Trichinella spiralis infection in voles alters female odor preference but not partner preference

Females may choose mates based on secondary sex traits that reflect disease resistance. Accordingly, females should be able to distinguish between unparasitized and parasitized males, and should prefer to mate with unparasitized individuals. Mate and odor preferences for uninfected males or males infected with the nematode, Trichinella spiralis, were examined among prairie voles (Microtus ochrogaster) and meadow voles (M. pennsylvanicus). In a 15-min odor preference test, only female meadow voles distinguished between bedding from parasitized and unparasitized conspecific males, and preferred to spend time with bedding from unparasitized males. Although T. spiralis infection influenced odor preference in female meadow voles, there was no effect of infection status on mate preference among either species. Testosterone and corticosterone concentrations were not different between parasitized and unparasitized males. However, among prairie voles, males that spent an increased amount of time with females during the mate preference test had elevated testosterone concentrations. Taken together, these data suggest that (1) female meadow voles can discriminate between unparasitized and parasitized males, (2) the effects of infection on steroid hormone concentrations may be masked by the effects of social interactions, and (3) parasites may represent a selective constraint on partner preference in voles; however, the life cycle of parasites may influence female preference and should be considered in studies of female preference. Received: 23 April 1998 / Accepted after revision: 25 October 1998     

A field test of the effects of familiarity and relatedness on social associations and reproduction in prairie voles

Inbreeding depression is a well-documented phenomenon. In animals, one means of avoiding the costs of inbreeding is through the recognition and avoidance of kin as mates. Prairie voles (Microtus ochrogaster) are short-lived, socially monogamous rodents that demonstrate inbreeding depression in the laboratory. Field data indicate that pair formation in nature is opportunistic but pairing among close relatives seems uncommon. We examined the role of relatedness and familiarity on prairie vole social associations and reproduction by placing adult voles into 0.1-ha enclosures with familiar siblings, unfamiliar siblings, and unrelated, unfamiliar conspecifics. Live-trapping data indicated that indices of social pair bonding were random with respect to relatedness and familiarity. Among females whose litters were sired by a single male, litters were significantly more likely to be sired by unfamiliar than familiar males, but the number of litters sired by males that were unrelated to their partner was not different from the number of litters sired by males that were related to their partner. Additionally, females that produced offspring with familiar siblings were significantly more likely to have litters with multiple paternity than females not producing offspring with familiar siblings. However, multiple paternity was not influenced by relatedness of sires. Finally, older individuals were more likely to produce offspring with each other than with younger individuals. Our findings suggest that prior association is a more important mechanism of inbreeding avoidance than phenotype matching for prairie voles mating under ecologically relevant conditions.     

Secondary sexual characters signal fighting ability and determine social rank in Alpine ibex (Capra ibex)

Social dominance is a fundamental aspect of male evolutionary ecology in polygynous mammals because it determines access to estrous females. As it is rarely possible to monitor marked individuals of known morphology, little is known about the determinants of male dominance. We studied the social structure of Alpine ibex males in Gran Paradiso National Park, Italy in 2003, 2006, and 2007. Dominance interactions produced a linear social hierarchy. In ibex males, body mass and horn length are key traits in male-male combat, and both increase with age. We explored the links between age, body mass, horn length, and social rank. Ibex males showed much age-independent phenotypic heterogeneity and rapidly growing males reached high rank at a younger age than slow-growing males. Because there is no trade-off between horn growth and longevity, fast-growing males may face weak potential costs of rapid growth and high fitness benefit of achieving high rank. Violent interactions were more likely to occur between males similar in both mass and horn length, independently of age. We suggest that only high-quality individuals can afford a strategy of rapid growth for both secondary sexual characters, and likely reap substantial fitness benefits.     

Male dominance,female choice and male copulatory behavior in two species of voles (Microtus ochrogaster and Microtus montanus)

Summary Female preferences for dominant males in prairie and montane voles were analyzed in two different test situations. In the first, prairie vole females preferentially spent time in proximity to dominant versus subordinate males which were housed behind a wire mesh screen. In a two-male tether test prairie voles females both preferentially associated and mated with dominant males. Montane voles, on the other hand, showed no preference in either test situation. The baseline copulatory behavior of naive montane vole males which became dominant differed significantly from those which became subordinate; no such differences were evident in the baseline copulatory behavior of naive praire voles. One hour following dominance testing, there was no difference evident between dominant and subordinate males of either species but there was a general facilitation of male copulatory behavior in both species, the effect being of greater magnitude in montane voles.     

