Featured Image Caption: Halloween is a time for spiders and social gatherings. Some spiders are social and share food that lands on their communal web (Image Source: “happy halloween” by JodyDigger is licensed under CC BY-SA 2.0).
Reference: Beleyur, T., Murthy, T. G., Singh, S., Somanathan, H., & Uma, D. (2021). Web architecture, dynamics and silk investment in the social spider Stegodyphus sarasinorum. Animal Behaviour, 179, 139-146. https://doi.org/10.1016/j.anbehav.2021.06.029
Trick or Treat
October is finally here, which means that Halloween is just around the corner. People of all ages will start to put together their fun and frightening costumes. Halloween is a very social holiday. Many celebrators make plans to trick-or-treat, carve pumpkins, or participate in costume contests. Some species of spiders – a Halloween icon – are social creatures, too, living in groups of tens to hundreds of spiders. Social spiders are very rare. Only about 25 out of more than 45,000 spider species are classified as being permanently social.
One social feature (arguably the best part) of trick-or-treating is sharing candy amongst friends and trading for the best sweets. Like trick-or-treaters, group-living spiders share treats that they collect. To catch the shared meals, each spider living in a group must chip in some silk to make a big communal web. Will sharing a web and food be a treat for all, or will some spiders get tricked into doing more of the work?
There are many advantages for social spiders that share a web. A shared web, for example, distributes egg care and predator surveillance between the members. Social spiders also share the responsibility of building the web. Larger groups can build grander webs that catch more sizable, nutritious prey. Silk takes a lot of energy for a spider to produce, so individuals in larger groups may contribute less silk than spiders in smaller groups. Receiving more food makes group-living a real treat, but having to provide more silk could be a trick. Researchers Thejasvi Beleyur and colleagues tested whether spiders living in larger groups receive the added benefit of contributing less silk.
Stegodyphus sarasinorum, one of the few species of social spiders, lives in India. S. sarasinorum builds communal webs in groups with upwards of hundreds of spiders contributing. It can take days for a group to build their web, but it can be maintained for multiple generations of spiders. These spiders either build two-dimensional webs on fences or more complex webs in vegetation. The researchers collected colonies of spiders from fences and brought them into the lab. For ten days, the spiders built webs in groups of 1, 5, 10, or 25 spiders. Each day, they measured the total length of the silk, the variation in density of silk across the web, and the mean hole size using photographs of the webs.
No Tricks, Only Treats
Hole size and web density did not differ by group size. However, webs were denser near the web retreat (the corner where the spiders gather) and decreased in thickness away from the retreat. The total silk length increased as groups increased in size since there were more spiders to contribute silk. However, no matter the group size, individual spiders invested a similar amount of silk. Thus, spiders living in a group are not saving energy by producing less silk. Perhaps the other benefits of group-living (like capturing larger, nutritious prey) are sufficient for sociality to have evolved in this species.
It appears that group-living is a treat for spiders of this species despite no reduction in silk contribution. Some real trickery might come into play if some spiders produced more silk than others. While the researchers could not track individual contributions, spiders lost equal amounts of weight throughout the experiment, suggesting they spent an equal amount of energy. Scientists still question how large groups of social organisms enforce equal work among members. Studying the complex social structures of animals gives us a better understanding of how animals have adapted to survive and pass on their genes. These behaviors have evolved to help animals thrive in their environment and inform the conservation of vulnerable species.
This Halloween season, remember to be like a social spider and take some time to be amongst friends and share your treats. And don’t forget to appreciate the spiders in your community!