The social spiders studied by Jonthan Pruitt of UC Santa Barbara make considerable sacrifices for their offspring. They have a remarkable suicidal co-operative care system, where adults liquefy their own bodies to feed the groups’ young. However it was not their breeding that Pruitt discussed, but rather the differing personalities of individuals within the spider groups.
While the spiders have no distinct castes, individuals do differ in their boldness. Bold individuals are rare (less than 5% of the population) but highly influential; in the laboratory, the addition of just one bold individual into a group can dramatically influence the boldness of the group as a whole, with significant effects on group survival.
Dr Pruitt wanted to test the influence of bold individuals in the spiders’ natural habitat of the Kalahari Desert. This was no easy task; Dr Pruitt calculated that in order to monitor survival over 7 months at his 4 field sites in arid regions, and 4 field sites in wet regions, he had to drive a distance equivalent to the circumference of Planet Earth!
At the arid field sites, the pattern of the lab was repeated; the addition of a single bold individual substantially affected the boldness of the colony as a whole and, moreover, these bolder colonies had significantly greater survival than their timid neighbours. However, in wetter regions the addition of bolder individuals had no effect on the colony.
To determine why bold individuals were only influential at arid sites, Dr Pruitt introduced bold individuals taken from arid sites into shy colonies at wet sites and vice versa. Bold spiders from arid sites had no effect on shy colonies at wet sites; conversely, the addition of bold spiders from wet regions did increase the boldness of shy colonies in arid regions. Indeed, Dr Pruitt found that shy spiders from arid regions will even be affected by the addition of bold individuals from completely different social spider species!
Rather than the phenotype of the bold individuals being key then, it is actually the phenotype of the shy spiders that affects the ability of the colony to respond to a bold individual. Spiders in the arid regions seem to have evolved to be more susceptible to the influence of bold individuals; they are natural followers. Intriguingly there is some evidence that ‘followership’ is a highly heritable trait, while boldness is not, further supporting this hypothesis.