). 5β‐Reductase activity was high throughout the telencephalon. Activities of all three enzymes changed over time in a region‐specific manner. In particular, aromatase activity in the rostral hypothalamus was decreased late in the breeding season, which may explain why T treatment at this time does not increase aggression. Changes in 5β‐reductase do not explain the effects of plasma T on aggressive behavior. © 1999 John Wiley & Sons, Inc. J Neurobiol 41: 176–188, 1999     

Androgens and estrogens induce seasonal‐like growth of song nuclei in the adult songbird brain

In seasonally breeding songbirds, the brain regions that control song behavior undergo dramatic structural changes at the onset of each annual breeding season. As spring approaches and days get longer, gonadal testosterone (T) secretion increases and triggers the growth of several song control nuclei. T can be converted to androgenic and estrogenic metabolites by enzymes expressed in the brain. This opens the possibility that the effects of T may be mediated via the androgen receptor, the estrogen receptor, or both. To test this hypothesis, we examined the effects of two bioactive T metabolites on song nucleus growth and song behavior in adult male white‐crowned sparrows. Castrated sparrows with regressed song control nuclei were implanted with silastic capsules containing either crystalline T, 5α‐dihydrotestosterone (DHT), estradiol (E₂), or a combination of DHT+E₂. Control animals received empty implants. Song production was highly variable within treatment groups. Only one of seven birds treated with E₂ alone was observed singing, whereas a majority of birds with T or DHT sang. After 37 days of exposure to sex steroids, we measured the volumes of the forebrain song nucleus HVc, the robust nucleus of the archistriatum (RA), and a basal ganglia homolog (area X). All three steroid treatments increased the volumes of these three song nuclei when compared to blank‐implanted controls. These data demonstrate that androgen and estrogen receptor binding are sufficient to trigger seasonal song nucleus growth. These data also suggest that T's effects on seasonal song nucleus growth may depend, in part, upon enzymatic conversion of T to bioactive metabolites. © 2003 Wiley Periodicals, Inc. J Neurobiol 57:130–140, 2003     

The effects of testosterone, estradiol, and dihydrotestosterone on male mouse (Mus musculus) ultrasonic vocalizations

Two experiments examined the effects of the free forms of testosterone (T), dihydrotestosterone (DHT), and estradiol (E₂) upon male mouse (Mus musculus) courtship vocalizations and seminal vesicle weight. In the first experiment, administration of T to castrated males was associated with a large number of vocalizations and large seminal vesicle weights, DHT was associated with large seminal vesicle weights but very few vocalizations, whereas E₂ was associated with low measures of both vocalization and seminal vesicle weight. In the second experiment, T and DHT had effects highly similar to those of the previous experiment; however, in contrast to the previous experiment, both low and high dosages of E₂ were associated with large numbers of ultrasonic vocalizations but small seminal vesicles. A mixture of E₂ and DHT was very similar to T in its effect upon both measures. We suggest from these results that hormonal mechanisms similar to those reported for rat sex behavior may interact with situational variables to determine the expression of male mouse courtship.     

Effects of quinestrol and levonorgestrel on prolactin serum concentration in lactating Mongolian gerbils (Meriones unguiculatus) and reproductive parameters of their offspring

The effects of the two sterilants, quinestrol (QE) and levonorgestrel (LNG) on serum prolactin (PRL) level in lactating Mongolian gerbils and reproductive parameters of their offspring were examined in the study. Both sterilants increased the serum PRL level in lactating gerbils. The body weight as well as weights of the ovary, testis, epididymides, and seminal vesicles were lower, whereas that of the uterus was higher in the pups originating from QE-treated mothers in comparison to controls. Histological ovarian sections of the offspring from QE-treated mothers contained only growing follicles, whereas their uterine sections showed a thinner endometrium, thicker myometrium, and greater epithelial-cell height than in controls. The histometrical testis characteristics as well as sperm concentration and motility of male pups from QE-treated mothers were lower compared to those of the control group. The serum gonadotropin levels of female pups from mothers treated with QE were lower, whereas the serum estradiol (E₂) and progesterone (P₄) levels were higher than in control gerbils. In contrast, serum gonadotropin and testosterone (T) levels of male pups from QE-treated mothers were lower compared to controls. LNG did not affect the examined parameters of the offspring. The offspring from QE-treated mothers was infertile, whereas the offspring from LNG-treated mothers was fertile. In summary, QE and LNG have a stimulatory effect on PRL level in lactating gerbils. It also appears that QE administered via milk to mothers affects reproductive processes of their offspring.     

