Search

Communicational behaviours by individuals provide information for not only the intended target(s) of the signal but any non-target individual(s) that may be nearby. For terrestrial mammals a major form of communication and social information is through odours via scent marking and self-grooming. Self-grooming is a ubiquitous behaviour in mammals with the function thought to primarily be centred on personal care. But it has been found in rodents that self-grooming will occur in the presence of social odours thus potentially serving a communicative role. For example, male meadow voles (Microtus pennsylvanicus) self-groom in the presence of a female conspecific odour but not a male conspecific odour. Most studies examining self-grooming as a form of olfactory communication have used single odour donors but in a natural environment individuals will come across complex social odour situations. Therefore, we examined how male meadow voles respond to complex social odours with regards to their self-grooming behaviour. We tested the hypothesis that self-grooming can act as a form of olfactory communication and that male meadow voles will control this behaviour measured by differences in self-grooming rates based on social contexts. Male meadow voles did not show differences in the amount of time spent self-grooming to social odours that contained a female and varying number of rival males (0, 1, 3, or 5) or if the social odour contained an acquainted or novel male. Male meadow voles did self-groom more to a social odour that contained a female and a younger male compared to when the social odour contained a female and older male. Male meadow voles appear to adjust their self-grooming behaviour based on the context of the social information. This may be a strategy that can maximize that individual’s fitness by adjusting how much information is provided to potential rivals and mates.

We hypothesized that male meadow voles adjust their odour preferences and sexual behaviours in response to the presence and number of male conspecifics they perceive to have visited a sexually receptive female conspecific. Male voles only preferred the odour of the female previously associated with 3 or 5 males to that of the unfamiliar female. Male voles also had a shorter latency to mate and a shorter mating duration when they were paired with the female that was previously associated with the bedding of 3 or 5 males compared to males paired with an unfamiliar female. Mating and reproductive success, however, were similar for males paired with either female. Thus, male voles use public information provided by scent marks of male conspecifics and adjust their responses in favour of a female that they perceive to been visited by several males, although she may represent a high risk of sperm competition.

Monogamous parents are predicted to invest equally in male and female offspring whereas polygynous parents in good condition are predicted to invest more in male than female offspring. Sex-biased parental investment can occur in three ways: (1) mothers and fathers invest different amounts of care in their offspring (effect of parent sex); (2) parents invest different amounts of care in male and female offspring (effect of offspring sex); and (3) one parent, but not the other, invests different amounts of care in male and female offspring (interaction between parent sex and offspring sex). Studies of parent–offspring interactions in rodents have focused on either effect of parent sex or effect of offspring sex, but not the interaction between parent sex and offspring sex, and most studies have examined only one species. We studied prairie voles (Microtus ochrogaster), a monogamous species, and meadow voles (M. pennsylvanicus), a polygynous (or promiscuous) species, under laboratory conditions designed to simulate field conditions. For each species, we recorded the frequency and duration with which mothers and fathers licked their male and female offspring. We found that meadow vole fathers licked male offspring for longer durations than female offspring. However, prairie vole fathers, prairie vole mothers, and meadow vole mothers did not lick male and female pups for different durations. From the standpoint of the pups, male prairie vole pups, female prairie vole pups, and female meadow vole pups were licked for longer durations by their mothers than by their fathers. In contrast, for male meadow vole pups there was no difference in the duration with which they were licked by mothers and fathers. We also detected effects of litter size: as litter size increased, the frequency and duration of pup licking decreased for mothers and increased for fathers. For duration (but not frequency) of pup licking, these changes were more dramatic in meadow voles than in prairie voles. Our data are generally consistent with predictions that monogamous parents, such as prairie voles, should invest equally in male and female offspring whereas polygynous (or promiscuous) parents, such as meadow voles, should invest more in male offspring when conditions are favourable. Our data also highlight the complexity of parent–offspring interactions in rodents and emphasize the need to examine whether male and female offspring within a species differ in their behaviour or ability to obtain parental care.

Vasopressin and its’ interactions with the vasopressin 1a receptor (V1aR) are important in the formation of social attachments and parental behavior in male mammals but there also is evidence that vasopressin is involved in maternal behavior in female mammals. Laboratory studies have shown that female rats with greater expression of V1aR in particular brain regions display more maternal behavior than those with less V1aR expression. Previous data showed that variation in neural V1aR expression in males of several species of rodents was influenced by the length of microsatellite DNA within the regulatory region of gene (avpr1a) encoding V1aR. In male prairie voles (Microtus ochrogaster), the neural expression of V1aR differs between individuals with longer versus shorter avpr1a microsatellites. Males with longer avpr1a microsatellites had greater V1aR expression in particular brain regions, spent more time with their female social partner, and licked and groomed pups more than males with shorter avpr1a microsatellite lengths. If avpr1a microsatellite length is correlated with V1aR expression in females in a similar fashion to that in male prairie voles, females with longer avpr1a microsatellites should have greater reproductive success since maternal care should be critical for offspring survival. In addition, female prairie voles may benefit by living with males that have longer avpr1a microsatellites due to increased male presence at the nest and paternal care. We tested these predictions with prairie voles from semi-natural populations. Females that had summed avpr1a microsatellite allele lengths greater than the median produced more litters and total offspring than females with summed avpr1a lengths less than the median. Females with summed avpr1a microsatellite allele lengths greater than the median also produced offspring sired by more males. The avpr1a length of male social and genetic partners did not influence female reproductive success. This is the first field study showing a relationship between avpr1a microsatellite allele length and female reproductive success in any species.

