Interaction of interaural cues and their contribution to the lateralisation of Mongolian gerbils (Meriones unguiculatus).

The main sound localisation cues in the horizontal plane are interaural time and level differences (ITDs and ILDs, respectively). ITDs are thought to be the dominant cue in the low-frequency range, ILDs the dominant cue in the high-frequency range. ITDs and ILDs co-occur. Their interaction and contribution to the lateralisation of pure tones by Mongolian gerbils was investigated behaviourally using cross-talk cancellation techniques for presenting ITDs and ILDs independently. First, ITDs were applied to pure tones with frequencies ≤ 2 kHz to the ongoing waveform, at the onsets and offsets, or in both the ongoing waveform and at the onsets and offsets. Gerbils could lateralise tones only if ongoing ITDs were present indicating that ongoing ITDs are decisive for the lateralisation of low-frequency tones. Second, an ITD was added to 2-to-6-kHz tones with varying ILD. Gerbils' lateralisation was unaffected by the ITD indicating that a large ILD provides a strong lateralisation cue at those frequencies. Finally, small ILDs were applied to 2-kHz tones with an ongoing ITD, pointing either to the same or opposing sides as the ITD. Gerbils' lateralisation was driven by the ITD but strongly affected by the ILD indicating that both interaural cues contribute to the lateralisation.

Everybody loses: intraspecific competition induces tragedy of the commons in Allenby's gerbils.

Interference competition may lead to a tragedy of the commons in which individuals driven by self-interest reduce the fitness of the entire group. We investigated this hypothesis in Allenby's gerbils, Gerbillus andersoni allenbyi, by comparing foraging behaviors of single vs. pairs of gerbils. We recorded strong interference competition within the foraging pairs. Competition reduced the amount of time the gerbils spent foraging, as well as foraging efficiency since part of the foragers' attention was directed toward detecting competitors (apparent predation risk). Single gerbils harvested significantly more food than the combined efforts of two gerbils foraging together. Competition reduced the success of both individuals within a pair by more than 50%, making this a case of the tragedy of the commons where each individual's investment in competition reduces the success of all individuals within the group, including its own. Despite their great costs, competitive behaviors will be selected for as long as one individual achieves higher fitness than the other. In nature, interspecific interactions, such as predation risk, may act to reduce and regulate the deleterious effects of intraspecific competition.

Adopted cognitive tests for gerbils: validation by studying ageing and ischemia.

Transient occlusion of common carotid arteries in gerbils is a simple and widely used model for assessing histological and functional consequences of transient forebrain ischemia and neuroprotective action of pharmaceuticals. In the present study we aimed to introduce additional behavioural tests as novel object recognition and food-motivated hole-board learning in order to measure attention and learning capacity in gerbils. For validating these cognitive tests the effects of ageing (4, 9 and 18 months) and those of transient forebrain ischemia induced by bilateral carotid occlusion at 9 months of age were investigated. Neuronal cell death was estimated in the hippocampus using TUNEL and caspase-3 double fluorescence labelling and confocal microscopy. Ageing within the selected range although influenced ambulatory activity, did not considerably change attention and memory functions of gerbils. As a result of transient ischemia a selective neuronal damage in CA1 and CA2 regions of the hippocampus has been observed and tested 4 days after the insult. Ischemic gerbils became hyperactive, but showed decreased attention and impaired spatial memory functions as compared to sham-operated controls. According to our results the novel object recognition paradigm and the hole-board spatial learning test could reliably be added to the battery of conventional behavioural tests applied previously in this species. The novel tests can be performed within a wide interval of adult age and provide useful additional methods for assessing ischemia-induced cognitive impairment in gerbils.

Effects of isolation-rearing on the development of social behaviors in male Mongolian gerbils (Meriones unguiculatus).