Sex ratios in high-density populations of the montane vole,Microtus montanus,and the behavior of territorial males

Summary Montane voles (Microtus montanus nanus, Rodentia: Muridae) were studied in unconfined populations and in an exclosure in open fields in Wyoming, USA. Field work involved capture-mark-recapture grids, tagging select individuals with irradiated wires, subcutaneous implantation of dye pellets, and behavioral observations.Territorial males were found in both high and low density populations. They were characterized as having scrotal testes, actively defended territories, residency over time, association with females, few scars, and scent-marking behavioral patterns.Males maintaining territories in high-density populations were nearly the only males in breeding condition. Five high-density populations were trapped in late fall, and age distributions were determined by an eye lens technique. There were two age cohorts of territorial males: those born earlier in the season and those which had overwintered. For each grid, the distribution of males by age was discontinuous at about 95 days. The ratio of older males to older females of comparable ages in these five populations varied between 1:2.1 and 1:3.8. The operational sex ratio was even more unequal.The observations of territoriality at such high densities did not support the hypothesis that territoriality and social rank are two poles of a behavioral continuum related to density.The system of polygyny found, wherein the male has a territory overlapping those of several females, does not neatly fit previous schemata considered favorable for the evolution of polygyny, but is considered adaptive in view of female movements and the labile response of female reproduction to the male.     

Effects of body mass,age, dominance and parasite load on foraging time of bighorn rams,<Emphasis Type="Italic"> Ovis canadensis</Emphasis>

In sexually dimorphic ungulates, males generally spend less time foraging than females, possibly because of difference in body mass or because of the energetic requirements of lactation. The relationship between body size and foraging time has received little attention at the intra-specific level, because few studies have documented activity budgets for individuals of known size. Bighorn rams are a good model to explore how body mass affects foraging time, because they range in mass from 55 to 140 kg. We examined how the foraging time of bighorn rams varied according to individual characteristics. We observed rams in a marked population and constructed time budgets during the 3 months preceding the rut. We determined ram social rank based on agonistic encounters and collected fecal samples to count lungworm larvae. Time spent foraging was negatively correlated with body mass. After accounting for age differences, larger rams spent less time foraging and more time lying than smaller rams. Among rams aged 6–12 years, dominants spent less time feeding than subordinates, while fecal output of lungworm larvae was negatively correlated with foraging time for rams of all ages. Body mass accounts for much of the individual variation in foraging time, suggesting that sexual dimorphism is important in explaining differences in feeding time between males and females.Communicated by P. Heeb     

Reproduction by subordinates in cooperatively breeding Arabian babblers is uncommon but predictable

In a genetic analysis of the mating system of cooperatively breeding Arabian babblers (Timalidae: Turdoides squamiceps), we identified which individuals in the population are breeding, and how reproductive success was distributed among group members with respect to their dominance rank, for both males and females. The population was characterized by an asymmetrical distribution of reproductive success; behaviorally dominant males produced 176 of 186 (95%) of the offspring in 44 social groups analyzed, and alpha females produced 185 of 186 (99.5%). We evaluated models of reproductive skew by examining genetic and demographic correlates of reproduction by␣subordinates. Subordinate (beta) males that sired young were more likely to be recent dispersers from their natal groups or members of newly formed groups than betas that did not reproduce. Breeding beta males had spent smaller proportions of their lives with the current alpha male and female as alphas than had beta males that did not sire young. One consequence of the linkage of dispersal with breeding in newly formed, nonnatal groups is that beta males that sired young had significantly lower genetic similarity to the alpha males in their groups (based on band-sharing coefficients using multilocus minisatellite DNA fingerprinting) than those that did not sire young. This pattern may occur generally in species in which group membership accrues both through nondispersal of young (forming groups of relatives) as well as through dispersal involving coalitions that sometimes include nonrelatives. Received: 22 July 1997 / Accepted after revision: 5 February 1998     

