Are smaller animals behaviourally limited? Lack of clear constraints in miniature spiders

Abstract

Because of scaling trends in physiology and morphology, very small animals are expected to suffer especially strong selection to reduce the cost of the central nervous system, which may make them more likely to sacrifice behavioural capacities to economize on nervous tissue. This [`]size-limitation' hypothesis predicts reduced behavioural capabilities in smaller animals. I tested this hypothesis by comparing web construction behaviour of young nymphs and adults of a very small orb-weaving spider, Anapisona simoni (young nymphs ~0.005 mg, adults ~0.8 mg), with those of relatives up to 104 times larger. In these comparisons I took advantage of the special opportunities offered by orb webs to study fine behavioural details during web construction, because the webs represent precise records of large numbers of behavioural decisions. Combining these results with those of a previous study, the size-limitation hypothesis was not supported: very small spiders failed to show three predicted trends, and they showed four other trends that were in directions opposite to those predicted by the hypothesis. Two additional intraweb comparisons (at least one of which was probably biased against equal performance by the smallest species) gave a mix of support and lack of support for the predictions, while only one interspecific difference supported the predictions. Other studies have shown that small spiders have relatively large central nervous systems for their body sizes, suggesting that they may maintain behavioural capabilities comparable to those of larger orb weavers by paying the material and metabolic costs of building and maintaining large volumes of nervous tissue. These considerations may have general consequences for the probability of evolving small body sizes and egg sizes in spiders and other animals.