Michael H. Ferkin

Male mammals respond to a risk of sperm competition conveyed by odours of conspecific males.

Sperm competition occurs when a female copulates with two or more males and the sperm of those males compete within the females reproductive tract to fertilize her eggs. The frequent occurrence of sperm competition has forced males of many species to develop different strategies to overcome the sperm of competing males. A prevalent strategy is for males to increase their sperm investment (total number of sperm allocated by a male to a particular female) after detecting a risk of sperm competition. It has been shown that the proportion of sperm that one male contributes to the sperm pool of a female is correlated with the proportion of offspring sired by that male. Therefore, by increasing his sperm investment a male may bias a potential sperm competition in his favour. Here we show that male meadow voles, Microtus pennsylvanicus, increase their sperm investment when they mate in the presence of another males odours. Such an increase in sperm investment does not occur by augmenting the frequency of ejaculations, but by increasing the amount of sperm in a similar number of ejaculations.

The effect of familiarity on social interactions in meadow voles, Microtus pennsylvanicus: a laboratory and field study

The role of familiarity in affecting the outcome of social interactions among meadow voles was investigated in both a laboratory and a field experiment. In the laboratory, captive meadow voles, Microtus pennsylvanicus, were exposed to a conspecifics odour. The voles were then placed into two groups: familiar and unfamiliar. Familiar voles were individuals who underwent staged dyadic encounters with the conspecific to whose odours they had been exposed. Unfamiliar voles were individuals who underwent paired encounters with conspecifics to whose odours they had not been exposed. In the field experiment, familiar voles were neighbours that were trapped within each others home ranges over two consecutive bi-weekly trapping sessions. Unfamiliar voles were individuals that were trapped on different trapping grids. The results of the laboratory and field studies were similar. Encounters between familiar females resulted in less agonistic behaviour and more amicable acts than encounters between unfamiliar females. In contrast, encounters between familiar males resulted in more agonistic behaviour than encounters between unfamiliar males. Familiarity did not affect the outcome of male-female interactions. These results are discussed in the framework of the social system of the meadow vole.

Meadow voles, Microtus pennsylvanicus, can distinguish more over-marks from fewer over-marks

Is it possible that voles have a sense of number? To address this question, we determined whether voles discriminate between two different scent-marking individuals and identify the individual whose scent marks was on top more often than the other individual. We tested whether voles show a preference for the individual whose scent marks was on top most often. If so, the simplest explanation was that voles can make a relative size judgement—such as distinguishing an area containing more of one individual’s over-marks as compared to less of another individual’s over-marks. We found that voles respond preferentially to the donor that provided a greater number of over-marks as compared to the donor that provided a lesser number of over-marks. Thus, we concluded that voles might display the capacity for relative numerousness. Interestingly, female voles were better able than male voles to distinguish between small differences in the relative number of over-marks by the two scent donors.

Cryptic Relatedness and the Opportunity for Kin Recognition in Microtine Rodents

Charnov and Finerty (1980) suggested that vole population density can be regulated by aggressive interactions that change with the relatedness of neighbors. As density increases, the relatedness of neighbors will decrease because of dispersal. Presumably, unrelated individuals will behave less amicably (more aggressively) toward one another. As density increases, dispersal will cause the relatedness of neighbors to decrease and therefore increase the levels of aggression in the population. Increased aggressive levels are postulated to cause the crash of numbers seen in vole population fluctuations. Inherent in this mechanism is the ability for voles to distinguish, and respond differentially between relatives and nonrelatives. As we will show, for this hypothesis to be correct, it must take into consideration cryptic relatedness, relatedness in which the members of the dyad cannot, in fact, determine that they are related. Familiarity, based on association prior to weaning, is the proximate cue that microtine rodents use to assess kinship (Blaustein et al. 1987a,b, Ferkin 1990). Familiar nestmates are recognized as close relatives, whereas unfamiliar nestmates are not identified as relatives (Schadler 1983, Gavish et al. 1984, Boyd and Blaustein 1985, Ferkin 1989, Ferkin and Rutka 1990). To date, there is no evidence to show that microtine rodents employ phenotype matching, recognition alleles, or the major histocompatibility complex to assess kinship (Yamazaki et al. 1980). Genetic relatedness, per se, plays no apparent role in kin recognition for microtine rodents (Bekoff 1981, Blaustein et al. 1987a,b, Ferkin 1990). Thus, the ability to assess genetic relatedness of unfamiliar relatives appears unlikely for microtine rodents. The persistence of kin recognition is dependent on repeated social contacts with close kin (Ferkin 1988, 1990). Although kin recognition and kin-directed behaviors, such as reduced agonism, sharing of nest sites, increased nepotism, and the avoidance of inbreeding, can operate without kin recognition if related individuals interact frequently (Hamilton 1964), repeated contacts with close relatives may not be a feature common to most microtine social organizations (Wolff 1985, Cockburn 1988). The opportunity to identify and be recognized as a close relative will be affected markedly by the type of mating system, population turnover rate, dispersal tendency, nesting patterns, and parental care displayed in a particular species. Therefore, the opportunity to identify and be recognized as a close relative will be affected markedly between microtine species. Microtine rodents that do not have protracted interactions with close relatives may not be able to recognize unfamiliar close relatives.

Adult-weanling recognition among captive meadow voles (Microtus pennsylvanicus)

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.

