As an individual who is part of a social species, you are likely aware of how others’ behaviour can influence your own and how you can influence others. This influence can be through a direct interaction, for example a single persuasive person can determine the verdict of a jury. One or two aggressive people can cause a crowd to turn violent. A single comedian can break the ice and make a room full of strangers relaxed and comfortable around each other. 

However, people also influence each other’s behaviour indirectly just by their presence. For example, you may behave differently whether you are in the presence of your friends, parents, co-workers, boss, or partner.

Within any group of animals (including humans), there may be particular individuals, so-called ‘keystone’ individuals, that have a disproportionately large effect on the other individuals around them. Of course, whether an individual is a keystone individual will depend on which environment it is in and who else is in the group of individuals it is with.

Evidently humans are not the only species whose behaviour is affected by other individuals. For example, crickets dance differently depending on who’s watching them, and cleaner fish provide a better service for their clients when there are other clients watching.  

Group-living animals can also have keystone individuals who change the dynamics of that group. For example, many spiders live in groups. The social spider Stegodyphus dumicola lives in colonies (which can have several hundred individuals) where they cooperate in their capture of prey, building of a web and care of young. When catching prey, a few bold individuals can lead the way and then many other individuals join in subduing the poor victim.

Andreas Modlmeier and colleagues from the University of Pittsburgh recently carried out an experiment in South Africa with these social spiders and found that the behaviour of young spiders was affected by the presence of adults.   

The researchers collected juvenile spiders and put them in either smaller or larger groups, with adults either being present or absent. They were put in containers and allowed to build a web.  They then dropped a cricket into the web and observed the spiders’ behaviour.

They found that when adults were present in a group, the group was much more likely to attack than groups where the juveniles were alone. This was especially the case in large groups. Interestingly, this is not because the adults were the ones who first attacked the prey; merely their presence seemed to stimulate the juveniles to attack faster.

So why might the presence of adults affect the juvenile’s willingness to attack prey? One possibility is that adults alert juveniles to the presence of the prey. However, it is also possible that the juveniles are just hungrier when there are adults present, as the adults have been eating the majority of the food the colony has had up until this point.

Either way, this experiment shows that when we’re looking at the behaviour of a social species, it’s important to take into account not only whether the individual is in a group or not, but specifically which individuals are around, as this can clearly affect behaviour


Modlmeier et al. (2015). Adult presence augments juvenile collective foraging in social spiders. Animal Behaviour109, 9-14.