C. J. Barnard

Male crickets increase sperm number in relation to competition and female size

Abstract There is evidence to suggest that males of various species can respond to the threat of sperm competition by varying the amount of sperm transferred during copulation. We tested this in two species of cricket, Acheta domesticus and Gryllodes supplicans (Orthoptera: Gryllidae) by varying the apparent threat of intermale competition experimentally. The results showed that males of both species increased the amount of sperm transferred as apparent competition increased and that male A. domesticus transferred more sperm when encountering larger females. The results also showed that male G. supplicans produced a larger spermatophylax when a larger ampulla was transferred, a relationship consistent with a sperm protection function.

The importance of kinship and familiarity in social interactions between mice

Abstract We investigated a range of social interactions between mice which differed in their degree of relatedness and familiarity. Unfamiliar half siblings (sharing paternity only) differed significantly from unfamiliar non-siblings (sharing neither mother nor father) in their tendency to perform aggression-related interactions and in the amount of passive body contact they showed. Differences between half and full siblings in patterns of interaction appeared to be due to differing degrees of familiarity with companions. Kinship effects disappeared completely when animals were allowed to become familiar. We discuss the functional significance of the familiarity and kinship effects we found, including differences between the sexes in the types of interaction showing kinship effects. Differences between adult and juvenile mice are also briefly discussed.

Risk-sensitive foraging in common shrews (Sorex araneus L.)

SummaryRisk-sensitive foraging theory predicts that predators which face starvation if there is a temporary shortfall in their food supply should choose feeding sites on the basis of variation in as well as mean expected reward rate. For a given mean reward rate they should choose high variance feeding sites (be risk-prone) if they are running below energy requirement, but low variance sites (be risk-averse) if they are running above.Common shrews presented with a choice between constant and variable feeding stations were more likely to visit the variable station when they were running below energy requirement and more likely to visit the constant station when they were running above. However, the tendency towards risk-aversion above requirement was greater than that towards risk-proneness below.When all shrews were considered together, the probability of visiting the variable station correlated negatively and continuously with intake relative to requirement.

Prey size selection and competition in the common shrew (Sorex araneus L.)

Summary1.We examine the idea that predators may sometimes use a rule of thumb (in this case prey size) to estimate relative prey profitability for optimizing diet selection. We provided adult common shrews with a choice between large and small pieces of mealworm, where large pieces were less profitable than small, because of handling time characteristics.2.The tendency for animals to prefer large prey depended on their encounter rate with large prey, but was not influenced by variation in encounter rate with small prey once a threshold encounter rate with large (about 0.03 encounters/s) had been reached.3.We also tested the hypothesis that a predator would be less selective when competing with other animals for food. Shrews showed a significant reduction in selectivity when an apparent ‘competitor’ was present during a test.

Rick-sensitive foraging in common shrews: an interruption model and the effects of mean and variance in reward rate

SummaryA model of risk-sensitive foraging based on a continuously foraging but interruptable predator is developed and tested in a simple choice experiment using common shrews.Given a choice of two feeding stations, shrews behaved in accordance with the broad predictions of the model. Having been trained below their estimated food requirement, shrewss preferred the station y elding the higher mean reward rate, but prefereence for a constant (constant reward rate) or risky variable reward rate) station was influenced by experience of variance in reward rate at the risky station.Preference was influenced most clearly by experience of reward rate variance during tests rather than training.

Social behaviour and susceptibility to infection in house mice (Mus musculus): effects of group size, aggressive behaviour and status-related hormonal responses prior to infection on resistance to Babesia microti

Associations between social rank, immunodepression and resistance to Babesia microti infection within single-sex groups of male house mice suggest rank-dependent suites of response involving different hormonal and immune changes in relation to aggressive behaviour and group size prior to infection. Reduced resistance among high-ranking males was associated with increased serum testosterone and corticosterone concentration and reduced serum immunoglobulin, but was independent of group size. Among low-ranking males, hormonal changes were not associated with resistance to B. microti but changes in corticosterone concentration and measures of immunodepression increased with group size and aggressive behaviour. The results concur with earlier findings suggesting differences between high- and low-ranking mice in their physiological responses to social experience and consequently reduced resistance to B. microti infection among high-ranking individuals.

Variation in the helminth community structure in bank voles (Clethrionomys glareolus) from three comparable localities in the Mazury Lake District region of Poland.

