Mice are one of those animals that practice communal nesting and communal nursing. Like African lions, free-tailed bats, greater ani birds and social spiders, female mice can take care of pups that are not their own. However, helping others has a cost and in a selfish world like ours, cooperation couldn’t stabilise and maintain if there were no advantages for helpers.
Photo credit: Sayler A and Salmon M (1971)
Mice are known to pool litters in a communal nest and take care indifferently of all pups. However, because litter sizes differ between females, an asymmetry in the benefit of cooperation will emerge and dams with small litters will invest more energy than dams with big litters. For example, if a female of 6 pups joins her litter with another one of 12 pups (for a total of 18 pups in the communal nest), she will provide care to 50% of the pups, so 9 pups in total instead of 6. It raises the question of “why females with smaller litters would participate to communal nesting and nursing?”.
Undoubtedly, cooperation for breeding provides a lot of benefits such as enhanced thermoregulation since grouping creates a warming micro-climate, as you can see with the spectacular penguin creches. In addition, two or more females is better than one to protect a nest against intruders. A female can leave the nest and go to forage while the other one stay at the nest. However, it seems more difficult to understand why female mice also nurse alien pups, especially when they have a small litter that could receive more milk if their mum was not feeding the others.
To answer to this question, researchers have made the analogy between cooperation in animals and the notion of public good, also called social good.
A public good is a resource or a common good provided by individuals that benefits to the whole group.
Although some females may “pay” more for the social good (i.e. provide more cares to the communal litters) at one point of time, there is nothing to prevent that next time she will have the bigger litter and thus, she will receive more help than she gives. This system could be compared to our health system for which we all pay taxes for the public good, until such time we beneficiate to the service. Cooperation for breeding could be maintained in the population only if the cost of nursing alien pups is overweighed by a higher lifetime reproductive success.
Net benefits are indeed greater in communal nesting. Although dams produce more milk, and thus lose more energy, for communal nursing than for their sole own litter, the total time spent by each female for caring pups is lower in communal than in solitary nursing. Studies also show that communally-nursed pups have a higher survival probability and are bigger than pups raised by their mother solely.
Of course, as with all public good systems, females are not prevented from cheating and exploitation. C57BL/6 and CBA/ca mice may be able to provide more milk to their own litter than to alien pups, they can also kill alien pups. However, female wild house mice do not always nurse communally when given the choice and they preferentially cooperate with sibling females while they may avoid previously unfamiliar or cheating females.
Communal breeding in other species:
Communal breeding in Greater Ani; Photo credit: https://phys.org/news/2016-01-birds.html
Communal breeding in free-tailed bats
An interesting article about communal nesting in the greater ani (Crotophaga major), a bird from Central and South America that lay their eggs in the same nest, provide cares to the shared clutch and perform egg-ejection as exploitation behaviour: https://phys.org/news/2016-01-birds.html
Ferrari, M., Lindholm, A.K., König, B., 2015. The risk of exploitation during communal nursing in house mice, Mus musculus domesticus. Animal Behaviour 110, 133-143.
Hager, R., Johnstone, R.A., 2007. Maternal and offspring effects influence provisioning to mixed litters of own and alien young in mice. Animal Behaviour 74, 1039-1045.
Manning, C.J., Dewsbury, D.A., Wakeland, E.K., Potts, W.K., 1995. Communal nesting and communal nursing in house mice, Mus musculus domesticus. Animal Behaviour 50, 741-751.
Rankin, D.J., Bargum, K., Kokko, H., 2007. The tragedy of the commons in evolutionary biology. Trends in Ecology and Evolution 22 (12), 643-651.
Riehl, C., 2011. Living with strangers: direct benefits favour non-kin cooperation in a communally nesting bird. Proceedings of the Royal Society of London B 278, 1728-1735.
Samuk, K., Avilés, L., 2013. Indiscriminate care of offspring predates the evolution of sociality in alloparenting social spiders. Behavioural Ecology & Sociobiology 67, 1275-1284.
Sayler, A., Salmon, M., 1971. An ethological analysis of communal nursing by the house mouse (Mus musculus). Behaviour 40 (1/2), 62-85.
Watkins, L.C., Shump, K.A.Jr., 1981. Behavior of the evening bat Nycticeius humeralis at a nursery roost. The American Midland Naturalist 105 (2), 258-268.
Weidt, A., Lindholm, A. K., König, B., 2014. Communal nursing in wild house mice is not a by-product of group living: females choose. Naturwissenschaften 101 (1), 73-76.