Rats can discriminate between the urine odors of genetically identical mice maintained on different diets.

Male Long-Evans hooded rats were tested in a habituation-dishabituation procedure for their ability to discriminate between the urine odors of male C57BL/6J mice maintained on two different diets. There were discriminable differences between the urinary odors of two individual mice maintained on different diets. The rats did not dishabituate when presented with urine odors from two individuals on the same diet or two odor samples from the same individual. These results indicate that individual urinary odors of genetically identical mice are influenced by diet. We discuss the hypothesis that diet may act together with genetic differences, commensal bacteria, and hormonal changes to convey olfactory information used for individual recognition.     

Selective depletion of bacteria alters but does not eliminate odors of individuality in Rattus norvegicus

To determine if odors of individuality are influenced by the removal of Gram-negative or Gram-positive gut bacteria, Long-Evans rats were trained in an operant olfactometer to discriminate between the odors of two individual conspecifics and their operant responses to three different odors in randomly presented probe trials were analyzed. Significantly more responses were made to the probe odors from two known individuals than to the probe odors from known individuals with their Gram-negative bacteria eliminated (Experiment 1) or their Gram-positive bacteria eliminated (Experiment 2). Responses to the probe odors from known rats with bacterial selectively depleted did not differ significantly from responses to probe odors from unknown rats. These results support the hypothesis that the urinary odor of an individual rat is altered by the removal of specific gut bacteria. In Experiments 3 and 4, subjects made fewer errors in learning to discriminate between the odors of the familiar rats whose bacteria had been selectively depleted than between the odors of unknown rats. This "savings effect" indicates that some components of the individual urinary odors were retained after the removal of specific gut bacteria. Thus, the eliminated bacteria were not totally responsible for the odors of individuality. The outcome of Experiments 3 and 4 also indicates that conclusions regarding the recognition of odors by rats should not be made on the outcome of probe trial experiments alone.     

Preferences of pre-weanling Long-Evans rats for anal excreta of adult males and females

Long-Evans hooded rat pups were reared with their dam alone, their dam and sire or their dam 12 hrs per day and their sire 12 hrs per day and tested for preferences for anal excreta from adult male and female rats when they were 18 to 20 days of age. In all rearing conditions the anal excreta of the rat's own dam was preferred to that of strange dams and the excreta of both own and strange dams was preferred to that of virgin females. Anal excreta from virgin females was preferred to no odor as was anal excreta of unmated males, strange sires and the rat's own sire. Only rats reared with their dam and sire preferred the odor of their own sire to that of other males. The results suggest that pre-weanling rats may attend to different dimensions of the odors of conspecifics, such as the species odor, maternal odor, and individual odor and that different rearing conditions may influence the odor dimension to which the rat attends.     

Olfactory discrimination and incentive value of non copulating and sexually sluggish male rats

Some apparently healthy male rats fail to copulate despite being tested on repeated occasions with receptive females and are called non copulating (NC) rats. NC rats sniff and lick the female genitals, and show normal erectile and ejaculatory functions and hormonal levels. Sexually sluggish (S) male rats take a long time to ejaculate or sometimes they don't achieve ejaculation when tested repeatedly with receptive females. The aim of the present study was to determine if NC and S males can discriminate sexually relevant olfactory cues such as urine from estrous or anestrous female and urine from sexually experienced males. We also tested odors like amyl acetate and mint using an olfactory discrimination test. In a second experiment we evaluated if a sexually receptive female has a preference for a copulating (C) male, for a NC male, or for a S male in a sexual incentive motivation test. This would let us determine if a NC and an S male are equally attractive than a C male to a sexually receptive female. The olfactory test revealed that C, NC and S males have the same ability to discriminate sexually relevant odors. As well, all males clearly discriminate non sexual odors like amyl acetate and mint suggesting that NC and S male rats do not have alterations in their olfactory system. With respect to the sexual incentive motivation test, females spend the same time in the incentive zone of the NC and C males. As well, females spent the same time in the incentive zone of S and C males. These results demonstrate that NC, S and C males are equally attractive to receptive females.     