Hierarchical dominance structure in reintroduced California condors: correlates, consequences, and dynamics

Populations of reintroduced California condors (Gymnogyps californianus) develop complex social structures and dynamics to maintain stable group cohesion, and birds that do not successfully integrate into group hierarchies have highly impaired survivability. Consequently, improved understanding of condor socioecology is needed to inform conservation management strategies. We report on the dominance structure of free-ranging condors and identify the causes and consequences of rank in condor populations by matching social status with the behavioral and physical correlates of individual birds. We characterized the hierarchical social structure of wild condor populations as mildly linear, despotic, and dynamic. Condor social groups were not egalitarian and dominance hierarchies regulated competitive access to food resources. Absence of kin-based social groups also indicated that condor social structure is individualistic. Agonistic interactions among condors were strongly unidirectional, but the overall linearity and steepness of their hierarchies was low. Although one aggressive male maintained the highest dominance rank across the 3-year observation period, there was considerable fluidity in social status among condors within middle and lower rank orders. Older condors were more dominant than younger birds and younger males supplanted older females over time to achieve higher status. Dominance rank did not predict the amount of time that a bird spent feeding at a carcass or the frequency that a bird was interrupted while feeding. Thus, younger, less dominant birds are able to obtain sufficient nutrition in wild social populations.     

Male mating opportunities affect sex allocation in a protrandric-simultaneous hermaphroditic shrimp

Sex allocation theory predicts phenotypic adjustments by individuals in their investments into the male and female reproductive function in response to environmental conditions. I tested for phenotypically plastic shifts in sex allocation in a protandric simultaneous hermaphrodite, in which individuals mature and reproduce as males first, and later in life, as simultaneous hermaphrodites. I predicted that initially maturing males should adjust the timing of maturation as hermaphrodites according to male mating opportunities mediated by population size of hermaphrodites. In a first experiment, males maintained with only one hermaphrodite reduced the time they spent as males in comparison to males maintained with no conspecifics, presumably because total reproductive output is maximized by two individuals being simultaneous hermaphrodites when the mating system is a pair. Conversely, males maintained in groups with two or more hermaphrodites increased the time they spent as males in comparison to single males. This delay in maturation was not an effect of resource depletion with increasing shrimp density because the growth rate of males did not differ among most of the experimental treatments. One hypothesis to explain this social mediation of sex allocation is that the smaller males are more successful in mating as males than are the larger hermaphrodites: it will pay reproductively for males to delay maturation as hermaphrodites in large but not in small groups. In agreement with this notion, a second experiment demonstrated that smaller males were four times more successful than were larger hermaphrodites in inseminating shrimps reproducing as females. The informative cue that males may use to perceive different group sizes deserves further attention.     

Active density-dependent habitat selection in a controlled population of small mammals

Density-dependent habitat selection has numerous and far-reaching implications to population dynamics and evolutionary processes. Although several studies suggest that organisms choose and occupy high-quality habitats over poorer ones, definitive experiments demonstrating active selection, by the same individuals at the appropriate population scale, are lacking. We conducted a reciprocal food supplementation experiment to assess whether voles would first occupy a habitat receiving extra food, then change their preference to track food supplements moved to another habitat. Meadow voles, as predicted, were more abundant in food-supplemented habitat than in others. Density declined when food supplements ceased because the voles moved to the new habitat receiving extra food. Although males and females appeared to follow different strategies, meadow-vole densities reflected habitat quality because voles actively selected the best habitat available. It is thus clear that behavioral decisions on habitat use can motivate patterns of abundance, frequency, and gene flow that have widespread effects on subsequent evolution.     

