Gry Gundersen

Causes and consequences of natal dispersal in root voles, Microtus oeconomus

To test the causes and consequences of variation in natal dispersal in root voles we released 53 matrilines (mothers with newly weaned litters) separately in field enclosures, during nine consecutive periods. The matrilines could disperse and distribute themselves among three pre-emptied habitat patches. Two dispersal measures were recorded: short-distance dispersal defined as individuals immigrating to a neighbouring patch, and long-distance dispersal defined as unsettled individuals captured along the fence of the enclosures. We analysed the role of social factors (i.e. maternal and litter characteristics), habitat quality (i.e. seasonal effect) and experimentally manipulated shape of the natal patch in dispersal. The consequences of dispersal were analysed with respect to the spatial distribution of kin, and to pregnancy in females and sexual maturation in males. Dispersal was unrelated to patch shape. In agreement with the inbreeding avoidance hypothesis, long-distance dispersal was male biased and philopatric males were most frequently reproductively inactive. Whilst young males avoided their mother, they seemed to disperse, settle and mature sexually independently of their sisters. In agreement with the resource competition hypothesis, young females avoided their mother and were most frequently reproductively inactive when residing in their mothers patch. We conclude that inbreeding avoidance was underlying the male dispersal pattern. For females, long-distance dispersal was most in agreement with the inbreeding avoidance hypothesis while short-distance dispersal could be explained by the resource competition hypothesis. (c) 1998 The Association for the Study of Animal Behaviour.

Male turnover reduces population growth: an enclosure experiment on voles.

Turnover of individuals is assumed to cause disruptions of social organization, followed by reduced reproduction and survival. We tested how male turnover (removal of resident males and their replacement by unfamiliar males) affected population performance in experimental root vole (Microtus oeconomus) populations. The treatment simulated predation of adult males, with the subsequent replacement by immigrants, and provided insight into the interaction between extrinsic (i.e., predation) and intrinsic (i.e., social organization) factors. We showed that recruitment and female survival dramatically declined and that reproduction commenced slightly later in treatment populations compared with control populations. The treatment nearly halved the population growth rate. We suspect that recruitment failed due to infanticidal immigrating males. Reduced female survival was particularly apparent in treatment populations in which females exhibited a high degree of spatial overlap. Our experimental results show how males may significantly shape population dynamics and suggest how predation and social factors interact mechanistically.

Effects of individual condition and habitat quality on natal dispersal behaviour in a small rodent

1. Individuals should benefit from settling in high-quality habitats, but dispersers born under favourable conditions have a better physical condition and should therefore be more successful at settling in high-quality habitats. 2. We tested these predictions with root voles (Microtus oeconomus) by a manipulation of individual condition through litter-size enlargement and reduction during lactation combined with a manipulation of habitat quality through degradation of the vegetation cover. We accurately monitored movements of 149 juveniles during a settlement and breeding period of 3 months. 3. The litter size treatment had long-lasting effects on body size, life-history traits and home range size, but did not influence dispersal behaviour. 4. Different stages of dispersal were influenced by habitat quality. In low-quality patches, females dispersed earlier, spent more time prospecting their environment before settling, and settlers had a smaller adult body size than in high-quality patches. Preference and competition for high-quality patches is likely adaptive as it increased fitness both in terms of survival and reproduction. 5. We found no interactive effect of individual condition and habitat quality on natal dispersal and habitat selection. 6. These findings suggest that immediate conditions are more important determinants of dispersal decisions than conditions experienced early in life.

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.

Demographic responses to a mild winter in enclosed vole populations

Mild winter weather causing snow to melt and ice to accumulate on the ground has been proposed to cause the decreased survival of individuals, and less pronounced cyclicity, of small rodent populations in Fennoscandia. However, detailed data linking ice accumulation to decreased winter survival is lacking. We live-trapped and monitored with passive integrated transponders enclosed populations of root voles (Microtus oeconomus) exposed to different amounts of ice accumulation through a mild winter. We studied how social behaviour and survival responded to snow melt and ice accumulation. Voles avoided ground ice by moving their home ranges, thus increasing home range overlap in enclosed populations experiencing more extensive ice cover. Winter survival was not affected by the amount of ice accumulation, and was only slightly reduced during ice formation in early winter. The lowest survival rates were found at the onset of snow melt in early spring. These results suggest that ice accumulation does not cause lower survival during mild winters, probably because plastic social behaviour enables root voles to reduce the negative effects of varying winter weather on survival. The mechanisms for lower survival during mild winters may operate during spring and be related to spring floods or increased susceptibility to predators.