main results

reproductive competition causes solitary-living

The first good empirical data from long-term studies (confirmed here with more years) and from field experiments as well as from population comparisons to show that reproductive competition is one of the main reasons for solitary-living.

Single Strategy

We developed the concept of the single strategy from our results that the fitness consequences of alternative reproductive tactics can differ between generations, depending on the generation specific ecological conditions.
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Social Flexibility

The concept of social flexibility was developed, which is a form of reversible phenotypic plasticity where the social system of an entire population can change facultatively as a function of individuals of both sexes changing their social tactics depending on ecological conditions.

Tactic change and Endocrine change

First eco-physiological studies demonstrating that individuals change their endocrine patterns when changing reproductive tactics, and that hormonal differences between tactics are seasonally dependent.

 The striped mouse (Rhabdomys pumilio) is a diurnal rodent from southern Africa with an adult body mass of 30-90g. Living in an open habitat and being easily habituated to observers, this is the only mouse species that can be easily observed directly in the field. As there are more mouse than any other mammal species, this is important if we want to understand mammalian natural behaviour.
It is famous for its social flexibility, being able to live solitary or in large groups of up to 30 adults of both sexes. Females can breed communally or solitary. Males can roam solitary over a large area or become the resident breeding male of a group. Offspring of both sexes can disperse when reaching adulthood, but typically remain as helpers at the nest, taking care of younger group members. Thus, this species offers us a great model to study reasons of solitary versus group-living and alternative reproductive tactics in both sexes. Being a small mammal, we can observe 12 groups with up to 200 individuals simultaneously in the field, and large colonies can be kept in captivity.
It’s an annual species: every year, a new generation of striped mice is born in spring (August-November), which has to survive the coming dry and food restricted summer and cold wet winter, before breeding the next spring. Less than 1% of individuals survive for a second breeding season. Food availability during the dry season differs dramatically for different generations. During the dry non-breeding season with low food availability, levels of testosterone and corticosterone decrease and we hypothesized that this decrease in hormone secretion functions to reduce metabolic costs (Schradin 2008). This is in agreement with observations of declining blood sugar levels during the dry season (Schradin et al. 2015). Thus, the striped mouse can be used to model how different generations adapt behaviourally and physiologically to droughts, the consequences for cognitive performance, and the resulting fitness consequences. Droughts occur each year in summer, and they have increased in frequency and intensity over the last 100 years (rs=-0.28, N=99 years, p=0.04; unpubl. data), and they are predicted to increase in the future (IPCC Fourth Assessment Report, Climate Change 2007). We were able to easily collect blood samples from hundreds of individuals each year, whose individual life histories were known.
The striped mice were the main study species at SKRS from 2001 until 2020, when the station was closed due to Corona for several months and all experienced staff had to leave. At the same time, South African legislation changed and biologists were no longer allowed to collect blood samples. Since 2021, we only monitor the study population by trapping, but without radio-tracking and observing focal groups. Instead, our studies now focus on the bush Karoo rat.