We tested the null hypothesis that populations of hosts trapped in isolated neighbouring locations showing comparable habitat quality, should support similar helminth parasite communities. The study was undertaken in a 2-week period in late summer in NE Poland in a single year, thereby eliminating seasonal and between-year variation in parasite burdens. A total of 139 Clethrionomys glareolus (bank vole) were sampled from 3 forest sites of similar habitat quality. Total species richness was 11 (6 nematodes and 5 cestodes) with 85.6% of the voles carrying at least 1 species and an overall mean species richness of 1.4. At the component community level, the fewest species of helminths were recorded from site 2 (n = 6, compared with 9 at each of the other sites), but site 3 had the lowest Berger-Parker Dominance Index and the highest Simpsons Index of Diversity. At the infracommunity level, site 3 had the highest mean no. of helminth species/vole, the highest mean Brillouins Index of Diversity but the lowest mean no. of helminths/vole. Voles from sites 1 and 3 differed in the nematodes that were most common (site 1, Heligmosum mixtum-95%; site 3, Heligmosomoides glareoli -79.3%). At site 2 no species exceeded 50% but prevalence of Syphacia petrusewiczi was higher than at the other sites. The prevalence of cestodes was too low to test reliably (12.%), but the highest prevalence of adult cestodes was recorded at site 1 (22.5% compared with 4.9 and 1.7% for sites 2 and 3 respectively). Host sex did not influence infection, but mean species richness increased with age. The different sites were responsible for most of the variation in our data, and the intrinsic factors (sex and age) were less important in shaping the component community structure of helminths. We conclude that even locations in relative close proximity to one another (13-25 km), selected on the basis of similar habitat quality, have rodent populations that differ in their helminth parasite communities, although for reasons other than the factors quantified in the present study.

Kin recognition and mate choice in mice: the effects of kinship, familiarity and social interference on intersexual interaction

Abstract Cross-fostered laboratory mice, Mus musculus , were tested for their ability to discriminate between individuals of the opposite sex on the basis of kinship. Subjects were presented with three sets of choices between familiar and unfamiliar test animals of five different degrees of relatedness. The three sets involved choices between (1) pots of soiled sawdust provided by test animals, (2) test animals confined within cages and (3) test animals in a freely mixing group. Analysis of time spent with different test animals or their odour cues and behavioural interactions between subject and test individuals showed evidence of kin discrimination by both sexes but discrimination was more pronouced in males and when subject and test animals were familiar. However, there was evidence of discrimination between unfamiliar animals in freely mixing groups. Males showed a preference for females of an intermediate degree of relatedness in terms of time spent and investigatory/contact activities but the little mounting behaviour that occurred appeared to be influenced by female proceptivity with females showing some preference for closely related (full/half-sibling) males. The difference in apparent preference between the sexes was similar to that reported in some other studies. There was no effect of male social dominance ranking per se on discrimination by females.

Environmental enrichment, immunocompetence, and resistance to Babesia microti in male mice.

Groups of male CFLP mice housed in cages furnished with shelves and nestboxes showed increased aggression and reduced resistance to an experimental infection of Babesia microti when compared with groups in unfurnished cages. Both a bystander measure of immunocompetence (serum total IgG concentration) and resistance to B. microti decreased as the number of attacks received by mice increased, but increased with the number of times individuals were recorded on shelves or in nestboxes. Serum concentrations of testosterone and corticosterone were generally downregulated in furnished cages; the absence of hormone-related reduction in resistance may have been due partly to this, but partly also to the apparent modulation of hormone concentrations in relation to concurrent immunocompetence. Some welfare implications of the results are considered.

Social behaviour, stress and susceptibility to infection in house mice (Mus musculus): effects of duration of grouping and aggressive behaviour prior to infection on susceptibility to Babesia microti

Unrelated and initially unfamiliar male CFLP mice, maintained for different periods in groups of 6, differed in both their rate of clearance of Babesia microti and the time taken to reach peak parasitaemia in relation to their aggressive behaviour within groups prior to infection. Males maintained in groups for shorter periods and showing more aggression within their group were slower to clear infection and males showing more marked external evidence of aggressive interaction reached a peak of parasitaemia sooner. Serum IgG and corticosterone analyses were consistent with increased aggression causing stress-induced immunodepression but relationships with aggression and social status were not simple. Males showing more aggression tended to enter their groups with higher levels of corticosterone and, to a lesser extent, reduced levels of IgG compared with other mice. The results thus suggest that increased susceptibility to disease may be a cost to males aggressively maintaining high social status.