Using computer modeling of the eastern slopes of the Andes, Zsóka Vásárhelyi and István Scheuring, researchers of the Institute of Evolution of the ELKH Cente for Ecological Research (CER), and Leticia Avilés, a researcher at the University of British Columbia, have investigated the distribution pattern of 'social spiders', which display extensive social behavior, and 'subsocial spiders', which display less cooperative behavior but provide maternal care for a longer period. It was established that the ecological characteristics of a given habitat fundamentally determine whether social or subsocial species live in that habitat. An article presenting the findings has been published in the journal American Naturalist.
Spiders are typically known as solitary and aggressive animals, yet there are a handful of spider species in the tropics where closely related individuals live in a common web. Their densely woven, three-dimensional web is built in groups, prey is captured and consumed together, and the offspring are cared for as a group. These species are collectively known as social spiders. Among other things, studying these spiders can contribute to a deeper understanding of the evolutionary and ecological background of social behavior.
Field specialists have observed that with increasing altitude and distance from the equator, social spider species are replaced by less cooperative, subsocial species, which, although characterized by prolonged maternal care, do not display extensive social behavior. The researchers were curious as to what creates this distribution pattern.
In the literature, this phenomenon is explained by two different hypotheses. According to the prey size hypothesis, it is only worthwhile to maintain a common web, i.e. to lead a social lifestyle, when there is a sufficient amount of large prey animals. As the size of the colony increases, the surface-to-volume ratio of the net decreases. However, this effect is offset by the fact that more populous colonies can capture larger prey.
On the other hand, according to the disturbance hypothesis, solitary individuals cannot reproduce in a stable manner when there are significant disturbance factors – such as wind, rain and predators. The reason for this is that the dense three-dimensional web characteristic of these species is very costly to maintain, and if the mother cannot take care of her offspring long enough – due to having to maintain the web, for example – the offspring will die before reaching sexual maturity. All this suggests that the distribution of social species is influenced by the size of the prey, and that of subsocial species by the degree of disturbance.
In their latest study, the researchers modeled the eastern slopes of the Andes on a computer, incorporating changing gradients (changes in the maximum size of prey animals and the degree of disturbance). Several virtual social and subsocial spider colonies of different sizes were 'transported' to this modeled habitat, and it was observed which species survived in which environment and where they proved to be more successful.
The model clearly illustrates that the combination of prey size and disturbance is indeed capable of forming the distribution patterns that occur in nature. The researchers showed that disturbance is not only responsible for the destruction of small colonies, but can also have a potentially stabilizing effect, even in the largest colonies. The results indicate that the ecological characteristics of a given habitat can have a significant impact on the social behavior of the species living there.
Publication:
Vásárhelyi, Zs., Scheuring, I., & Aviles, L. (2022). The ecology of spider sociality – A Spatial Model. American Naturalist, 199:6 https://doi.org/10.1086/719182