Social isolation has been shown to modify behavioral patterns and neuroendocrine function in many species. In this study, we examined the influence of post-weaning social isolation on the behavioral development and fecal testosterone and corticosterone levels of male Mongolian gerbils. In Experiment 1, isolation-reared (IR) males spent more time engaged in social-sniffing. IR males also showed more aggressive and anxiety-related behaviors than group-reared males (GR; three animals per cage). In Experiment 2, a screen-divided-reared condition (SDR) was examined in addition to GR (two males per cage) and IR males. In the SDR group, two subjects were separated by a wire mesh screen allowing for sensory communication and limited physical contact but not direct social interactions. SDR males showed similar patterns in sniffing and anxiety-related behaviors to IR males; however, an increase in aggression was not observed. No differences in fecal testosterone and corticosterone levels were found among males in different rearing conditions. These results suggest that post-weaning social isolation affects behavioral development without affecting testosterone and corticosterone levels. The behavioral changes observed may therefore be regulated by different mechanisms; increases in social-sniffing and anxiety-related behaviors may be caused by the deprivation of direct social interactions, whereas the enhancement of aggressive behaviors may be caused by the lack of sensory stimulation and limited physical contact with other individuals.

The different baseline characteristics of cognitive behavior test between Mongolian gerbils and rats.

The Mongolian gerbil is a popular laboratory animal useful across many research fields. In the area of cognitive behavioral research the gerbil have been shown exhibit an anxiety-like profile on the elevated plus-maze, and they could be useful as an animal model for testing anxiolytics and antidepressants. However, there are few reports that thoroughly describe the behavioral characteristics of the gerbils in common cognitive behavior tests. In the present study, we used 7 behavior tests to detect the baseline characteristics of the gerbils and compare them to the Sprague Dawley rats. Collectively, the gerbils showed significantly different behavior characteristics in the open field test, elevated plus maze, grip strength, social interaction and fear conditioning compared to the rats. However, no difference was found between gerbils and rats in sucrose preference or Barnes maze test. The data showed that the Mongolian gerbil exhibited higher social interaction and exploratory activity, but lower conditioning fear and grip strength compared with the rats. These results indicate that the gerbil may be a sensitive animal model in behavioral brain research particularly in the areas of anxiety and fear.

Forward masking in gerbils: the effect of age.

We investigated forward masking in 21 gerbils as a function of age (5-47 months) using 400ms maskers at 40dB SPL and a 20ms, 2.85kHz probe presented 2.5ms after the masker. Elevated thresholds for the unmasked probe were only observed in animals older than 3 years. Unmasked thresholds showed no significant age-dependent hearing loss in animals below 3 years of age. In these animals without peripheral hearing loss, we found a significant age-dependent increase of masker-induced threshold shift. A regression analysis revealed that threshold shift increased from 23dB in 1 year old gerbils to 37dB in 3 year old animals. Increased forward masking in these animals with no sign of peripheral hearing loss points to a central processing deficit.

Male expulsion in cooperative Mongolian gerbils (Meriones unguiculatus).

Mongolian gerbils are territorial rodents of steppes and semideserts of Mongolia and Northern China. At least under laboratory conditions, the family life is characterized by long-term harmonious and short-term aggressive periods in which some members are expelled. The purpose of this study was to clarify which features of male offspring promote their expulsion. Therefore body mass, absolute and relative weight of testes and epididymis, histology of testes and testosterone concentrations were analyzed in males derived from five families living in semi-natural enclosures with food and water ad libitum. Males expelled (EFM, n=40) had significantly higher relative testes and relative epididymis weight and higher testosterone concentrations in harmonious periods than it did non-attacked ones (IFM, n=13). A stepwise discriminant analysis has established the relative testes weight as decisive for the affiliation to EFM or IFM. Based on this parameter, 71.7% of the offspring could be classified correctly. Moreover, histological results showed that 50.0% of integrated and 29.4% of the expelled family members had incomplete spermiogenesis, indicating a reduced fertility. It is concluded, that males do compete for reproduction within the families which triggers the expulsion of the males with the highest reproductive capacity.

Gerbils exhibit stable open-arms exploration across repeated testing on the elevated plus-maze.

Repeated testing on the elevated plus-maze (EPM) leads rats and mice to avoid the open-arms of the apparatus. The effect of multiple exposures to the EPM on the behavioral profile of gerbils is unknown. In this study, young and middle-aged gerbils were exposed to the EPM and four retests were carried out 24, 48, 72 and 96h after the first trial in order to determine whether animals exhibited open-arms avoidance. In addition, groups of young and middle-aged gerbils were exposed to the EPM for 20-min followed by a 5-min retest trial 24h apart to analyze the effect of a prolonged exposure to the EPM on open-arms exploration during first trial and retest. Gerbils exhibited high exploration of open-arms during the first trial and progressive locomotor decrease across repeated testing. Unlike previous reports for rats and mice, young gerbils showed a stable open-arms exploration both across multiple exposures and during a prolonged exposure to EPM. Middle-aged gerbils also exhibited a stable open-arms exploration during retest prior to the 20-min test. Results suggest a reliable repeated test paradigm for the EPM using our proposed methodology for gerbils.