Preferences of pre- and post-weanling long-evans rats for nest odors

Three experiments were conducted on the preferences of infant rats for nest odors. Pre-weanling rats preferred their own nest odor to nests of strange litters and showed low preferences for the nest odors of virgin females. Nest odors of unmated males were investigated less than the pup's own nest odor but more than the odors of virgin females. Male and female rats did not differ in odor preferences. Rats reared with both their dam and sire did not differ in their odor preferences from rats reared with their dam alone when tested at 16–20 days of age, but when tested at 33–38 days of age they showed a greater preference for male odors. Post-weanling rats did not show a preference for the nest odors of lactating females and began to show sex differences in their preferences.     

The role of the hippocampal system in social odor discrimination and scent-marking in female golden hamsters (Mesocricetus auratus)

Female golden hamsters (Mesocricetus auratus) received aspiration lesions of the parahippocampal region (PARA) or electrolytic lesions of the fimbria-fornix (FNX) and were tested for their (a) discrimination between odors of individual males in a habituation-discrimination task, (b) preference for male over female odors, and (c) scent-marking in response to conspecific odors. Both lesion groups habituated to repeated presentations of a male's odor. However, only FNX females discriminated between scents of individual males, whereas PARA females did not. Neither lesion eliminated female preferences for male odors. Females with FNX lesions showed decreased levels of scent marking, but those with PARA lesions had more subtle deficits. Thus, the PARA, but not the subcortical connections of the hippocampus, is critical for discrimination of the odors of individuals.     

Lesions centered on the medial amygdala impair scent-marking and sex-odor recognition but spare discrimination of individual odors in female golden hamsters.

The medial amygdala (Me) has been implicated in various social behaviors that depend on chemosensory cues, but its precise role in discriminating and learning social odors is not known. Female golden hamsters (Mesocricetus auratus) received electrolytic lesions of the Me or sham surgery and were tested for their ability to (a) discriminate between odors of individual males in a habituation-discrimination task, (b) show preferences for male over female odors in a Y maze, and (c) scent-mark in response to male and female odors. All females discriminated between scents of individual males. In contrast, Me lesions eliminated female preferences for male odors in a Y maze. Females with Me lesions also showed a substantial reduction in vaginal marking and virtually no flank marking in response to odors. Thus, the Me in female hamsters is critical for differential investigation of opposite-sex odors and for scent-marking behavior but is not involved in discrimination between odors of individuals.     

Conspecific odor investigation by gray short-tailed opossums (Monodelphis domestica)

Gray short-tailed opossums (Monodelphis domestica) are small marsupials, which have recently become the subjects of numerous laboratory investigations. While these opossums have well-developed olfactory systems and complex scent-marking behaviors, the significance of their use of odors in conspecific communication is still poorly understood. Investigation of body odors by male and female opossums was examined in the present study. Males investigated flank and urine odors of nonestrous adult females significantly more than controls, but not urine from sexually inexperienced juvenile females or urine of females at cytological estrus. Since in this species females have an induced estrus, it would be advantageous for males to investigate and follow the odors of urine of diestrous females, which become receptive in proximity to males. Female opossums investigated odors of male mandibles and suprasternal glands significantly more than controls but not odors of male urine. We suggest that the use of glandular secretions is more common and more effective than urine for intraspecific communication between gray short-tailed opossums: In the semiarid conditions inhabited by the opossums, glandular secretions are less volatile and are effective for longer periods than urine and would be of greater value in intraspecific communication if, as suggested in the literature, these opossums are nomadic and meet one another infrequently.     

Effects of vomeronasal organ removal on individual odor discrimination, sex-odor preference, and scent marking by female hamsters

Removal of the vomeronasal organ (VNX) did not eliminate the ability of female hamsters to discriminate between individual male's flank gland or urine odors in a habituation/discrimination task nor did it impair preference for male odors over female odors from a distance. Vomeronasal organ removal did reduce overall levels of investigation of flank gland odor in the habituation/discrimination task. Although VNX females did not show severe impairments in the frequency of either flank or vaginal marking in response to odors, they did show an abnormal pattern of marking. VNX females, unlike shams, did not flank mark more to female odors than to male odors, nor did they vaginal mark more to male odors than to female odors. Thus, the vomeronasal organ in female hamsters appears to be important for differences in scent marking toward male and female odors, but is not essential for discrimination of individual odors or for preferences for male over female odors.     