The environment experienced by pups during lactation (nutrition and maternal behaviour) can contribute not only to sexual development, but also to individual differences in offspring sexual behaviour. We tested the hypothesis that female offspring of meadow vole dams (Microtus pennsylvanicus) that were 30% food restricted (FR) during days 1–7 of lactation (FR 1–7), days 8–14 of lactation (FR 8–14), or days 15–21 of lactation (FR 15–21) show negative effects on their food intake, growth, and the three components of sexual behaviour (attractivity, proceptivity and receptivity) as compared with female offspring of control dams. With the exception of age 29 days or age 34 days, the body weights of female offspring of FR 1–7 dams and FR 8–14 dams between days 21 and 48 were lower than the body weights of female offspring of FR 15–21 dams and those of control dams. Female offspring of FR 1–7 dams maintained a lower body weight than the other female offspring throughout the study. Female offspring of FR 8–14 and FR 15–21 dams produced odours that were less attractive to males than odours produced by those of FR 1–7 and control dams. Female offspring of FR dams and control dams did not differ in their measures of proceptivity and receptivity. However, the total amount of time allocated for copulation by males was shorter for those males that were paired with female offspring of FR 1–7 dams than it was for those that were paired with the female offspring of FR 8–14, FR15–21, and control dams. The results of this study, coupled with those found in a study conducted on the male offspring of FR dams, indicate that for female offspring days 1–7 and for male offspring days 8–14 of lactation are the time periods during which food restriction of dams had the greatest impact on deficits in sexual behaviour and body mass in meadow voles.

Many female terrestrial mammals undergo postpartum estrus (PPE), a state of heightened sexual attractiveness to male conspecifics relative to females not in PPE (REF females). PPE and REF females of several species may use over-marks to maintain territories and attract and indicate interest in males as potential mates. In our first two experiments, we determined if the response of male meadow voles, Microtus pennsylvanicus, to the top- and bottom-scent donors of a same-sex over-mark was affected by the reproductive state of the scent donors. We found that after exposure to a same-sex over-mark in which the female scent donors were in the same reproductive state, males spent more time investigating the mark of the top-scent female. Males, however, spent more time investigating the mark of the PPE female to that of a female not in PPE, independent of the position of either female’s scent mark in the over-mark. The third and fourth experiments determined if the response of males to the female scent donor of a mixed-sex over-mark was affected by whether she is the top- or bottom-scent donor. In three of the four comparisons, males spent more time investigating the mark of a novel PPE female (not in the over-mark) to that of the top- or bottom-scent female. In addition, males spent similar amounts of time investigating the mark of a novel female not in PPE and that of a top-scent PPE female. However, males spent more time investigating the novel REF female’s mark compared to that of a bottom-scent PPE female. The results are discussed in terms of the natural history and mating strategies of male meadow voles.

Many terrestrial mammals will choose the path that contains evidence of conspecifics with whom they would like to encounter, such as a potential mate, while avoiding a path that will lead them to encounter a threat, such as a same-sex conspecific or predator. We tested hypotheses about the space use of meadow voles, Microtus pennsylvanicus, in an arena containing a short arm and a long arm that both lead to the bedding of a sexually receptive opposite-sex conspecific. Subjects were tested under three conditions: in an empty arena (experiment 1); in an arena containing the scent mark of a known predator, the mink, Mustela vison (experiment 2); in an arena containing the scent mark of a same-sex conspecific (experiment 3). Male voles placed into an empty arena preferred to take the shorter of two paths to reach the bedding of a sexually receptive female; females did not show a preference in the length of the path to reach the bedding of a sexually receptive male. Male and females showed no preference for the short or the long path to reach the bedding of a potential mate, independent of the placement of the scent mark of a mink in the long or short path. Females and males showed no preference for the short or the long path to reach the bedding of a potential mate, independent of the placement of the scent mark of a same-sex conspecific in the long or short path. Males however, were less likely than females to enter a path if it contained the scent mark of a same-sex conspecific. The paths that male and female voles take to reach an opposite-sex conspecifics may be associated with sex differences their responses to risks of predation and same-sex competition.

Differences in the protein content of individuals may affect their response to signals produced by opposite-sex conspecifics and how opposite-sex conspecifics respond to the individual's own signals. Many terrestrial mammals use over-marks to communicate with potential mates. In this study, we determined whether over-marking behavior is affected by the protein content of the top- and bottom-scent donors of an over-mark. We tested two hypotheses by performing two experiments on meadow voles, Microtus pennsylvanicus, in which the top- or bottom-scent donors of an over-mark were fed a diet containing either 9, 13, or 22% protein; these protein concentrations are similar to those that voles may consume in free-living populations. In experiment 1, we varied the protein content of the top-scent voles but not that of the bottom-scent donor. We tested the hypothesis that top-scent donor voles fed a diet high in protein content deposit more scent marks and more over-marks than do top-scent donor voles fed diets lower in protein content. In experiment 2, the top-scent voles were fed a 22% protein diet but the protein content of the diet of the bottom-scent donor varied. We tested the hypothesis that the top-scent donor will deposit more scent marks if the bottom-scent vole was fed a diet high in protein content than if it was fed a diet lower in protein content. Protein content of the top-scent vole's diet did not affect the number of scent marks and over-marks it deposited. Likewise, the protein content of the bottom-scent vole did not affect the number of scent marks and over-marks deposited by the top-scent vole.

