The role of numerical competence in a specialized predatory strategy of an araneophagic spider

Abstract

Although a wide range of vertebrates have been considered in research on numerical competence, little is known about the role of number-related decisions in the predatory strategies of invertebrates. Here, we investigate how numerical competence is expressed in a highly specialized predatory strategy adopted by the small juveniles of Portia africana when practicing communal predation, with the prey being another spider, Oecobius amboseli. Two or more P. africana juveniles sometimes settle by the same oecobiid nest and then share the meal after one individual captures the oecobiid. Experiments were designed to clarify how these predators use number-related cues in conjunction with non-numerical cues when deciding whether to settle at a nest. We used lures (dead spiders positioned in lifelike posture) arranged in a series of 24 different scenes defined by the type, configuration and especially number of lures. On the whole, our findings suggest that P. africana juveniles base settling decisions on the specific number of already settled conspecific juveniles at the nest and express a preference for settling when the number is one instead of zero, two or three. By varying the size of the already settled juveniles and their positions around the nest, we show that factors related to continuous variables and stimulus configuration are unlikely explanations for our findings.

Representation of different exact numbers of prey by a spider-eating predator

“…The numerical competency of spiders has been considered in two previous studies, one on P. africana [68] and the other on Nephila clavipes, an orb-web spider [69]. As with this study, a natural-history perspective was a strong feature in both of these earlier studies.…”

“…Portia africana juveniles (body length 3 mm or less) were used in the other earlier study [68], with experiments designed to simulate encounters by P. africana juveniles with Oecobius amboseli, this being a small spider (body length of adults 2-3 mm) that builds small silk nests on stones and on the walls of buildings. The prey-capture routine adopted by P. africana juveniles is to settle beside an occupied oecobiid nest and then, by using their legs for manipulating the nest silk, to make signals to which the resident oecobiid responds by fleeing from its nest.…”

“…In experiments, P. africana juveniles were presented with lures made from dead oecobiids and dead conspecific individuals [68,75] and the sizes and arrangements of the dead prey and conspecific individuals were varied systematically. It was found that P. africana juveniles expressed their strongest predisposition to settle at occupied oecobiid nests when only a single other P. africana juvenile was present beside a nest occupied by an oecobiid.…”

Numerosity representation is encoded in human subcortex

“…Mosquitofish can perform various numerical tasks including picking larger shoals (38), choosing more companions (39) or mates (40), or discriminating larger quantities (41), and guppies have been shown to differentiate ratios with levels of precision similar to that of undergraduate college students (42). Moreover, number processing is not restricted to vertebrates as both honeybees (43) and spiders (44) possess some ability to process number, as well. Taken together, the evidence from the nonprimate literature suggests that the ability to process number may be relatively ubiquitous in the animal world.…”

Visual Discrimination Learning in the Jumping Spider Phidippus regius

“…Arthropods appear able to perform a variety of complex tasks, ranging from spatial navigation (Menzel et al, 2005;Wehner, 2003) to object perception (van Hateren, Srinivasan, & Wait, 1990), learning (Hammer & Menzel, 1995), numerical competences (Cross & Jackson, 2017;Dacke & Srinivasan, 2008;Nelson & Jackson, 2012) and even more complex skills, such as self-recognition (Cammaerts & Cammaerts, 2015) and the cultural spreading of learned abilities (Alem et al, 2016). Among arthropods, a family of spiders has caught the interest of scientists: Salticidae.…”