The vomeronasal organ is involved in discrimination of individual odors by males but not by females in golden hamsters

The vomeronasal organ (VNO) has a wide variety of functions in terrestrial vertebrates, some of which involve responses to classical pheromones whereas others do not. We examined the role of the VNO in discrimination of individual differences in odors of male and female golden hamsters using a habituation paradigm. Removal of the VNO resulted in elimination of the ability of male hamsters to discriminate between some individually distinctive odors (e.g., flank gland secretion), but not others (e.g., urine). In females, such lesions had no effect. The type of test trial also influenced the results; in test trials employing a single, novel odor, removal of the VNO in males did have an effect but in test trials in which both the novel and the familiar odor were presented, VNO removal had no effect. It is concluded that (a) there is a sex difference in the role of the VNO in the discrimination of individual odors, (b) the role of the VNO in discrimination of individual odors varies from odor to odor, and (c) deficits due to VNO removal are more readily observed in more difficult tasks.     

Female discrimination of male odors correlated with male genotype at the T locus: A response to T-locus or H-2-locus variability?

Female house mice (Mus musculus), derived from several populations of wild-caught mice, were tested for their ability to discriminate between males whose genotype at the T locus was +/+ and those whose genotype was +/t, using odor cues alone. Females spent more time near the odors or +/+ males than near the odors of +/t males. This preference was independent of the T-locus genotype of the female and the particular type of t allele carried by either the male or the female. A female's preference, however, did appear to be related to the genotype of her parents. Females with one +/t parent were more likely to prefer +/t males than were females whose parents were both +/+. In a second experiment 18 females were tested with odors from soiled bedding of recombinant males whose genotype varied at the T locus but who were similar at the H-2 locus. As a control, these 18 females were also tested with bedding of wild-derived +/+ and +/t^{w semilethal} males. Females tested with recombinant males preferred odors of males not carrying lethal t alleles over those of males carrying two lethal t alleles, indicating that T-locus variability, not H-2-locus variability, is responsible for odor differences between +/+ and +/t males. Female responses to odors of recombinanat males did not differ from those to odors of +/+ and +/t^{w semilethal} males. Responses of mice to odor differences associated with T-locus variability may have evolved independently of responses to odor variability associated with the H-2 locus.This work was supported by NIH Grant HD 15997-01.     

Sex and strain differences in the developmental activity profile of the rat tested over clean vs home cage bedding

In the first of two experiments, CD rat litters were used to characterize activity patterns obtained in a size-adjustable, single photodetector chamber. Beginning on postnatal Day 10 or 12, pups were tested repeatedly over clean bedding (C) or over bedding removed from each pup's home cage (HC). In C rats of both sexes and in HC females, short-term activity levels peaked on Day 16. However, HC males displayed an earlier and even greater elevation in activity from Day 12 to 16. This overall pattern was found in rats tested either every second or fourth day. In the second experiment, Long-Evans pups were assigned to each testing condition (C vs HC) and activity measured beginning on Day 12. Peak levels were seen in all Long-Evans rats on Day 16 and only females showed significant alterations as a function of bedding condition. When overall activity levels of the two strains were compared, significant differences were found on Days 12, 24, 30, and 120 in males, and 12, 24, and 30 in females. Significant differences between strains in activity as a function of bedding condition were found in males on Days 12, 20, 24, and 120 and in females on Days 12, 30, and 60. These data confirm the generality of a developmental hyperactivity phase in isolated juvenile rats. However, different patterns of hyperactivity were found in male vs female rats across strains. CD males were more active in the presence of HC olfactory cues, while in Long-Evans rats, female activity was affected more by bedding condition.     