An assessment of behavioral aging in the Mongolian gerbil.

Male Mongolian gerbils (Meriones unguiculatus) 14-54 months old (n = 77) were evaluated in a battery of psychomotor (open field, locomotor, and runwheel activity, rotorod performance) and learning (one-way active avoidance in a straight runway and in 14-unit T-maze performance) tests. Body weight and seizure activity were also monitored. According to linear regression analysis, runwheel activity decreased with age; and the number of errors in the 14-unit T-maze increased as a function of age (ps < 0.05). None of the other behavioral measures or body weight were significantly correlated with age. This gerbil strain (Tumblebrook Farms; West Brookfield, MA) tended to be very prone to seizures with 64% of the gerbils experiencing at least one seizure while being tested. Seizures tended to occur when the gerbil was exposed to a novel situation (e.g., initial weighing, placement on the rotorod). An age-related decline in some aspects of psychomotor and learning performance was observed, suggesting the gerbil as an additional mammalian model of aging. The high incidence of seizure activity presented a complicating and confounding variable to the interpretation of the results of the behavioral tests used in the present study. Interventions to control seizure activity (e.g., systematic, controlled breeding; adaptation to apparati) in this model will likely increase its viability as a mammalian model of aging.

Pair-bond disruption in Mongolian gerbils: effects on subsequent social behaviour.

Gerbils are social rodents which form stable male-female pairs. These pair bonds appear to be based on prolonged association rather than an exclusive mating relationship. However, both sexes contribute to territorial defense and pup rearing. Therefore, pair-bond disruption may be expected to have consequences for subsequent behaviour. This hypothesis was examined in a series of laboratory studies. Behavioural consequences of pair-bond disruption were seen in both sexes and the optimum housing parameters were found to be to pair males and females for 5 weeks and to examine their behaviour 1 week after pair-bond disruption. Two further studies using these parameters were conducted to examine the combined influence of pair-bond disruption and the sex of the animal being interacted with. Results across all studies revealed a consistent pattern. Females showed increased immobility in contact, that is, freezing upon another animal's approach, and altered cage-orientated behaviour, regardless of the sex of the animal they were interacting with. In contrast, males showed decreased social investigation and offense and initiated fewer social interactions. However, these effects were only seen in interactions with other males. No changes in social behaviour were evident in interactions between pair-bond-disrupted males and females. The behavioural strategies of pair-bond-disrupted males and females in a social situation therefore differ. From these studies, it may be concluded that pair-bond disruption has consistent and reproducible effects on subsequent social behaviour in gerbils and that there are important sex differences in the behavioural expression of this.

A multivariate assessment of spontaneous locomotor activity in the Mongolian gerbil (Meriones unguiculatus): influences of age and sex.

A multivariate assessment of the spontaneous locomotor activity of male and female Mongolian gerbils (Meriones unguiculatus) was obtained using a Digiscan automated animal activity monitoring system. Spontaneous motor activity data were collected over 1 h (5-min samples) for groups of male and female gerbils ranging from 26-341 days of age (26, 38, 62, 116, 151, 172, 196, 247, and 341). Variables examined included: total distance travelled, average distance per movement, average speed, number of horizontal movements, time in horizontal movement, time per horizontal movement, number of vertical movements, time in vertical movement, and time per vertical movement. Age had a significant effect on spontaneous activity; all measures of horizontal activity increased from preadulthood (26 and 38 days) and remained relatively constant thereafter for adults (62+ days). Vertical activity (rearing) measures were found to increase from the 62-day-old group to the 151- and 172-day-old groups and then decrease among the older groups (196+ days). Across the 12 samples, within sessions, all horizontal and vertical activity measures (except average speed) declined for both males and females. Habituation was more rapid for the preadults than for the adults on all horizontal measures except average distance per movement. No consistent sex differences in locomotor activity were found.