Summary Domestic mammals exhibit diversity in suckling behaviour, yet little is known about suckling in most wild species, including rodents with tenacious nipple attachment. This behaviour, whereby young cling tightly to nipples, has been interpreted as an adaptation to competition within and among litters for nipples and milk. Comparative studies of suckling behaviour of tenacious and non-tenacious species are lacking, however, as are studies placing patterns of suckling in a phylogenetic context. We compared suckling in prairie voles (Microtus ochrogaster; tenacious nipple attachment), woodland voles (= pine voles, M. pinetorum; tenacious nipple attachment) and meadow voles (M. pennsylvanicus; non-tenacious nipple attachment). We hypothesized that suckling behaviour of meadow voles differs from that of the other two species. We found that meadow vole pups display higher frequencies of nipple attachment, shorter durations of nipple attachment (significantly different from prairie voles only), more frequent nipple-switching, and no preference for nipple pairs (prairie vole and woodland vole young preferred the hindmost nipples). Mapped onto a phylogeny, our data suggest a suite of behavioural characters associated with suckling in prairie and woodland voles (tenacious nipple attachment, preference for hindmost nipples, and infrequent nippleswitching) and highlight diversity of suckling behaviour among closely related species.

How an individual responds to scent marks and the information they receive from them may be affected by whether the scent marks are encountered singly or as part of an overmark, two overlapping scent marks. We tested the hypothesis that meadow voles that have encountered an area containing over-marks and single scent marks from two conspecifics respond preferentially to the donor that was last to deposit its scent marks compared to the donor that was first to deposit its scent marks. Males spent more time investigating the mark of the second female-scent donor than that of the first female-scent donor after they encountered 60% over-marks and 40% single scent marks from these donors. Females spent more time investigating the scent mark of the second male-scent donor than that of the first male-scent donor after they encountered at least 40% over-marks and 60% single marks. Interestingly, the proportions of over-marks that male and female voles needed to encounter to display a preference for the mark of the second scent donor over that of the second scent donor were similar to the proportion of scent marks males and females use to over-mark the scent marks of a same-sex conspecific.

Rodent pups of many species emit both ultrasonic vocalizations (USVs) and calls spanning into a lower frequency range, audible to humans (AUDs), yet there has been little systematic comparison of these different call types, or analyses of how they might differ in signal function. Here the spectral and temporal characteristics of USV and AUD pup calls are described for the prairie vole (Microtus ochrogaster), a model used in studies of monogamous mating and biparental care, and a species with an unusually large functional and anatomical representation of auditory cortex. Findings provide a detailed description of each call type, including changes to a number of USV characteristics over pup development. Adult attraction to the playback of these two call types was tested in groups that differed in regards to sex and reproductive status. Separate groups included virgins as well as paired animals of each sex, tested at time points either prior to the birth of pups (associated with either mid or late gestation), or at postpartum time points (parents of 2–3 day olds and parents of 8–9 day olds). Adults were attracted to both USVs and AUDs, but group differences were found only in the attraction to USVs. Males were more attracted to USVs than females, and there were interactions of sex and reproductive status. Results suggest that pup vocal communication is quite sophisticated, and that USVs and AUDs may have different communicative functions.

Male mammals are attracted to the scent marks of sexually receptive female conspecifics. Male voles spend more time investigating the scent marks of female voles in postpartum oestrus (PPE), a heightened state of sexual receptivity that occurs following the delivery of a litter, compared to those of female voles that are not in PPE, but in a moderate state of sexual receptivity. However, both types of females will attract male conspecifics to deposit their scent marks near those deposited by these females. The scent marks deposited by these males may indicate how many males have visited this female, which may affect how attractive she is to other males. In the present study, we exposed male meadow voles, Microtus pennsylvanicus, to the scent mark of a PPE female and a female that was not in PPE, a reference female (REF female); the scent marks of 0, 1 or 5 males were placed adjacent to the scent marks of these two female scent donors. In doing so, we tested three hypotheses. The first hypothesis is males will spend less time investigating the scent mark of a female that has more scent marks of male conspecifics adjacent to it compared to that of a female that has fewer scent marks of male conspecifics adjacent to it. The second hypothesis is the converse of the first hypothesis. The third hypothesis is males will spend more time investigating the scent mark of a PPE female than that of a REF female, independent of the number of scent marks of other males adjacent to them. Overall, our data suggests that a combination of factors may influence a male's preference for the scent marks of potential mates. Most tests suggest that males will respond preferentially to a female if she has more male suitors than another female, independent of the reproductive state of either female. If however, the number of male suitors is the same for each female, males tend to prefer the scent mark of the female that is in a more heightened state of sexual receptivity.