Two-way shuttlebox avoidance conditioning in three DAB rat stocks

Avoidance conditioning in FH rats, characterized by a peripheral serotonin deficiency, was compared with that in two stocks presumed to be their ancestors, Long-Evans (LE) and Wistars (W). Escapes, conditional avoidance response (CAR), and activity (locomotion) of the animals were studied in a two-way shuttlebox. FH females made fewer escapes and more CAR's, and were more active than both LE and W females. The behavioral differences among the males of the three stocks were not statistically significant. A CNS serotonin deficiency may be involved in the conditioning and activity differences among the females of the three stocks. Possible variation between sexes should be considered in investigating levels of CNS serotonin in FH rats, as well as other stocks.     

A comparison of the contribution of the major histocompatibility complex (MHC) and Y chromosomes to the discriminability of individual urine odors of mice by Long-Evans rats

A go/no-go operant task was used to assess the ability of male Long-Evans rats to discriminate between the urine odors from pairs of intact MHC congenic mice (C57BL/6-H-2K^b/J and C57BL/6-H-2K^bm1/ByJ), intact Y congenic mice (DBA1 and DBA1.C57BL10-Y), and castrated Y congenic mice of these two strains. The MHC congenic strains differ in alleles of the H-2 K locus, while the Y congenic strains differ in the nonrecombining part of the Y chromosome. Analysis of the number of correct responses to a criterion of 85% correct on each block of 20 trials revealed that the ability of the subjects to discriminate between urine odors did not differ whether samples were from pairs of intact MHC congenic mice, intact Y congenic mice, or castrated Y congenic mice. These findings are consistent with the hypothesis that individually unique urine odors may be influenced both by genes in the nonrecombining part of the Y chromosome and by genes in the major histocompatibility complex of chromosome 17. These odors are not androgen dependent. Such urinary chemical signals may be involved in pregnancy block (the Bruce effect), aggression, and other mouse social behaviors.     

Not all rat strains are equal: differential unconditioned fear responses to the synthetic fox odor 2,4,5-trimethylthiazoline in three outbred rat strains

Predator odors induce unconditioned fear in rats; however, the synthetic predator odor 2,4,5-trimethylthiazoline (TMT) either elicits robust fear behavior (e.g., freezing) or no fear responses at all. The authors investigated whether this is due to the use of different outbred rat strains. TMT induced robust freezing in Sprague-Dawley and Long-Evans rats but not in Wistar rats. All 3 strains avoided TMT, but Wistar rats were less sensitive to TMT. Wistar rats are capable of freezing; all 3 strains displayed the same amount of odor-cue conditioned freezing. Thus, TMT is a robust unconditioned fear stimulus in rats, and prior negative results from other laboratories were due to the choice of a rat strain (Wistar) that is less responsive to TMT.     

Odor modulation of ventral scent gland marking in the Mongolian gerbil

Gerbil odors were allowed to accumulate in individual home cages for several months in order to provide stable olfactory stimuli that might modulate the number of ventral scent gland marks produced by both male and female Mongolian gerbils. Male marking levels for most test conditions were 2-3 times larger than those in females. In general, the relative patterns of responsiveness to particular odors were similar in both sexes. The number of scent marks in home cages was significantly smaller than the number obtained in clean cages. This suppression was not strongly dependent on the odors emanating from an individual's own ventral gland sebum. Marking levels in the home cages of other gerbils were substantially higher than those seen in the test animal's own cage and were maximal in the home cages of other males. Behaviorally receptive females marked at significantly higher levels in home, clean, and conspecific cages than did nonreceptive females. Conspecific urine odors, especially those from females, increased marking levels significantly over those obtained in home cages. In males, home cage marking was significantly enhanced by the presence of a female. Females, on the other hand, marked at similar levels in home cages whether a potential mating partner was present or not. Both sexes showed significant reductions in marking levels in another's cage if the resident animal was present. These results suggest that when sufficient gerbil-related odors are available to a test animal, its rate of scent-marking is altered, depending on the particular odor environment. Some gerbil-related odors increase marking levels, whereas others decrease marking. Scent-marking then, like other olfactory guided behaviors and odor preferences, is context specific.     

