Two current models seek to explain reproduction of subordinates in social groups: incentives given by dominants for peacefully remaining in the group (reproductive skew model) or incomplete control by dominants. These models make different predictions concerning genetic relatedness between individuals for the distribution of reproduction and the stability of cooperative breeding associations. To test these models and to further explore the relationships between reproductive skew, genetic relatedness, and investment of each participant we performed behavioural observations of female wood mice (Apodemus sylvaticus) raising pups communally. Our results do not support previous models. Differences in lifetime reproductive success were significantly greater within mother/daughter pairs than within pairs of sisters or unrelated females. Subordinate females of either breeding unit did not differ in their direct reproduction. Calculations of inclusive fitness based on our results predict the following. 1) Communal nests should occur only when ecological circumstances prevent solitary breeding. 2) Subordinate females gain the highest inclusive fitness joining their mothers; they also showed the highest nursing investment. 3) Mothers should accept daughters, who have no opportunity for solitary breeding. 4) Dominant sisters and unrelated females should reject subordinate females, because cooperative breeding reduced their reproductive success. However, that those breeding units nevertheless occur can be explained by our results that such females significantly reduced nursing time which may help them save energy for future breeding cycles. Our data demonstrate that both genetic relatedness and 'investment skew' are important in the complex evolution of reproductive skew in cooperative breeding.

Emigration mechanisms in feral house mice – a laboratory investigation of the influence of social structure, population density, and aggression.
Gabriele Gerlach
Behavioral Ecology and Sociobiology 39 (1996) 159 – 170

Emigration in small mammals may be strongly related to social factors but direct observations of emigrants are rare. Feral house mice (Mus domesticus) were studied using a population cage system that allowed continuous observation of individually marked animals. Mice that left their natal cage and took up residence in cages that could only be reached by crossing a water barrier were defined as emigrants. Six pairs of house mice with their litters were placed in the system, and data on aggressive interactions, body weight, reproduction, mortality and emigration were collected daily. Both sexes emigrated, but males did so twice as often as females. Population density was not correlated with the frequency of aggression, and had no effect on the weight of emigrating individuals. Male emigrants suffered more aggression before emigration than their non-emigrant brothers of the same age; they were aggressively driven out by other males, predominantly by the father. Female emigration depended on the female's chances of reproduction. The probability of a female reproducing decreased with increasing birth order. Females born in a late litter, who therefore had only low reproductive chances, dispersed earlier than those of early litters. Resident males were reproductively suppressed. Male offspring had two different strategies for attaining top rank. They could develop rapidly and reach sexual maturity early on, but face competition with the father, risking being forced to emigrate. Alternatively, they could develop slowly, stay within their family and wait for a chance to take over the dominant position. It is concluded that emigration in male and female feral house mice is caused by intrasexual competition.

Fragmentation of landscape as a cause for genetic subdivision of bank vole populations.
Gabriele Gerlach and Kerstin Musolf
Conservation Biology 14 (2000) 1 – 10

We studied the barrier effects of various roadways on genetic subdivision of bank vole (Clethrionomys glareolus) populations. Allele frequencies, genetic variability, and genetic distances of natural populations were calculated based on polymorphism of 7 microsatellite markers. We compared bank vole populations in control areas without such barriers with animals from both sides of a country road, a railway, and a highway, all roadways older than 25 years. Using F- and R-statistics, we demonstrated significant population subdivision in bank vole populations separated by the highway, but not in populations on either side of the other roadways or in the control area. Correlations between geographic and genetic distances were revealed by an extended method based on a Mantel analysis. This allowed us to measure genetic barrier effects and express them as additional geographic distances. For instance, statistically significant differences in allele frequencies in all 7 loci examined existed among populations in southern Germany and Switzerland, which are separated by the Rhine River and Lake Constance. The real geographic distance between bank vole populations in Konstanz and those in Lengwil, Switzerland, is 6 km. According to this analysis the genetic barrier effect of the Rhine could be defined as an additional distance of 7.7 km.