Testosterone, paternal behavior, and aggression in the monogamous California mouse (Peromyscus californicus)

Testosterone (T) mediates a trade-off, or negative correlation, between paternal behavior and aggression in several seasonally breeding avian species. However, the presence or absence of a T-mediated trade-off in mammals has received less attention. We examined the relationship between paternal behavior and territorial aggression in the biparental California mouse, Peromyscus californicus. In contrast to seasonally breeding birds, T maintains paternal behavior in this year-round territorial species. Castration reduced paternal behavior, whereas T replacement maintained high levels of paternal behavior. We hypothesize that T is aromatized in the brain to estradiol, which in turn stimulates paternal behavior. In contrast to paternal behavior, aggressive behavior was not reduced by castration. Interestingly, only sham males showed an increase in aggression across three aggression tests, while no change was detected in castrated or T-replacement males. Overall, trade-offs between aggression and paternal behavior do not appear to occur in this species. Measures of paternal behavior and aggression in a correlational experiment were actually positively correlated. Our data suggest that it may be worth reexamining the role that T plays in regulating mammalian paternal behavior.     

Paternal aggression in a biparental mouse: parallels with maternal aggression

Environmental and social factors have important effects on aggressive behaviors. We examined the effect of reproductive experience on aggression in a biparental species of mouse, Peromyscus californicus. Estrogens are important in mediating aggressive behavior so we also examined estrogen receptor expression and c-fos for insights into possible mechanisms of regulation. Parental males were significantly more aggressive than virgin males, but no significant differences in estrogen receptor alpha or beta expression were detected. Patterns of c-fos following aggression tests suggested possible parallels with maternal aggression. Parental males had more c-fos positive cells in the medial amygdala, and medial preoptic area relative to virgin males. The medial preoptic area is generally considered to be relatively less important for male-male aggression in rodents, but is known to have increased activity in the context of maternal aggression. We also demonstrated through habituation-dishabituation tests that parental males show exaggerated investigation responses to chemical cues from a male intruder, suggesting that heightened sensory responses may contribute to increased parental aggression. These data suggest that, in biparental species, reproductive experience leads to the onset of paternal aggression that may be analogous to maternal aggression.     

Differential effects of testosterone metabolites in the neonatal period on open-field behavior and lordosis in the rat

Two experiments were done to compare the effects of neonatal exposure to testosterone and its major metabolites, dihydrotestosterone (DHT) and estradiol (E₂), on the development of sex differences in open-field behavior in the rat. In Experiment 1 female rats administered either testosterone propionate (TP), DHT, or estradiol benzoate (EB) were found as adults to have low activity scores, more typical of adult males, when compared to the high scores of oil-treated females. In Experiment 2 the adult open-field behavior of female rats treated neonatally with testosterone or the metabolites was compared to that of male rats treated from Day 1 to 10 of life with the aromatizing enzyme inhibitor, androst-1,4,6-triene-3,17-dione (ATD). These same animals were later tested for lordotic behavior after gonadectomy and priming with EB and progesterone. All male animals and female animals exposed neonatally to testosterone or to either of the metabolites had suppressed open-field activity scores compared to oil-treated females. However, the lordotic behavior of females exposed to DHT and of males exposed to ATD was not defeminized and was comparable to that of oil-treated females. These observations were discussed in terms of a role for the androgenic actions of testosterone in establishing sex differences in nonreproductive behavior in the rat.     

The Association Between Male Offspring Aggression and Paternal and Maternal Behavior of Peromyscus Mice

Parents influence offspring aggression through genetic and non‐genetic mechanisms, although the latter are less well understood. To examine potential non‐genetic effects of parents on offspring, we cross‐fostered the highly aggressive and biparental California mouse (Peromyscus californicus) and the less aggressive, less parental white‐footed mouse (Peromyscus leucopus). In‐fostered animals within each species were used as controls. We examined associations between the foster parents’ behavior and aggression of the fostered male offspring in resident–intruder (R–I) and neutral arena aggression tests. When both species and fostering groups were combined, R–I aggression of offspring was positively associated with paternal time spent retrieving pups. In contrast, aggression in a neutral arena was negatively associated with a composite score of maternal behavior. We discuss how our findings regarding paternal retrievals may explain previously reported effects of cross‐fostering on male aggression.     

Action of androgen and estrone implants on sexual behavior and reproductive organs of castrated male rabbits

Sexually mature but inexperienced male rabbits were castrated, immediately implanted with either testosterone (T), estrone (E₁), dihydrotestosterone (DHT), T + E₁, or DHT + E₁, and tested for male sexual behavior. Other castrates were not implanted, and testing was either begun immediately (Ca-I) or delayed for 4 weeks (Ca-D). Intact males served as controls (C). Latency to mount a teaser female and to ejaculate into an artificial vagina was tested twice in a morning three times per week for 8 weeks. Then, animals were sacrificed, and reproductive organs were measured. The Ca-I group responded slowly to sexual training and ceased nearly all sexual activity by 8 weeks, whereas the Ca-D males seldom displayed interest in the teaser female. Intact controls and the T and T + E₁, groups all responded to the teaser and mounted and ejaculated within a few seconds. DHT and E₁, individually maintained the sexual activity of castrates equivalent to that of C for 4–5 weeks, but the time required to mount and, particularly, to ejaculate increased thereafter. The results with DHT + E₁ were equivocal in that castrates with this hormone combination sustained sexual activity equivalent to that of the controls for 7 weeks, but one animal in particular became sexually inactive the last week of the experiment. Penis weight was at least partially maintained in all implanted castrates. Accessory sex gland weight was smallest in the DHT group and was significantly increased in the T + E₁ and DHT + E₁ groups. The largest ejaculates of fluid were obtained in the group receiving E₁ alone. These results may be interpreted to indicate a role of both androgen and estrogen centrally and peripherally in the rabbit.     