Meadow voles, Microtus pennsylvanicus, self-groom before, during and after copulation, which may convey olfactory information to nearby conspecifics. Since males who self-groom at high rates were found to be attractive to females, it is possible that the copulating male may attempt to increase his attractiveness over that of other males who are present or nearby. In that the presence of other males affects sperm investment and can be used by males as an indicator of sperm competition, we tested the hypothesis that the presence of scent marks of other males near a sexually receptive female affects the self-grooming behaviour of males that encounter them. We did so by pairing a male and a female vole in the presence of the odours of one male conspecific, five male conspecifics, or no male conspecifics. The amount of time male voles self-groomed was not affected by the risk or intensity of sperm competition. We also tested the hypothesis that self-grooming behaviour of males differed depending on whether it was performed before, during, or after copulation. Male voles differed in the amount of time and the location on their body that they self-groomed before, during, and after copulation.

Abstract Promiscuous mating is common in female rodents; however what role the female plays in this choice of mates is not clear. Also, whether MMM occurs in the reportedly socially monogamous prairie vole, Microtus ochrogaster, and what role mate-guarding plays in deterring MMM is not known. We conducted two experiments to determine if female prairie voles that were not mate-guarded would copulate with multiple males during a given oestrous period. In experiment 1 using females that were unpaired, we allowed females to choose among three males that were tethered and unable to interact with one another, thus eliminating male-male competition and mate guarding. MMM occurred in 55% of 47 trials. Females mated most often with males with whom they spent the most time, thus social preference was a good predictor of sexual preference. The tendency to mate with multiple males increased over time, thus the length of time a male mate guards can affect paternity. In experiment 2 with females that had been paired with a male and were in post-partum oestrus, 5 of 12 (42%) females mated with more than one male and 3 of 12 (25%) females deserted their paired mate and paired with a new novel male. Thus multi-male mating was similar for paired and unpaired females. Our results suggest that female prairie voles that are not mate-guarded will mate with multiple partners.

Seven matriarchal breeding groups of common voles (Microtus arvalis) were studied to test bonding between females when given the option to leave their family group, and the hypothesis that they explore more in areas with odours from related individuals compared to areas scented with odours from unrelated individuals.Young females were observed more frequently in the proximity of their mother and were recorded more often with their sisters than alone. Except for reproductive females, all females were seen more frequently in the initial home area than in areas of exploration. Females frequented an area scented with the odour of related females more often than one with the odour of unrelated, unfamiliar females, but only shortly before dark. In most test groups only the adult females reproduced although their daughters had reached the age of sexual maturity. The results indicate that young female M. arvalis tend to stay with their mother and may delay their own reproduction. When exploring the vicinity of the burrow young females tend to prefer areas with the scent from related females and can recognize odour cues after being separated from the odour donor for 3.5 weeks. This behaviour may minimize aggressive encounters, leading to dispersal toward related individuals.

Abstract Female common voles live in groups in large burrows whereas males are probably solitary and promiscuous. At high population density, when burrows become a limiting factor, some females are forced to emigrate from their group. To investigate whether these emigrants could share new common burrows with unrelated neighbours, we analysed in the laboratory burrowing and social behaviour in dyads of unrelated wild females before and during their cohabitation. They were compared to wild male dyads. In solitary voles, no sexual difference in burrowing pattern was observed but females built a burrow more quickly than males. In both sexes, the first encounters happened in one of the burrow and were of aggressive nature; during them intruders used burrowing during offensive approaches. While males displayed hierarchy and nested for themselves, females rapidly became friendly and nested together. They reshaped their burrows in response to social nesting and had more complex burrows than solitary females or intolerant males. We may conclude that in common voles at least unrelated non breeding females may contribute to the formation of social groups. To investigate whether, within a matriarchal group, females may co-operate with their own daughters in building a new common burrow, we analysed burrowing behaviour in dyads comprising either an experienced wild mother with one of her naive daughters or two naive familiar daughters. No co-operative behaviour could be detected but experience in burrowing speeded up the building. The results strongly suggest that, within a short time, young females may improve their burrowing.

A quantitative description of the copulatory behaviour of two species of closely related voles (M. agrestis and M. arvalis) is presented. Special attention is paid to the separate contributions of males and females to the time course of events in order to make predictions about the problems which would arise in interspecific copulation attempts. The species were similar with regard to mounting latency, number of ejaculations and durations of pauses between mounting series after ejaculation. Durations of mounting series were shorter in M. agrestis than in M. arvalis. The duration of a series is determined by the number and length of intermount intervals (IMIs) and nonejaculatory mounts and duration of the ejaculatory mount. The number of mounts was greatest and their duration was shortest in M. arvalis, leading to equal total mounting time in the two species. IMIs were longest in M. arvalis and, because the number was also greatest in M. arvalis, the total IMI time per series was longest in M. arvalis. Consequently, the mounting series was longest in M. arvalis. Durations of mounts were exponentially distributed; therefore, the tendency of males and females to terminate mounts could be estimated as constants. In both species, females showed a much higher tendency to terminate mounts than did males. In M. arvalis, both males and females showed higher termination tendencies than in M. agrestis. Males of the two species did not differ in their tendencies to terminate IMIs. Females of M. arvalis, however, aggressively resisted mounting more often than did M. agrestis females, leading to relatively long IMIs in M. arvalis. M. agrestis males often ejaculated during a single mount; M. arvalis males did so after at least five mounts. In accord with this species difference, it could be shown that mount terminations (or the experiences connected with them) negatively influenced the excitation state of M. agrestis males; i.e., the number of mounts prior to an ejaculation was greater (and the total mounting time was longer), the shorter the time the mounts lasted (i.e., the higher the females termination tendency was). In M. arvalis the number of mounts was smaller, the shorter the mounts lasted. In this species, high termination tendencies of the female correlated with much resistance to mounting (long IMIs). The combination of short mounts with long IMIs seems to have a positive influence upon the males. Thus, M. agrestis females delay ejaculation by a high termination tendency of mounts, whereas M. arvalis females seem to accelerate ejaculation by similar actions. Males and females appear to be very well adapted to each other: the M. agrestis female is relatively tolerant and the male ejaculates sooner, the more tolerant she is; the M. arvalis female is rejective and the male ejaculates sooner the more rejective she is, at least within the limits of rejectivity of the observed females. Females differ such that rejectivity in M. agrestis equals tolerance in M. arvalis, so that interspecific copulations are not likely to be successful. It is hypothesized that the rejectivity in M. arvalis fits in with the gregarious way of life of this species. For the female may select the fittest males and/or rejectivity by the female and the positive effect of it upon the male may serve as an incest preventing mechanism.