The nature and frequency of social interactions among free-living prairie voles (Microtus ochrogaster)

Hypotheses relating the behavior of voles to their population cycles often assume that the rate of social interaction increases with population density. To test this assumption, we examined the frequency of social interactions in a population of prairie voles (Microtus ochrogaster) over a 7-year period. In addition, we characterized space use by resident animals, patterns of visitation by nonresidents to nests, and participants in social interactions. Social groups within the population typically displayed little overlap in their use of space, even at high population densities. Nevertheless, nonresidents, particularly wandering males, were captured as visitors at nests. The number of visits per social group did not increase in a simple linear manner with population density and was particularly variable when there were fewer than 100 animals/ha. At such times, more single females and fewer pairs received visits from males than expected based on the frequency of occurrence of these groups in the population; a similar pattern was noted during periods of high population density (≥100 animals/ha) but the comparisons failed to reach statistical significance. Furthermore, at high population density, more communal groups received visits from females than expected. Patterns of visitation to communal groups were influenced by the number of adult male residents (winter only), but not by the number of adult female residents or presence of philopatric female offspring. These data indicate that the frequency of social contact in prairie voles does not increase linearly with population density and is influenced by the spacing and possible mate-guarding behavior of resident animals. Received: 7 January 1998 / Accepted after revision: 16 May 1998     

Territory ownership and familiarity status affect how much male root voles (Microtus oeconomus) invest in territory defence

Neighbour–stranger discrimination occurs when individuals respond with more aggression to strangers than to territorial neighbours—a phenomenon termed the “dear enemy phenomenon” (DEP). We investigated the DEP with male and female root voles (Microtus oeconomus Pallas 1776) using field dyadic arena tests conducted in enclosures where we could test for the effects of familiarity (familiar versus stranger), ownership (resident versus intruder status) and resource-holding potential (body mass) on territorial behaviours. The results showed that males put more effort into territorial defence than females, and males could discriminate between neighbours and strangers. In males, aggressiveness was influenced by a significant two-way interaction between treatment and ownership. Male residents were more aggressive towards stranger intruders than towards neighbour intruders, while male intruders were less aggressive towards stranger residents than towards neighbour residents. In females, neither treatment nor ownership status had a significant effect on aggressiveness. Familiar males performed more social behaviours but less non-social behaviours than stranger males. Furthermore, there was a clear dominance hierarchy between residents and intruders in stranger dyads, with the male territory holders dominating the intruder in pairwise interactions. To our knowledge, these results demonstrate for the first time DEP in a small mammal with a known pedigree and present the first evidence for “prior resident advantage” in voles. We argue that both ownership status and familiarity status affect how much an individual invests in territory defence. The benefits of neighbour–stranger discrimination for male root voles and the absence of neighbour–stranger discrimination in female root voles are discussed.     

Kin recognition in rainbowfish (Melanotaenia eachamensis): sex, sibs and shoaling

Living with relatives can be beneficial to individuals via the evolution of kin-directed altruism, but this is tempered by the increased risk of inbreeding. Therefore, in social species, the ability to recognise relatives can be highly advantageous. This study focuses on kin discrimination in the Lake Eacham rainbowfish, Melanotaenia eachamensis, an endangered freshwater species from north-east Queensland, Australia. First, I examined kin recognition abilities when a combination of both chemical and visual recognition cues was available. When given a choice of shoaling with same-sex groups, females spent significantly longer with full-sibs rather than half-sibs, full-sibs rather than non-relatives and half-sibs rather than non-relatives. Males spent significantly longer shoaling with full-brothers versus half-brothers, but showed no other shoalmate preferences. Second, in the presence of only chemical cues, females did not discriminate among groups of different levels of relatedness, but males showed a non-significant tendency to associate with full-sibs rather than non-relatives. Male shoaling behaviour seemed to be more influenced by factors other than relatedness, e.g. intra-sexual competition. Finally, I found that the shoaling preferences of females changed when exposed to groups of males. Females preferred to associate with non-relatives rather than half-brothers and non-relatives rather than full-brothers. There was no significant difference in the time spent with half-brothers versus full-brothers. Taken together, my results suggest that females have very good kin recognition abilities. They prefer to shoal with female relatives but avoid male relatives, and so are able to balance the benefits of nepotism and the costs of incest. Received: 2 May 2000 / Revised: 26 July 2000 / Accepted: 30 July 2000     

Melanin-based colour polymorphism signals aggressive personality in nest and territory defence in the tawny owl (Strix aluco)

