Size, Sex and Squirrels

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Schindler, S., Neuhaus, P., Gaillard, J., & Coulson, T. (2013). The Influence of Nonrandom Mating on Population Growth The American Naturalist, 182 (1), 28-41 DOI: 10.1086/670753

The body size of an animal influences its survival, fertility, and mating chances. In addition, who is reproducing and with whom determines how a species evolves. Despite the important fact that body size and mating decisions shape the population and the course of evolution, there have been limited methods so far to study how both together affect population growth.

Columbian Ground Squirrel with nest material.  (c)Peter Neuhaus

Columbian Ground Squirrel with nest material. (c)Peter Neuhaus

A team of scientists from the UK, France, and Canada has developed a model that tracks body size evolution and allows the mating pattern to be changed. They studied the consequences of altering the mating behaviour from random to non-random mating and analysed the effect of the three most prominent departures from random mating. The first scenario is when mates tend to be of similar size, the second when they are very different in size, and the third when large males are preferred by all the females.

The scientists applied their model to the Columbian ground squirrel, as it is an ideal model species with plastic mating behaviour which includes the potential for any of the three scenarios to occur. Interestingly, they found that the effect of the mating pattern alone on population growth rate is rather small. But the rate can be substantially affected when non-random mating co-occurs with sex differences. These findings are of great importance, since the majority of species shows sex differences, for instance when males live shorter lives, females invest more energy into offspring or mothers influence the phenotype of their offspring more than fathers do. Researchers can now address these issues with the new model.