[Twenty individuals belonging to the rodent species Glaucomys volans, Microtus californicus, M. pennsylvanicus, Neotoma lepida, Peromyscus californicus, P. crinitus, P. floridanus, and Thomomys bottae, and the didelphid species Marmosa mitis were studied in activity-wheel enclosures in which they could control ambient illumination in 10 steps (darkness to 200 lux) by instrumental means. At each illuminance level we determined the time spent running (active time) and not running (inactive time), running speed, number of visits, and visit lengths. During activity, most individuals were highly selective for illuminance. Behaviour not only was generally species specific, but conspecifics often gave identical results. Since the illuminance preferences generally correlated well with habits and habitat preferences in the wild, volitional control of illumination tests probably give a reliable indication of the overall state of adaptation of the visual system of small nocturnal mammals; such tests could serve as Independent guides for Interpreting retinal histology. Both the mouse opossums and gopher spent most of their active time in total darkness and much of the remaining time at level 2 (0.7% of starlight). For the flying squirrels, active time generally peaked at level 2 or 3 (2.1 or 7.5 × 10-5 lux), for the Florida mice it peaked at level 5 (50% of the light on a clear moonless night), and for the California mice at level 7 (about 3 times full moon). lndividual woodrats and canyon mice had the most spread between the positions of their peaks - levels 5 to 7 for the former and 5 to 8 for the latter. Meadow voles were the only animals to have dual illuminance preferences (darkness and level 4). These dual preferences may be related to the fact that meadow voles need to feed at regular intervals and are active around-the-clock in the wild. Components of persisting physiological cycles, particularly digestion, may have influenced the degree to which visual and non-visual factors determined to voles' illuminance preferences; thus, non-visual factors may have predominated during primarily digestive phases and visual ones during primarily non-digestive ones. Eight of the animals showed preference shifts to dimmer levels during inactivity; 3 of these 8 showed marked shifts to darkness. For 13 of the 20 animals, running was fastest in the illuminance range preferred for activity. The results for the gopher and flying squirrels accord with our hypothesis that the proportionality of the running speed of certain rodents to illuminance is the result of selection for individuals that run as fast as navigational safety permits during twilights. Thus, factors that lead to such selection would not be expected to play a significant role for gophers and flying squirrels, and these two species did not exhibit the speed-illuminance dependence., Twenty individuals belonging to the rodent species Glaucomys volans, Microtus californicus, M. pennsylvanicus, Neotoma lepida, Peromyscus californicus, P. crinitus, P. floridanus, and Thomomys bottae, and the didelphid species Marmosa mitis were studied in activity-wheel enclosures in which they could control ambient illumination in 10 steps (darkness to 200 lux) by instrumental means. At each illuminance level we determined the time spent running (active time) and not running (inactive time), running speed, number of visits, and visit lengths. During activity, most individuals were highly selective for illuminance. Behaviour not only was generally species specific, but conspecifics often gave identical results. Since the illuminance preferences generally correlated well with habits and habitat preferences in the wild, volitional control of illumination tests probably give a reliable indication of the overall state of adaptation of the visual system of small nocturnal mammals; such tests could serve as Independent guides for Interpreting retinal histology. Both the mouse opossums and gopher spent most of their active time in total darkness and much of the remaining time at level 2 (0.7% of starlight). For the flying squirrels, active time generally peaked at level 2 or 3 (2.1 or 7.5 × 10-5 lux), for the Florida mice it peaked at level 5 (50% of the light on a clear moonless night), and for the California mice at level 7 (about 3 times full moon). lndividual woodrats and canyon mice had the most spread between the positions of their peaks - levels 5 to 7 for the former and 5 to 8 for the latter. Meadow voles were the only animals to have dual illuminance preferences (darkness and level 4). These dual preferences may be related to the fact that meadow voles need to feed at regular intervals and are active around-the-clock in the wild. Components of persisting physiological cycles, particularly digestion, may have influenced the degree to which visual and non-visual factors determined to voles' illuminance preferences; thus, non-visual factors may have predominated during primarily digestive phases and visual ones during primarily non-digestive ones. Eight of the animals showed preference shifts to dimmer levels during inactivity; 3 of these 8 showed marked shifts to darkness. For 13 of the 20 animals, running was fastest in the illuminance range preferred for activity. The results for the gopher and flying squirrels accord with our hypothesis that the proportionality of the running speed of certain rodents to illuminance is the result of selection for individuals that run as fast as navigational safety permits during twilights. Thus, factors that lead to such selection would not be expected to play a significant role for gophers and flying squirrels, and these two species did not exhibit the speed-illuminance dependence.]