Habituation to individual odors occurs following brief,widely-spaced presentations

The possibility that male guinea pigs (Cavia porcellus) habituate to individual female conspecific urine odors following very brief, widely-spaced presentations was tested. On day 1 of test week 1, each male was exposed for 2 min to urine of one female, on day 3 to urine of a second female and on day 5 to urine of a third female. The number of seconds investigating the samples was recorded. This same procedure was repeated for 3 more weeks. On weeks 5 through 8, three novel females provided urine samples for testing which was conducted in an identical manner. On week 9, urine samples of the original 3 donors was tested. Results indicated that male guinea pigs habituated to urine odors presented for only 2 min 7 days apart and that dishabituation occurred following presentation of samples from novel donors. These data demonstrate an exquisite sensitivity to female urine odors, a long memory for individual differences in these odors, and a consistency over time in the composition of an individual's urine.     

Development of sex differences in the response of spiny mouse pups to adult male odors

Using a three-choice preference test, olfactory-mediated investigatory activity in response to adult male urine odor was examined in a precocially active rodent, the spiny mouse (Acomys cahirinus) aged between 3-26 days. Temporally related sex differences were seen in the time spent in the presence of the odors of father's or unfamiliar adult male's urine, or distilled (control) water. Neither male nor female pups discriminated between odors from the father and strange adult males. After the first olfactory test, when the pups were aged between four and six days, male pups strongly preferred to stay in the vicinity of urine odors of adult males, whereas female pups avoided odors of adult males and remained in the enclosure with the control odor source. To our knowledge this is the first time that such a behavioral sex difference related to olfaction has been shown to occur in young rodent pups. We suggest that the sexually dimorphic response of the pups is associated with the development of later sex differences in behavior.     

Sexually dimorphic enhancement by estradiol of male urinary odor detection thresholds in mice

We asked whether sex and adult estrogen exposure influence the detection thresholds for urinary odors used by mice to guide their social behaviors. Gonadectomized (GDX) male and female mice were trained on a two-choice food-motivated task to determine detection thresholds for male urinary odors. There was no significant sex difference in the detection of these odors by GDX subjects without hormone replacement. However, during treatment with estradiol benzoate (EB), GDX females, but not GDX males, showed an enhanced ability to detect these odors. To investigate a possible mechanism for this effect, the authors measured GDX females' odor-sampling behavior (sniffing) by monitoring intranasal pressure transients during performance of the urinary odor detection task with and without EB treatment. Under both hormone conditions, females decreased their sniffing frequency as the urinary odor concentration decreased, with this decrease being significantly greater while GDX females received EB. Thus, estradiol enhanced detection thresholds for male urine in a sex-specific manner, and this enhanced sensitivity in females was correlated with altered odor-sampling behavior.     

Rat sex and strain differences in responses to stress

Sensitivity to stress has been linked to the development of a variety of physical and psychological disorders. Studies to-date have focused on extreme stress phenotypes, have studied mostly male responses, have used limited dependent variables, and have included a limited number of measurement time points. The present experiment was designed to address these limitations. Feeding, body weight, open-field activity, acoustic startle reflex (ASR), and prepulse inhibition (PPI) responses of adult male and female Sprague-Dawley and Long-Evans rats to daily immobilization stress (20 min/day) were evaluated for 3 weeks. Stress significantly decreased feeding and body weight of males but generally not of females. Effects were greatest in Long-Evans males. Stress decreased 15-min activity levels for males on Stress Day 1, but not on other days. Stress did not affect 15-min activity levels of Long-Evans females but decreased 15-min activity levels of Sprague-Dawley females on every measurement day. ASR responses to stress differed based on rat strain; percent PPI responses differed based on rat strain and sex. Stress increased startle responses of Sprague-Dawley males and females but not of Long-Evans males and females. Stress reduced PPI of Long-Evans females on every measurement day but not of other groups. These findings indicate that strain and sex of rat is important to consider in evaluating behavioral and physiological responses to stress.