Nest and territory defence are risky and potentially dangerous behaviours. If the resolution of life history trade-offs differs between individuals, the level of defence may also vary among individuals. Because melanin-based colour traits can be associated with life history strategies, differently coloured individuals may display different nest and territory defence strategies. We investigated this issue in the colour polymorphic tawny owl (Strix aluco) for which plumage varies from dark to light reddish melanic. Accordingly, we found that (1) our presence induced a greater response (flying around) from dark-coloured than light-coloured females and (2) dark reddish males suffered lower nest predation rates than light-coloured males. In experimentally enlarged broods, the probability that females reacted after we played back the hoot calls of a stranger male was higher if these females were lighter reddish; the opposite pattern was found in experimentally reduced broods with dark parents being more reactive than light parents. Finally, darker females alarmed more frequently when paired with a light than with a dark male, suggesting that partners adjust their behaviour to each other. We also tested whether colouration is used as a signal by conspecifics to adjust the level of their defensive behaviour. Accordingly, breeding females responded more vigorously to a dark than a light reddish stuffed tawny owl placed beside their nest. We conclude that melanin-based colouration is a signal of alternative nest and territory defence behaviour that depends on ecological factors.     

Changes in the activity budgets of yellow baboons (Papio cynocephalus) during sexual consortships

Summary The energetic cost of guarding estrous females has been assumed to be a potentially major factor in limiting the duration of male consortships in a number of polygamous species. This issue has been examined in the present study by assessing changes in the usual activity budgets of 13 male and 20 female yellow baboons which occurred during their consortships.The percent of time adult males spent feeding was strongly related to their agonistic rank: Higher-ranking males fed proportionately more than lower-ranking males while not in consort, and they showed a relatively greater decrease in feeding time during consortships.The proportion of time females spent feeding was related to their agonistic rank and (to a lesser degree) to their parity. Higher-ranking females fed more than lower-ranking females outside of consortships, and showed a relatively greater decrease in time spent feeding while in consort. Nulliparous females tended to feed more than parous females while not in consort; during consortships, the feeding time of nulliparous females decreased, while that of parous females increased slightly.The activity of male and female consort partners was most coordinated during the fertile cycle stage of the female partner, and least coordinated once the female had passed the fertile stage.Although reductions in feeding time during consortships were generally much more pronounced in males than in females, neither males nor females appeared to attain the undisturbed optimal feeding level for their sex while in consort.     

Age- and state-dependent reproductive effort in male mountain goats, Oreamnos americanus

In mammalian polygynous mating systems, male reproductive effort consists mainly of male–male competition and courting of females, which entail substantial somatic costs. Males are thus expected to adjust their reproductive effort according to their age and condition. In this study, we examined how activity budgets of male mountain goats (Oreamnos americanus), a polygynous ungulate, varied with age in a marked population over two periods: (1) summers 1995–2006 and (2) ruts 2004–2006. We then assessed if the proportions of time spent in male–male competition and courtship behaviors were influenced by age-specific body mass and social rank during the rut. Males spent most of their time foraging and resting during summer, and rested more and foraged less with increasing age. During the rut, pronounced shifts in activity budgets occurred as juveniles (1–2 years) increased time spent foraging, whereas adults (≥3 years) increased standing and time spent in social interactions at the expense of foraging. At old age, reproductive effort either stabilized or decreased slightly, providing weak support for the ‘mating strategy–effort’ hypothesis, predicting that courtship behaviors should peak in prime-aged males. Age-specific body mass did not affect time spent in male–male competition, but was positively related with time spent in courtship behaviors, providing support for the ‘individual quality’ hypothesis, predicting that males with more resources at the start of the rut should spend more time in mating-related activities. Age-specific social rank did not affect reproductive effort. Surviving to prime age while increasing mass each year should thus allow male ungulates to gain greater ability to court estrus females.     