Dyadic encounters and odour preference tests were carried out to determine the behavioural mechanisms mediating weanling-adult interactions in meadow voles, Microtus pennsylvanicus. Encounters between weanlings and the adult(s) which they were reared contained few agonistic behaviours. Encounters between weanlings and an unfamiliar adult, independently of genetic relatedness, contained many agonistic acts. Encounters involving female weanlings were less agonistic than encounters involving male weanlings. Wcanlings often preferred the odour of adults with whom they would have an amicable encounter, whereas weanlings avoided the odour of adults with whom they would have an agonistic encounter. However, female weanlings preferred the odour of their RA father to the odour of other adult males. Adults could not recognize their own offspring if they were reared apart. Social recognition was based on familiarity by association rather than phenotype-matching. The data suggest that adult agonism toward weanlings may be a behavioural mechanism regulating sex-biased dispersal patterns.

1. Microtus agrestis and M. arvalis are two very similar rodents with widely overlapping geographical ranges. One expects strong competition between them. The aim was to study the relationships between habitat choice, adaptation, intra- and interspecific social behaviour and the effects thereof in terms of occupied space. Knowledge of this network may elucidate the type and methods of competition between these species. 2. Field trapping was carried out in the Netherlands. In large regions inhabited by only one of the species, M. arvalis was absent in wet grasslands and was rare in very tall vegetation. M. agrestis was common there. Both species were common in dry meadows with sufficient but not very tall vegetation. This habitat is the arena for competition in regions where the species are sympatric. There, a habitat displacement was found. The distribution of M. agrestis shifted to tall and that of M. arvalis to short vegetation. Displacements were more pronounced if the species lived at a close distance in one field; in these fields, the species were well separated by a sharp boundary between short and long vegetation. These observations suggest that species interaction led to habitat displacement. 3. In the laboratory, behavioural data relevant to choice of and adaptation to habitat were collected in an experimental set-up with tunnels as well as open space. Of the two species, M. agrestis spent more time in open space and behaved less carefully there according to a number of nonsocial behavioural parameters. Furthermore, M. agrestis preferred an illuminated tube; the other species a dark one. Large M. agrestis drank twice as much water at higher temperatures than large M. arvalis. These differences indicate that, of the two species, M. agrestis depends less on burrows and is better adapted to long vegetation that provides better protection from predators, lower temperatures and slower drying of dew- and raindrops. The greater dependence of M. arvalis on burrows is consistent with its absence in very wet meadows. 4. As the habitat displacement in the field depended on the distance between the two species, it is possible that interspecific social behaviour promotes that displacement. The social behaviour of Microtus is described and compared with descriptions from the literature. Aggression is subdivided into the rather static and harmless upright context and the quick, mutilating wrestling context. Confrontations were carried out in three different set-ups. The first confrontation series of noncopulating male-female pairs of the same or of different species were arranged in a simple tube with a small cage with water and food at each end. Aggression consisted almost exclusively of the upright context. M. agrestis was - both towards conspecifics and members of the other species - the more aggressive but also the more avoiding species. During 50 min after confrontation, M. agrestis pairs and hetero-specifics never passed each other and were spaced out. The residents were more aggressive and fled less than did the visitors; this indicates intra- and interspecific territoriality. M. arvalis pairs, however, passed each other often ; they were infrequently aggressive, irrespective of being at home or not. No distance-increasing effect of the upright context could be demonstrated, but passing each other and huddling was prevented; this aggressive behaviour apparently serves as territorial defense. Formation of groups (i.e. huddling) with nonaggressive members proceeded more quickly in M. arvalis than in M. agrestis. Heterospecific groups were uncommon, even after a few days. Social grooming prevailed in the intermediate stage of the process of group formation, characterized by decreased aggression and increased huddling. 5. The second series of confrontations involved isosexual pairs that were confronted in a square set-up of 4 m2 with complex runways. No clear sex differences emerged. M. agrestis pairs only used upright aggression and most of the pairs slowly formed groups. M. arvalis pairs, however, not only used the upright context but also fought fiercely (wrestling context) ; in most cases, one pair member died after one or a few days. Heterospecific pairs only used the upright context and mostly became well segregated spatially, after which aggression was rare. M. agrestis tended to be more avoiding, but some of the confrontations were inconclusive because of immediate good segregation. Heterospecific group formation was exceptional. From the tube and square confrontations, the following is concluded. (a) Noncopulating male-female pairs of the same species form groups after mild aggression. (b) Isosexual pairs of M. agrestis may also form groups ; however, there are indications in the literature that at least large males do so less than we found in the square. Isosexual M. arvalis pairs fight strongly, leading to the death of one opponcnt. (c) Pairs of different species, irrespective of sex, fight mildly but avoid strongly. M. agrestis is the more avoiding species. Group formation is exceptional. These laboratory results indicate that avoidance and segregation are the main mechanisms in interspecies interference competition. 6. In a third series of confrontations, displacement experiments were carried out in order to study avoidance and segregation in relation to the environment. Isosexual pairs of different species were confronted in a set-up with preferred tunnels and an un-favoured open space with restricted hiding opportunity. After confrontation, M. agrestis more often took domain in the open space than did the other species. This occurred either abruptly shortly after one or a few nonaggressive encounters or unobserved in the course of 24 h. The experiment indicates that habitat shift can be due to interspecific encounters that need not involve aggression. That M. agrestis shifted more often may be related to its slightly lesser preference for tunnels in comparison to the other species; heterospecific encounters strongly enhanced this differential preference. 7. Results are reviewed together with the literature on other small rodent species. The classic distinction between exploitation and interference competition is discussed. Examples of increasing intensities of interference between species and their spatial effects are given, viz., (a) opportunistic interference in overlapping home ranges without much space division and (b) systematic interference leading to contiguous home ranges, habitat shift or contiguous geographic distributions. Except for perfect cases of the last type, exploitation and interference concur. The profound habitat displacement in M. agrestis and M. arvalis indicates effective interspecific competition. It is explored to what extent species differences in habitat choice and presumably adaptative characteristics contribute to the explanation of habitat differences. The greater dependence of M. arvalis on burrows and the greater need for water of large M. agrestis seem to be clues being used when choosing a habitat. These needs as well as the differences in adaptive traits described under (3) are in agreement with the natural habitats to which each species is displaced. Analysis of possibly competitive social behaviour led to a greater emphasis on spacing out and group formation and suggested a distinction between the upright and wrestling contexts in aggression. The effect of the upright context (the only class of aggression in most encounters) is not so much direct repulsion of the opponent or conquering its space but rather territorial defense, blocking passage and prevention of huddling. The wrestling context leads to repulsion and conquest. As far as is knovn, the species are basically similar with respect to group life and spacing out, but there are several indications that M. arvalis lives more gregariously than does M. agrestis. This can be related to the more scattered food in tall vegetation, the habitat to which the latter, less gregarious species restricts itself when the other species lives nearby. There is evidence for species recognition and reproductive isolation. Though interference has primarily spatial effects, the likely ultimate factor in competition is food, possibly protein in particular. In the field, interspecific proximity leads to habitat displacement in both species. It is concluded that this is not only due to exploitation that had led to the local extinction of one of the species that initially lived together. It is not likely that the species, when meeting in the field, "await" extinction. It is hypothesized that it depends on the habitat in which they meet which species will evade. It is predicted that the species which is less adapted to a local habitat will move out after behavioural interference.

