Fortune Favors the Bold Spider
Leadership can play an important role of a population dynamics, but is it the strength of the leaders or the willingness of the followers that has more influence? (Image Credit: Bernard Dupont, CC BY-SA 2.0)
Selection for Collective Aggressiveness Favors Social Susceptibility in Social Spiders (2018) Pruitt et al., Current Biology, doi: 10.1016/j.cub.2017.11.038.
Socially-influential leaders can have a large effect on the actions of any group. Think of that one person in your life that everyone looks to when it’s time to make a decision; whether it’s something trivial like where to go for dinner, or something more important like whether or not to take that job on the other side of the country, these individuals make a large impact in their social circles. This can also be seen in the natural world, like the alpha of a wolf pack, or the matriarchs of an elephant troop or an orca pod. These focal individuals greatly influence the actions and success of their groups.
In order to determine not only how important these influential individuals are, but also how much the “social susceptibility” of the followers matters, the researchers in this paper used a species of social spider in two different habitat types. By using both arid and wet environments and analyzing both sides of the influence coin, this study was able to accurately determine the importance of both influencing and being influenced in different ecosystems.
Did You Know: Social Spiders
You read correctly, spiders can also be social organisms. Like termites and ants, these social spiders live in colonies of a few thousand individuals. Most of the spiders are female, and they do most of the work within the colony, such as prey capture, web maintenance, and parental care.
Bold individuals are defined as those that put themselves at risk, and they make up anywhere from 3% to 10% of the total colony. Although only one in ten spiders (at most) are bold, their willingness to take risks mean that they are more likely to collect prey items and influence the other, less bold spiders to participate in prey capture.
How it Works
The authors set up 242 colonies of spiders (Stegodyphus dumicula), each containing 19 shy spiders and 1 bold spider, and the boldness of the spider varied from slightly more bold than the shy to vastly bolder than the shy spiders. To measure how much of an influence these bold individuals had, the authors presented simulated prey to the colony and recorded how many spiders responded. They simulated prey by vibrating the nest. They vibrated the nest using a vibrator. I’m not joking.
In order to determine what effect influential, bold spiders had on 1) colony foraging success 2) colony survival and 3) colony reproductive success, the authors set up multiple colonies which experienced various degrees of rainfall. This design allowed for the authors to determine 1) how these influential spiders influenced the colony dynamics in general and 2) how these dynamics were different in arid, prey-sparse environments and wet, prey-rich but also predator-rich environments.
To determine if the leader-follower dynamic was more dependent on either influential, bold leaders or easily-influenced, shy followers the authors set up artificial colonies in the lab. These colonies had either bold leaders from arid sites mixed with shy followers from wet sites, or bold leaders from wet sites mixed with shy followers from wet sites.
What They Found Out
At the arid sites, colonies with a bold spider at the boldest end of the spectrum were 300% more aggressive than those with a bold spider from the other end. Interestingly, there was no difference in the aggression levels between colonies with and without bolder individuals in the wet sites. This means that the shy spiders only altered their foraging habits to fit those of the bold spider in the prey-sparse, dry environments. The authors found a similar result on colony survival and reproductive success, in that only the colonies with bolder spiders in the arid environments were more successful than those without bolder spiders.
In the artificial colonies, the shy spiders from the wet sites were not influenced by the bold spiders from the arid sites. However, the bold spiders from the wet sites were able to influence the shy spiders from the dry sites. What this tells us is the shy spiders from wet sites are not easily influenced to change their foraging habits, while shy spiders from dry sites are subject to the influence of a bold leader.
While the experiment was carefully controlled and well-executed, there are some differences between naturally-occurring spider colonies in arid and wet environments. Spider colonies in arid environments are 40% larger than their wet counterparts, but the individual spiders are themselves smaller. In addition, wet environment colonies have a larger proportion of bold individuals than arid colonies (5.4% compared to 3.6%). Because the authors set up equal colony sizes in the two different habitats (as any scientist should do to ensure a good experiment), they may have affected the natural dynamics of these spiders.
This paper and the experiments within have shown that the leader-follower dynamic is dependent not on the strength of a bold and influential leader, but on the willingness of the shy followers to be influenced. While social spiders differ from other organisms in many respects, it is interesting to think what this could mean for the rest of the animal kingdom. We tend to think of the strong and charismatic leader of a group as the most important part of the equation, but it may be that their influence only matters because the followers are “looking” for some direction in their own lives.