Effects of food deprivation and metabolic fuel utilization on food hoarding by jirds (Meriones shawi).

Food hoarding plays an important role in the energetic repertoire of a variety of mammalian species. Both food hoarding and food intake have been examined in rodents using several energetic challenges including food deprivation, treatment with metabolic fuel blockers, and enhancement of fuel storage. In the present experiment, we examined food hoarding by female jirds (Meriones shawi), a desert rodent species occupying the arid steppes and desert regions of Egypt. Jirds are prodigious hoarders in the field; however, virtually nothing is known about their hoarding within controlled laboratory settings. In the present study, the effects of food deprivation as well as alterations in metabolic fuel utilization (i.e., 2-deoxy-D-glucose and isophane insulin) on food hoarding and food intake were tested in female jirds using a simulated burrow system. Jirds decreased body mass and increased food consumption following either 32 or 56-h food deprivation. Food hoarding, however, was virtually abolished after food deprivation and treatment with 2-DG. In contrast, isophane insulin treatment had no effect on food consumption or hoarding in this species. Taken together, the present results suggest that total body mass (fat), rather than short-term metabolic fuel utilization, regulates both food consumption and hoarding in female jirds. In addition, these results provide a novel set of appetitive responses to these energetic challenges in small mammals.

Laboratory studies of antipredator behavior in the Mongolian gerbil (Meriones unguiculatus): factors affecting response attenuation with repeated presentations.

A series of experiments was conducted to study the properties of the attenuation of responding to repeated presentation of overhead visual transients in the gerbil (Meriones unguiculatus). Results suggest that this attenuation consists of habituation to the repeated association of a potentially threatening sensory stimulus with an increasingly familiar spatial context. The results of these experiments further suggest that the likelihood of eliciting fleeing is a conjoint function of the degree of risk posed by an overhead sensory transient and the degree of safety that is to be gained by fleeing. Results are discussed in the context of the ecology of predator recognition and the structural organization of the rodent visual system.

Kinship and familiarity as factors affecting social transfer of food preferences in adult Mongolian gerbils (Meriones unguiculatus).

Experiments were carried out with Mongolian gerbils (Meriones unguiculatus) to assess whether a socially mediated acquisition of diet selection exists in this species. Results showed that a gerbil was influenced in its diet choices by information extracted during a brief period of interaction with a familiar conspecific that had recently eaten a novel food. Data revealed that the acquisition of a food preference from a conspecific depends on the existence of a social bond between the interacting gerbils. Either genetic relatedness (being brother or sister raised in different litters) or familiarity (being bred in the same litter or being member of a reproductive pair) is necessary for the transfer of information. Unfamiliar and unrelated observer gerbils did not selectively choose their demonstrator's food.

Pavlovian conditioning of social-affiliative behavior in the Mongolian gerbil (Meriones unguiculatus).

Gerbils learned to approach a spatial-olfactory stimulus that signaled access to their pairmate. Experiments 1 and 3 used a discrimination procedure in which 1 conditioned stimulus (the CS+) was presented immediately before access to the pairmate and another (the CS-) was presented alone. Both male and female gerbils came to approach the CS+ sooner than the CS- and spent more time near the CS+ than the CS-. Discrimination learning was facilitated by making the CS+ and CS- spatially distinct (Experiment 3). Learning also was demonstrated in male gerbils, using a between-subjects design with a single CS. Pairing the CS with the opportunity for social interaction resulted in greater approach to the CS within 10 trials than presenting the CS and social opportunity in an unpaired fashion (Experiment 2). These findings demonstrate social-affiliative learning in the Mongolian gerbil. Similarities and differences between these findings and sexual conditioning effects in other species are discussed.

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.

Age-related changes on marking, marking-like behavior and the scent gland in adult Mongolian gerbils (Meriones unguiculatus).