A laboratory study of the aggressive behaviour of the vole (Microtus agrestis) has been made as part of an investigation of the consequences of CHITTY's hypothesis concerning the cause of vole population cycles. Adult male voles will attack other voles, whether male or female. Some males are more successful in fights than others and they come to be the dominants of the groups to which they belong. Females in advanced pregnancy or those nursing a litter will attack any animals which come near the nest, including dominants that had formerly chased them. Before a male attacks another vole he may move in a jerky fashion through the cage. The jerky movements possibly indicate conflict between aggression and flight. Or an aggressively motivated male may stay in one small area of a cage with his body hunched and with hair erected. While staying at the same spot his legs may make walking movements (marking time), he may turn round on the spot (waltzing), or still retaining the hunched posture and erected hair, he may travel with frequent changes of direction and with short, rapid, leg movements through part of the cage (dancing). This behaviour seems also to indicate activation of both the aggressive and the flight drives, the walking movements being part of the appetitive behaviour of both drives. Waltzing and the frequent changes in direction of movement in dancing, can be regarded as representing the taxis component of attack (moving towards an opponent) and of escape (moving away from an opponent). If an aggressive vole succeeds in catching a subordinate, he may settle upon him and inflict severe wounds. However, a subordinate, before being caught, may retaliate by turning and facing the pursuer. The subordinate then lunges with his incisors bared at the oncoming aggressive animal, or, squatting on his hindquarters, squeals loudly each time the aggressive animal comes near. The retaliation of a subordinate often causes a dominant to retire. Occasionally a subordinate vole that is being chased stops suddenly with his tail erected sharply. The dominant may still be quite near, yet there will not be an attack. This posture of the subordinate may subserve appeasement, as may also the supine posture sometimes assumed by subordinates immediately in front of dominants. When a dominant retires from a retaliating subordinate, he may go to another part of the cage and dig in the sawdust with his fore and hindlegs. This behaviour seems to be a displacement activity, the digging being autochthonously employed in food seeking, tunnel construction and defaecation or urination. The displacement digging is probably caused by the activation of the flight drive in an animal whose aggressive drive is highly activated. Following such digging activity, the dominant animal may re-approach the subordinate inducing it to flee, or the subordinate's repeated retaliation may once more cause the dominant to retire and to do some further displacement digging. In between the successive approaches of an aggressive dominant to a retaliating subordinate, the subordinate may briefly brush its nose with its fore paws. This also seems to be a displacement activity, autochthonously forming part of toileting. Displacement toilet can be interpreted as being caused by slight activation of the aggressive drive of an animal whose flight drive is highly activated.