SCENT MARKING IN MEADOW VOLES AND PRAIRIE VOLES: A TEST OF THREE HYPOTHESES

Most terrestrial mammals deposit scent marks to communicate with conspecifics. We examined the scent marking behaviour of meadow voles and prairie voles, species with different mating systems and social organizations, to determine whether voles scent mark according to the targeting response, the avoidance response, or the shotgun response. The targeting response occurs when the second scent donor deposits more of its scent marks in an area marked by the first scent donor than in an unscented area. The avoidance response occurs when the second scent donor deposits more of its scent marks in an unscented area than in an area marked by the first scent donor. The shotgun response occurs when the second scent donor deposits a similar number of its scent marks in an area containing scent marks of a conspecific and in an area containing no conspecific scent marks. We allowed voles simultaneous access to an arena containing two arms: one of the arms was scented by a conspecific and the other arm was unscented. We recorded the number of marks deposited by the voles in each arm and the amount of time they spent investigating marks deposited previously in the scented arm. Our data provide no support for the avoidance response, but provide support for the shotgun response and the target response. Species and sex differences in the scent marking behaviours of voles when they encounter the scent marks of conspecifics are discussed within the framework that scent marking responses depend on the voles social organization and mating system, and that these responses may reflect the tactics males and females use to attract mates and compete with same-sex conspecifics.

A battery of olfactory-based screens for phenotyping the social and sexual behaviors of mice

We provide a battery of screens that reflect the social and sexual behavior displayed by both captive and feral mice. Some screens focus on measuring aspects of chemical communication, providing information about whether or not mice will interact and if they do so, predict the nature of the interaction. Other screens measure direct interactions between target mice and same- and opposite-sex conspecifics, providing an indication of the social status and sexual responsiveness, respectively, of target mice. The battery of screens yields a high-throughput bioassay of a mouses relative social status, competitive ability, social discrimination, and sexuality. These traits are essential elements of the socio-sexual behavior of mice as well as humans. Thus, by identifying phenotypic deviants for complex behaviors we will allow geneticists to map behavioral abnormalities onto specific chromosomes and increase the efficacy of genetically altered mice as models for human behavioral disorders.

Effects of previous interactions and sex on over-marking in meadow voles

The effects of winning and losing on over-marking behaviour of mammals, a behaviour associated with competition, are not known. The current study tests the hypothesis that after having a staged dyadic encounter with a same-sex conspecific, individuals adjust the proportion of scent marks they use to over-mark the marks of same-sex conspecifics. Female meadow voles that won their encounter used a higher proportion of their marks to over-mark the marks of female conspecifics than did females that either lost their encounter, were evenly matched, unfamiliar, or had no previous paired encounter. Females that lost their encounter used a lower proportion of their marks to over-mark those of female conspecifics than did females that either won their encounter or females that were evenly matched, unfamiliar, or had no previous paired encounter. Females that were evenly matched, unfamiliar, or had no previous paired encounter used a similar percentage of marks to over-mark those of another female. Male meadow voles, however, independent of whether they won, lost, were evenly matched, unfamiliar, or had no previous paired encounter used a similar proportion of marks to over-mark those of male conspecifics. The role of over-marking and the effects of previous social experience are discussed.

Increased sperm numbers in the vas deferens of meadow voles, Microtus pennsylvanicus, in response to odors of conspecific males

Sperm competition occurs when the sperm of two or more males compete to fertilize the egg/s of a particular female. Males of some species respond to a high risk of sperm competition by increasing the number of sperm in their ejaculates. Males may accomplish such a response by increasing the intensity or duration of contraction of the cauda epididymidis and vas deferens. During emission (first phase of the ejaculatory process), the vas deferens receives sperm from the cauda epididymidis and propels the sperm to the urethra. In this paper, we tested the hypothesis that males exposed to a high risk of sperm competition mobilize larger numbers of sperm from the cauda epididymidis to the vas deferens before initiation of copulatory behavior. This accumulation of sperm in the vas deferens would result in a larger number of sperm in the ejaculate. To test this hypothesis, we exposed male meadow voles, Microtus pennsylvanicus, to either low or high risks of sperm competition using soiled bedding of conspecific individuals. At three different times after this exposure (15, 30, or 60xa0min), we removed both vasa deferentia and counted the sperm within them. We found a significant increase in sperm numbers in the vas deferens of males after 30xa0min of being exposed to a high risk of sperm competition. The lower sperm numbers after 15 and 60xa0min of exposure suggest that the observed response is relatively slow and that sperm mobilized to the vasa deferentia may return to the cauda epididymides if ejaculation does not occur some time after the observed response. Our results indicate that the physiological response that may result in high sperm numbers in the ejaculate in relation to high risk of sperm competition can occur before initiation of copulatory behavior.

Seasonal differences in self-grooming in meadow voles, Microtus pennsylvanicus

We determined whether seasonal differences exist in the amount of time meadow voles, Microtus pennsylvanicus, self-groom when they encounter the scents of conspecifics. To do so, we used voles that were born and reared under long photoperiod (LP) and short photoperiod (SP). LP voles represent those found in free-living populations during the spring and summer breeding season, whereas SP voles characterize those found in free-living populations during the fall and winter nonbreeding season. Experiment 1 showed that LP male and female voles self-groomed more in response to odors of LP opposite-sex conspecifics as compared to those of other LP and SP conspecifics, suggesting that they may be self-grooming to signal sexual interest or excitement to potential mates. Experiment 2 demonstrated that SP males self-groomed more in response to scents of LP female voles and those of SP males as compared to scents of LP males and SP females, whereas SP females spent similar amounts of time self-grooming in response to scents of LP males, LP females, SP females, and SP males. These seasonal differences in self-grooming may reflect differences in the messages produced by groomers when they broadcast their odors as well as differences in the meaning of such odors to opposite-sex conspecifics. Alternatively, these data may be associated with seasonal differences in sexual motivation of the groomers when exposed to scents of particular conspecifics.