“Social interactions play a crucial role in the lives of organisms, including humans. They influence varied aspects of an individual’s life history, physiology and behaviour, and therefore are the focus of interest in fields as diverse as medicine, immunology, evolution, behavioural ecology, population biology and theoretical biology. Individuals benefit from social interactions, but at the same time compete over access to resources,” said Barbara König, Professor Emerita at the Department of Evolutionary Biology and Environmental Studies, University of Zurich. “A challenging situation here is that the social environment is characterised by a high degree of flexibility and intrinsic unpredictability and is thus considered to be the most complex and fluctuating component of an individual’s environment. Our research focuses on social interactions in an evolutionary framework and uses an integrative approach to understand how differences in the social environment lead to differences in social interactions that translate into fitness differences between individuals.”
STIAS fellow König was delivering the third public webinar of the first semester. Her research interest is animal behaviour, investigated from an evolutionary perspective. She studied biology at the University of Konstanz and after fellowships and positions in Australia, Switzerland, Germany and India, joined the University of Zurich in 1996 as head of Animal Behaviour from where she retired as Managing Director of the Department of Evolutionary Biology and Environmental Studies in 2020. She has served as president of the German Zoological Society, was Head of the Evaluation Board of the Alexander von Humboldt Foundation, as well as being a Member of Academic or Scientific Boards of, among others, the Max-Planck Society, Austrian Academy of Sciences, Volkswagenstiftung, and the Danish National Research Foundation. She was also Secretary General of the International Council of Ethology. She remains a Board Member of the Swiss Academy of Natural Sciences and of the Zurich Zoo.
With her group, König aims to understand the evolution of social behaviour as well as how interactions with conspecifics (animals that belong to the same species) structure groups and populations, processes that are relevant for conservation. Working in the field and laboratory, they use behavioural, ecological, physiological and molecular genetic methods to achieve a comprehensive understanding of the causes and consequences of social behaviour in mammals.
“Being a STIAS fellow is a nice exemplification of the work I’m doing,” she said. “Five months ago we didn’t know each other but we met for lunch, discussed our work, shared ideas and spaces. We have seen that social interactions have benefits. But we have also seen that they have the potential for conflict, exploitation and other costs as seen in Europe in this time.”
Social co-operation is ubiquitous across organisms from simple bacteria through to mammals but the cost and benefits vary depending on group size, group composition and the traits encountered in group members. Such traits differ according to age, sex, reproductive status, health and dominance, and interactions evolve through natural selection. “Individual reproductive success is the crucial currency here,” said König. “Reproduction counts more than survival – rather a short, brilliant reproductive career than a long life with low or no reproductive success.”
A mouse tale focused on females
König reported on findings from a long-term data set from a population of wild house mice (Mus musculus domesticus) her group has been following since 2003 to investigate general principles underlying the evolution of sociality and its endocrine and neural mechanisms. The free-living mice inhabit a barn, their usual habitat, where food and water are provided as well as nest boxes. Tissue samples taken for genetic analysis and transponders recording their movements provide important information.
König highlighted communal offspring care among the females to illustrate how decision making and social partner choice affect the dynamics of social groups and the entire population. Communal nursing in house mice was first described over 60 years ago occurring when two (sometimes more) females from the same social group pool their litters and nurse all.
House mice are polygynandrous with males and females having multiple mating partners; but there is strong competition for reproduction with 50% of the individuals never producing pups. Females live in social groups with 6 to 20 females (relatives and non-relatives) during their lifetime and 50 – 70% of the litters are nursed communally.
“Lactation is costly,” said König, “to raise a litter of seven pups requires a mother to produce a total of 100 ml of milk, of an energy equivalent of 1100 kJ. More milk produced leads to a higher weaning weight of offspring but delays the birth of the next litter – so there are also reproductive costs.”
If this is the case, then why do the females give away milk? Is communal nursing just a consequence of sharing nest boxes or is it a selective behaviour? And what about some underlying conflicts or risks – do all partners share the same investment in communal nursing?
The group found some surprising results. Communal nursing has fitness costs – an increased number of communally nursed litters results in lower lifetime reproductive success. Pups raised in communal nests also have higher mortality – due to female infanticide. Since the females share the cost of lactation according to the combined litter size they don’t benefit equally when they differ in how many pups they contribute to the communal nest – a smaller litter means a higher per capita investment.
“This means females experience a social dilemma,” said König. “The Tragedy of the Commons, as introduced by Garrett Hardin in 1968, where each individual maximises their own benefit from a public good, with the costs being shared by all group members, leading to the overuse of those public goods.”
This might explain why they also found that females nurse litters communally less often than would be randomly expected – they are choosy, not taking all opportunities. They make specific partner choices that reduce conflict – preferring to join a familiar partner with a young litter, which might lower the probability of discrimination and might mean they have more information on their partner. They further prefer to nurse communally in groups with relatives because the costs of being exploited are lower among relatives, with whom they share genes. Female house mice thus decide how much to be involved in co-operation – some females (the more dominant ones) start and remain solitary, others are communal from the beginning, while some change their habits across progressive litters.
This means communal and solitary nursing are condition dependent alternative reproductive tactics. Although females gain higher reproductive success when nursing solitarily, females co-operate to increase the probability to reproduce under conditions of high competition. The decision rules they apply for social partner choice result in a mutualistic outcome. “It’s an example of social selection – selection resulting from social interaction.” König pointed out that this goes back to ideas raised by the American theoretical biologist Mary Jane West-Eberhard. “Individual fitness is not only determined by own phenotype but by the phenotype of the social partner.”
In discussion, König pointed to other examples of female co-operation from the animal world including communal nursing in pigs, other rodents and lions, and bats or giraffes who establish maternity groups or crèches but don’t all nurse. “The unusual thing in mice is the high investment in terms of milk.”
She also discussed the inevitable impulse to extrapolate this to human behaviour. “We scientifically study house mice not humans, but understanding the general principles underlying social behaviour in different animal species will help to understand the evolution of human social behaviour.”
Michelle Galloway: Part-time media officer at STIAS
Photograph: Noloyiso Mtembu