Short-tailed voles know the area of their range in detail and this knowledge is maintained by regular exploratory activity, which occurs at frequent intervals in familiar surroundings as well as in new situations. Observations of this behaviour in voles, suggest that is is an instinctive behaviour pattern. There is an exploratory drive present for the animal to know the topography of its surroundings. The appetitive phase of the drive, called reconnaissance behaviour, takes the form of more or less random movements through the range, during which the animal reacts to anything different from that previously experienced and confirms old objects, so that memory is refreshed. The presence of a "new object" elicits investigation which may follow a set pattern. In the vole this is approach, nose and touch with withdrawals at some stage. The drive is satisfied by reception of sensory stimuli from objects or situations after a certain time of exposure to them. The periodic recurrence of reconnaissance behaviour is believed to be due to a fading of the effects of the sensory stimuli on the animal.

The purpose of this study was to determine conditions under which neonatal Microtus produced ultrasounds, to analyze the structure of these ultrasounds, and to investigate the probable function of ultrasounds in communication between the microtine parents and neonates. Five species of Microtus, M. pennsylvanicus, M. montanus, M. californicus, M. longicaudus, and M. ochrogaster, were trapped and maintained in the laboratory. In a first study a total of 2I neonates were subjected to I2 different conditions for 30 sec to determine in a qualitative manner whether the neonates emitted few or many ultrasounds. Results of this study showed that microtine neonates produced ultrasounds under stressful conditions in which an important factor eliciting ultrasounds was cold stress. Ultrasounds were given by pups in the field when their nests were opened. In a second study, neonates from four different litters of each species were used as subjects for analyzing the structure of ultrasounds. Two of the litters were sampled every other day beginning at day one and ending at day ten. The other two litters were sampled only once. In two species, M. californicus and M. ochrogaster, recordings were continued until pups reached I8 days of age. The investigator stimulated pups to produce ultrasounds by suspending them from their tails. Six parameters of the ultrasounds, namely the ultrasonic set, pulse duration, interpulse duration, frequency pattern, frequency of the fundamental, and bandwidth of the fundamental, were analyzed. Results of study 2 showed that all species gave ultrasounds of a similar pattern. The pattern consisted of ultrasounds of similar length and relative intensity repeated at short regular intervals. Typically, from two to ten calls constituted each ultrasonic set. Following each ultrasonic set, a pause occurred before another ultrasonic set began. In all five species, four variations in the ultrasonic calls were detected. These four types of ultrasounds were named the plain whistle, the whistle chirp, the wavering whistle, and the hoarse whistle. Certain structural features of the ultrasounds changed significantly with age. The interpulse duration decreased systematically with age. The bandwidth of the fundamental increased regularly with age in three species. The frequency of the fundamental decreased inconsistently, but in all species the frequency was significantly higher at day one than at day ten. Changes in the pulse duration were not significant. Four species, M. pennsylvanicus, M. montanus, M. californicus, and M. longicaudus, produced ultrasounds which were remarkably alike in pulse duration, interpulse duration, fundamental frequency, and frequency pattern. In contrast, ultrasounds produced by M. ochrogaster were structurally different from calls given by the four similar species. Structural differences in ultrasounds produced by M. ochrogaster support morphological data that M. ochrogaster is not closely related phylogenetically to the other four species. As microtine pups passed ten days of age, the structural properties of ultrasounds began to deteriorate and increasingly fewer ultrasounds were given. The relative intensity, pulse duration, and interpulse duration became irregular and structural breaks occurred frequently in the fundamental. Adult microtines produced few or no ultrasounds when they were subjected to the same conditions which caused neonates to emit ultrasounds. Structural features of ultrasounds produced by neonatal Microtus suggest that neonates giving these calls would be difficult to localize by predators. Therefore the probable adaptiveness of ultrasounds is that they function in short range communication between the mother and pups without revealing the location of the pups to predators. As pups mature and are able to escape from predators, it may no longer be necessary for their survival that they communicate by sounds which are difficult to localize. This may in part explain the disappearance of ultrasounds as pups grow older since there is no reason to retain an acoustic signal which no longer has survival value. Study 3 tested the communicative importance of ultrasounds produced by neonatal Microtus. The test subjects were all parents and included nine pairs of M. montanus, six pairs of M. californicus, and five pairs of M. ochrogaster. Responses of parents were observed in a Y-maze, and all tests were conducted in a dimly illuminated room. Parents were subjected six times to three test conditions, namely no stimulus, olfactory stimulus (non-vocalizing pup), and a pup producing ultrasounds. Parents could give no responses, incorrect responses, or correct responses to the olfactory and ultrasonic stimuli. Visual cues were eliminated by partitions between the non-vocalizing or vocalizing pups and the parent. This experiment tested the importance of ultrasonic as opposed to olfactory cues in initiating retrieval in microtine parents. Parents showed significantly more correct responses to the pups giving ultrasounds than to the non-vocalizing pups. Adults rarely gave an incorrect response to the pups producing ultrasouds. In contrast adults responded incorrectly and correctly about the same number of times to the olfactory stimulus. Results of the experiment provided evidence that ultrasounds function in communication between the microtine parents and neonates.