The meek shall inherit the Earth – as long as it’s crowded

It’s not just humans who have personalities. Lots of animals exhibit consistent individual behavioural differences, which are inheritable. But as we might expect evolution to favour the optimal personality, why is there such diversity? University of Groningen ecologists used a natural experiment with Great Tits to find out. Their results were published on 29 February in the journal Ecology Letters.

Personality differences have been found in many species. Individuals can be shy, real explorers or aggressive. What is striking is that such species maintain this diversity, despite evolutionary pressure. ‘We figured a general evolutionary process must account for this’, says Marion Nicolaus, postdoctoral researcher at the Groningen Institute for Evolutionary Life Sciences (GELIFES).

As many personality traits have an effect on social interaction, changes in the social environment could explain the diversity in personality. Nicolaus and her colleagues from GELIFES and the Max Planck Institute decided to see whether this was true in a natural experiment. The group had been studying a population of Great Tits in the Lauwersmeer nature reserve for several decades already. ‘We knew there were year-to-year variations in bird density, but we also had individual life history records for a great many birds.’

Over a period of five years, they assessed the birds’ success, which they defined as their survival. During the winter, they caught birds and assessed their personality type. ‘We kept them overnight, and then put them in an artificial environment, a room with several trees. Then we observed their exploratory behaviour for two minutes.’ Previous work has shown that this is a robust way of determining the individual’s personality type, and that it strongly correlates with aggressive behaviour in nature. ‘Furthermore, this personality trait is heritable.’

Once the data had been compiled, the results showed that in years with a high density, the shy-slow birds were more successful than the aggressive-fast types. ‘One explanation is that the number of social conflicts the fast birds experience has a negative impact on them’ says Nicolaus. ‘They burn out basically, for example due to high levels of stress hormones.’ The fast birds were more successful in years in which fewer birds competed for territory in the nature reserve.

The scientists ruled out the possibility that the birds had simply changed their social tactics and become shyer in high-density years. ‘We were able to do so because we had individual life histories for the birds in the study, and their annual personality assessments.’

The results explain why Great Tits exhibit persistent personality diversity: it is driven by changes in their social environment. This appears to be a general evolutionary mechanism that could explain personality diversity in many species, perhaps even in humans, says Nicolaus. As the authors mention in their conclusions, psychologists believe that in growing human populations, competitive environments will favour shy, non-aggressive individuals.

So with population numbers rising, does that mean the human race will become more peaceful? That might be taking the results a bit too far. Nicolaus: ‘What we can say is that your performance depends on who else is around.’

Reference: Density fluctuations represent a key process maintaining personality variation in a wild passerine bird. Marion Nicolaus, Joost M. Tinbergen, Richard Ubels, Christiaan Both and Niels J. Dingemanse