Effects of repeated pup exposure on behavioral,neural, and adrenocortical responses to pups in male California mice (Peromyscus californicus)

In biparental mammals, the factors facilitating the onset of male parental behavior are not well understood. While hormonal changes in fathers may play a role, prior experience with pups has also been implicated. We evaluated effects of prior exposure to pups on paternal responsiveness in the biparental California mouse (Peromyscus californicus). We analyzed behavioral, neural, and corticosterone responses to pups in adult virgin males that were interacting with a pup for the first time, adult virgin males that had been exposed to pups 3 times for 20 min each in the previous week, and new fathers. Control groups of virgins were similarly tested with a novel object (marble). Previous exposure to pups decreased virgins' latency to approach pups and initiate paternal care, and increased time spent in paternal care. Responses to pups did not differ between virgins with repeated exposure to pups and new fathers. In contrast, repeated exposure to a marble had no effects. Neither basal corticosterone levels nor corticosterone levels following acute pup or marble exposure differed among groups. Finally, Fos expression in the medial preoptic area, ventral and dorsal bed nucleus of the stria terminalis was higher following exposure to a pup than to a marble. Fos expression was not, however, affected by previous exposure to these stimuli. These results suggest that previous experience with pups can facilitate the onset of parental behavior in male California mice, similar to findings in female rodents, and that this effect is not associated with a general reduction in neophobia.     

From here to paternity: Neural correlates of the onset of paternal behavior in California mice (Peromyscus californicus)

In a minority of mammalian species, including humans, fathers play a significant role in infant care. Compared to maternal behavior, the neural and hormonal bases of paternal care are poorly understood. We analyzed behavioral, neuronal and neuropeptide responses towards unfamiliar pups in biparental California mice, comparing males housed with another male (“virgin males”) or with a female before (“paired males”) or after (“new fathers”) the birth of their first litter. New fathers approached pups more rapidly and spent more time engaging in paternal behavior than virgin males. In each cage housing two virgin males, one was spontaneously paternal and one was not. New fathers and paired males spent more time sniffing and touching a wire mesh ball containing a newborn pup than virgin males. Only new fathers showed significantly increased Fos-like immunoreactivity in the medial preoptic nucleus (MPO) following exposure to a pup-containing ball, as compared to an empty ball. Moreover, Fos-LIR in the bed nucleus of the stria terminalis (STMV and STMPM) and caudal dorsal raphe nucleus (DRC) was increased in new fathers, independent of test condition. No differences were found among the groups in Fos-LIR in oxytocinergic or vasopressinergic neurons. These results suggest that sexual and paternal experiences facilitate paternal behavior, but other cues play a role as well. Paternal experience increases Fos-LIR induced by distal pup cues in the MPO, but not in oxytocin and vasopressin neurons. Fatherhood also appears to alter neurotransmission in the BNST and DRC, regions implicated in emotionality and stress-responsiveness.     

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.     

Presence of the father and parental experience have differentiated effects on pup development in Mongolian gerbils (Meriones unguiculatus)

Conflicting data exist on the importance of the father and parental experience during development in rodents. The purpose of this study was to investigate the effects of these two variables on development in Mongolian gerbils. Forty pairs of males and females with a litter size of between 4 and 7 pups were used as subjects. Twenty couples had no experience in raising young. After the birth of their pups, four experimental groups were formed: (I) inexperienced mother and father; (II) inexperienced mother; (III) experienced mother and father and (IV) experienced mother. When the pups reached 10 days of age, pup and parental behavior was recorded in experimental sessions of 15 min on 11 consecutive days. Through the statistical analysis it was found that the presence of the father significantly increased the physical contact between pups and parents and that pups opened their eyes earlier in comparison to the groups without the father. On the other hand, parental experience had a significant influence on the behavior of the pups (locomotion inside and outside the nest, and self-grooming). The results of this study suggest that parental experience and the presence of the father have differentiated effects on development in Mongolian gerbils.     

Marking behavior is innate and not learned in the Mongolian gerbil.

We studied whether marking behavior in Mongolian gerbils would be innate or learned behavior. The marking behavior was defined as "animals rubbing their abdominal scent glands on small protruding objects". Between 21 and 90 days of age, Mongolian gerbils, which were kept under such conditions that they would be unable to learn this behavior, were observed at intervals of 5-15 days to find out if there were signs of the behavior or not. Six male and four female Mongolian gerbils were used for observing. Neonate Mongolian gerbils during the age of 3 to 28 days were fostered by ICR mother mice. Weaning Mongolian gerbils were then individually kept away from the others. Marking behavior was observed in 2 out of 6 males at 50 days of age and 2 of 4 females at 60 days and the mean frequency of the marking behavior for 10 min was 3.5 in the males and 5.0 in the females. These results suggest that marking behavior was innate and not learned behavior in Mongolian gerbils.