Marking behavior, marking-like behavior [3], and changes of the scent glands were observed in aged Mongolian gerbils. In Experiment 1, changes in the marking and marking-like behavior with aging were evaluated in adult male and female Mongolian gerbils of an inbred strain aged 6 to 36 months. The frequency of marking behavior in males was significantly higher than females throughout the observation period except at 36 months of age. On the other hand, frequency of marking-like behavior in males, but not in females decreased with aging, significantly. In Experiment 2, changes of the scent gland in adult males and females aged 6 to 36 months were morphologically evaluated. Macroscopic examination revealed an increase in the size length and width of the glands of males aged 12 months and females aged 6 months. Histologically the glands of all the males and females aged 6 months developed moderately or well. Some of the 12-month-old males and females showed acinar atrophy of the glands, and all the females aged 18 months or more had highly atrophied scent glands. From these results, we concluded that there is no relationship between the changes of marking behavior and those of the scent glands in aged male Mongolian gerbils, and assume that marking behavior in aged animals does not have an important meaning as marking. In Experiment 3, marking and marking-like behavior in castrated adult Mongolian gerbils aged 16 weeks were observed. The result showed that marking behavior, not marking-like behavior was inhibited after castration. From these findings, we consider that generally marking behavior in Mongolian gerbils consists of androgen-dependent marking behavior and androgen-independent marking behavior (marking-like behavior).

Time of day and stress response to different stressors in experimental animals. Part I: Golden hamster (Mesocricetus auratus Waterhouse, 1839).

The present paper describes the effects of animal house routine stressors on adult golden hamsters during activity time (2 hrs after lights off) and rest time (2 hrs after lights on). In addition, for determination of norm values, the circadian rhythms of the stress indicators heart rate, core body temperature and general activity of unstressed animals were telemetrically registered via implanted transmitters. The three circadian patterns of the nocturnal golden hamster under L:D = 12:12 were unimodal with a main peak after lights off. The physiological norm values (mean over 24 hours +/-SD) were: heart rate 324 +/- 18 bpm, core body temperature 37.5 +/- 0.5 degrees C and activity 114 +/- 123 units/5 min. The mean body temperature of females was significantly higher (0.4 degree C) and its mean activity level was significantly (40%) lower than that of males. The stress responses were dependent on the time of day and on the kind of stressor. The stress responses were significantly stronger during the rest time of the animals (i.e. light period), and it resulted in the subsequent ranking of stressors: handling < vaginal smear < intruder/resident confrontation < cage changing < grouping. There were no sex-dependent stress response differences. The results of this study were compared with identical investigations on the social Mongolian gerbil (J. Exp. Anim. Sci. 1996/97; 38: No. 3).

Ventral marking in the male Mongolian gerbil reflects present and future reproductive investments.

Ventral scent-marking in the male Mongolian gerbil, Meriones unguiculatus, may be related to reproductive potential. In four experiments involving nine groups, males' marking during 5-min. open field tests correlated with their social and reproductive status. The lowest levels of marking occurred among individually housed or subordinate males of a pair, followed by males exposed to ovariectomized females, and dominant males of a pair. Higher levels of marking occurred with vasectomized males living with intact females or following the removal of males from a nuclear family. The most frequent marking occurred when males were housed with pregnant females and with females and their offspring. The proximate stimuli leading to high marking appear to be the sexual availability of reproductively competent females.

The effect of different working definitions on behavioral research involving stereotypies in Mongolian gerbils (Meriones unguiculatus).

Many sources of variation in animal experiments are related to characteristics of the animal or its husbandry conditions. In ethologic studies, observational methods can also affect interexperimental variation. Different descriptions for a behavior can lead to divergent findings that may be incorrectly attributed to other factors if not recognized as stemming from a classification dissonance. Here we discuss 2 observational studies in Mongolian gerbils (Meriones unguiculatus). The first study describes how data vary when 2 different working definitions are used for stereotypic digging: WD(mor), a definition based on a morphologic description of the behavior, and WD(12), a definition that relies mainly on a duration criterion of digging bouts (greater than 12 s). The total duration and number of stereotypic bouts were 22.0% and 63.1% lower, respectively, when WD(12) was applied compared with WD(mor). However, strong correlations existed between data generated by WD(mor) and WD(12), indicating that the 2 definitions yielded qualitatively similar results. The second study provides the first report that laboratory gerbils develop stereotypic behavior that is characterized by alternating bouts of digging and bar-gnawing. Of the 1685 stereotypy bouts investigated, 9.1% comprised both stereotypies, 87.6% consisted of digging only, and 3.3% consisted of bar gnawing only. Working definitions that neglect combined stereotypies can result in considerable underestimation of stereotypic behavior in Mongolian gerbils.