Space use,longevity, and reproductive success in meadow voles

Summary We addressed the question of how reproductive success (RS) was limited in the shortlived but highly fecund meadow vole, Microtus pennsylvanicus. In so doing, we asked how differential space use patterns could affect longevity and hence RS in each sex. The sample comprised all voles achieving sexual competency over the course of a 40-week breeding season in a live-trapped population in Manomet, MA USA. Matrilineal families were determined using a radionuclide labelling technique; paternity was estimated using a maximum likelihood model. Individual RS was defined as the number of offspring successfully recruited into the trappable population per adult. We found that the variance in RS among female meadow voles was greater than the variance among males. In an attempt to explain this pattern, reproductively successful individuals were compared to reproductively unsuccessful individuals with regard to survivorship, maximum body weight achieved, and spatial mobility. The only difference between fathers and reproductively unsuccessful males was that fathers were heavier. In contrast, mothers differed from unsuccessful females in every measurement. Females lived longer than males, and mothers lived longer than either fathers or reproductively unsuccessful females. The observed differences in longevity may have been largely the result of differences in levels of mobility, assuming more mobile voles were more susceptible to predation. Mothers were significantly more site tenacious than were either males or unsuccessful females. These patterns explain the distribution of RS in our population if predation differentially affects male and female meadow voles. The meadow vole is the only non-polyandrous vertebrate reported to date in which the variance in RS among females exceeds the variance in RS among males.     

Sneak copulations in long-tailed macaques (Macaca fascicularis): no evidence for tactical deception

Sexual competition is highly prevalent within multi-male multi-female primate groups and may lead to copulations in absence of potentially interfering bystanders. Such avoidance of bystanders may result from tactical deception or from simpler mechanisms such as taking advantage of encountered situations without bystanders, operant conditioning or a peripheral positioning of non-alpha males. We investigated which individuals are avoided as bystanders, how individuals react to the presence of bystanders and whether copulation partners separate themselves from the group in a tactical way. Our observations of a group of 15 female and seven male long-tailed macaques housed in three interconnected, but visually separated compartments revealed that both males and females can interrupt sexual behaviour and that bystanders of both sexes were avoided during copulations (n?=?256). The strength of the effect of bystanders tended to decrease with the dominance rank of male bystanders, but did not depend on the dominance rank of female bystanders. The audience effects of non-alpha individuals did not depend on the strong audience effect of the alpha male in combination with proximity with the alpha male. The effects that we found for separate bystanders suggest that sexual competition concerns rank dependent male–male competition and rank independent female–female competition. Additionally, both male and female copulation partners paid attention to the presence of bystanders and conducted fewer copulation solicitations in their presence. The timing of a male and female’s separation from the group suggests that exploitation of the peripheral position of non-alpha males, and not tactical deception, may cause these audience effects.     

Genital morphology linked to social status in the bank vole (<Emphasis Type="Italic">Myodes glareolus</Emphasis>)

Since genital morphology can influence the outcome of post-copulatory sexual selection, differences in the genitalia of dominant and subordinate males could be a factor contributing to the fertilisation advantage of dominant males under sperm competition. Here we investigate for the first time if penile morphology differs according to male social status in a promiscuous mammal, the bank vole (Myodes glareolus). In this species, dominant males typically achieve higher reproductive success than subordinates in post-copulatory sexual selection, and male genital morphology is complex, including both a baculum (os penis) and penile spines. Our results show that despite no difference in body size associated with male social status, baculum width is significantly larger in dominant male bank voles than in subordinates. We also found evidence of positive allometry and a relatively high coefficient of phenotypic variation in the baculum width of male bank voles, consistent with an influence of sexual selection. By contrast, baculum length and three measures of penile spinosity did not differ according to male social status or show evidence of positive allometry. We conclude that dominant male bank voles may benefit from an enlarged baculum under sperm competition and/or cryptic female choice and that differences in penile morphology according to male social status might be important but as yet largely unexplored source of variation in male reproductive success.     

Scent mediation of intraspecific,interspecific, and intergeneric agonistic behavior among sympatric species of voles (Microtinae)

Summary The hypothesis that the posterolateral scent glands of male microtine rodents (Muridae) direct the attacks of other males was tested.Montane voles (Microtus montanus) and sagebrush voles (Lagurus curtatus) were established as residents in terraria, and conspecific and heterospecific males, from which posterolateral scent glands had been unilaterally removed, were introduced. More attacks by residents were aimed at the glandular sides than at the glandless sides. Male M. montanus also attacked the unilateral hip gland of longtail voles (M. longicaudus), the growth of which had been induced by exogenous hormone inasmuch as this species does not normally possess these glands. Resident male M. longicaudus did not similarly attack the induced glands of conspecific males.The findings are related to demographic patterns, mterspecific relationships, and the evolutionary development of posterolateral scent glands among microtine rodents.