Plasticity and cognition in spiders

Abstract

Spiders can be a particularly important model for the study of cognition. Their close interaction with niche-constructed environmental features, such as webs, cocoons, draglines or retreats, allows for the experimental manipulation of these silken structures, and thus for a controlled study of the cognitive machinery that underlie the use and construction of these structures. There are contrasting theories about cognition, and we explore particularly the opposition between the traditional approach, the one that requires information to be processed solely within the central nervous system (CNS), and the extended cognition approach, which is less restrictive. Here we review the literature on spider cognition with an eye to the experimental data that allows the contrast between these theories of cognition, and conclude that spiders evolved to process information prior to reaching the nervous system: they use their webs to decide whether to attack or not a prey item, and we can experimentally alter their decision by manipulating web properties, such as radii tension. The experimental manipulation of web threads also alters the attentional state of the web building spider so that she predictably ignores important cues for decisions taken during the building process. Together, the experimental evidence shows that spiders extend their cognitive machinery outside the bounds of their CNS, making use of the external silken structures to offload cognitive processing. This insight may help to explain graded changes in brain/body allometry, because smaller animals could rely more on extended cognition so as not to be behaviourally limited by a relatively small brain. Extended cognition could also help explain the emergence of new levels of organisation, particularly the transition from solitary to social life. In general, extended cognition emerges as a natural bridge between two traditionally separate research agendas: the area of cognitive development (learning mechanisms) and that of evolution through natural selection.