Multiple paternity and similar variance in reproductive success of male and female wood mice (Apodemus sylvaticus) housed in an enclosure.
Susan Bartmann and Gabriele Gerlach
Ethology 107 (2001) 889 – 899

The mating system and variance in individual reproductive success in wood mice (Apodemus sylvaticus) were analysed genetically and using observational studies within a large cage system in an outdoor enclosure. Four experimental groups contained four males and four females, each individually marked with a transponder - small computer chips injected under the skin - allowing individual detection of animals underground or within nest boxes without disturbance. Probability of paternity was analysed by comparing frequencies of cohabitation of males and females. In addition, DNA microsatellite analysis revealed reproductive success of each individual. Multiple paternity was found in 85 % of all litters, which were sired by up to all four males. Males with a greater body mass possibly indicative of a higher rank sired more offspring than those with lower body mass. Interestingly, variance in reproductive success of males and females did not differ. There was no indication that paternity could be assessed by the time males resided with a female shortly before she became pregnant. Our results indicate that a promiscuous mating system is probable in wood mice

Impact of social ties on dispersal, reproduction and dominance in feral house mice (Mus musculus domesticus).
Gabriele Gerlach
Ethology 104 (1998) 487 – 499

According to the ‘Social cohesions hypothesis’ it was tested in feral house mice (Mus musculus domesticus) if there is a relationship between social ties an individual has to other family members and its further role within the family. As it is predicted by the hypothesis individuals which did not form strong social ties were assumed to be the first who emigrate.

House mice were studied using a population cage system that allowed continuous observation of individually marked animals. Data on time staying together with other animals (social ties), aggressive interactions, body weight, reproduction, and emigration were collected daily. The results could be summarized as follows:

• Male emigrants were less integrated in cohorts of male littermates compared to their brothers of the same age. These male cohorts seemed to protect single males from attacks by the dominant male. No difference could be observed in social ties to other family members.
  
• After weaning there was no difference in social ties of male and female offspring. But after sexual maturation social ties of males decreased significantly while those of females remained almost constant.
  Female emigrants showed the same intensity of social ties as their resident sisters.
  
• No difference could be found between social ties of females becoming pregnant and their non-reproductive sisters of the same age. Reproduction or reproductive suppression could not be explained by having more or less contact with other reproductive females.
  
• Dominant males spent least of all time with other family members.
  
• The social cohesions hypothesis has to be rejected in analyzing proximate causes of emigration. In house mice, male emigration was caused by aggression of the dominant male in competition for the top rank within the group. This was enhanced by a lower integration in the group of same-aged brothers but it had nothing to do with a lag of integration into the family at all.
  

Molecular phylogeny of European muroid rodents based on complete cytochrome b sequences
Yvonne Martin, Gabriele Gerlach, Christian Schlötterer and Axel Meyer
Molecular Phylogenetics and Evolution 16 (2000) 37 – 47

Phylogenetic relationships among 18 species of mainly European muroid rodents that belong to three different subfamilies, were estimated using complete sequences of the mitochondrial cytochrome b gene. The inferred monophyly of the subfamilies Murinae (mice and rats) and Arvicolinae (voles, lemmings, and muskrats) is in agreement with previous studies. Within the Murinae the morphology-based division of the genus Apodemus into three subgenera is supported by these DNA sequence data. The relationships among the different genera of the Murinae were generally poorly resolved, and the relationships of Micromys and Acomys to the other murine genera remained unresolved. Within the subfamily Arvicolinae the relations of the genera Arvicola, Clethrionomys and Microtus remained tentative with our data. However, within the Microtus group there is a good molecular support for the phylogenetic relationships. These findings might suggest that the origin of the different murine and arvicoline lineages was rapid, suggesting a radiation and fast speciation.

Further Publications:

Harr B., Musolf K., and Gerlach G., 2000. Isolation of DNA microsatellite primers in wood mice (Apodemus sylvaticus, Rodentia). Molecular Ecology 9, 1664 – 1665.

Gockel J., Harr B., Arnold W., Gerlach G., Schlötterer C., and Tautz D., 1997. Isolation and characterization of microsatellite loci from Apodemus flavicollis (rodentia, muridae) and Clethrionomys glareolus (Rodentia, Cricetidae). Molecular Ecology 6, 597 – 599.

Gerlach G., 1996. Emigration mechanisms in house mice – a laboratory investigation of the influence of social structure, population density, and aggression. Behavioral Ecology and Sociobiology 39, 159 – 170.

Gerlach G., 1990. Dispersal mechanisms in a captive wild house mice population (Mus domesticus Rutty). Biological Journal of Linnean Society 41, 271 – 277.