Miniature spiders (with miniature brains) forget sooner

Abstract

Miniature animals have tiny brains and should therefore face cognitive limitations. There is little supporting evidence for this expectation, however. We focused on memory information content and retention time, which likely subtend a broad range of cognitive abilities. Our study species, a web spider, allowed us to use a simple assay of working memory: how spiders search for prey they have captured and lost. We used an ontogenetic approach, taking advantage of variation in body size and the concomitant variation in brain size across instars in a single species. This approach eliminates possible confounding variation from species differences in ecology. Small spiders were the most highly motivated to search for lost prey and made the clearest discrimination of prey size. However, when we introduced a delay between memory formation and memory use, search time decreased more steeply in small spiders than in large spiders. Small spiders also performed less additional searching after their primary bout. Thus, the retention of working memory, but not its content, was limited in small spiders with small brains. We suggest that animals that evolve miniature sizes sacrifice not the ability to perceive and acquire information, but rather the ability to retain information over time in working memory. This may, in turn, limit their ability to relate behavioural decisions to their consequences.