Poke but donʹt pinch: risk assessment and venom metering in the western black widow spider, Latrodectus hesperus

The capacity to assess threat, and modulate defensive behaviour accordingly, is widespread among animals. Although numerous studies have demonstrated that venomous animals possess control of venom expenditure in predatory contexts, few have examined venom metering in defensive contexts. Here, we investigated venom use in the context of threat assessment in the western widow spider, Latrodectus hesperus, a synanthropic, medically relevant species found throughout western North America. In experiment 1, we subjected wild-caught adult females (N = 43) to single (low threat) and repeated (medium threat) prods with a gelatin ‘finger’, and repeated pinches between two gelatin fingers (high threat). Poking at low threat elicited primarily avoidance responses (‘move’ and ‘retract’), repeated prodding at moderate threat stimulated increased silk-flicking, and pinching at high threat provoked significantly more biting. In experiment 2, we pinched spiders (N = 20) to induce bites of three successive targets (parafilm-covered tubes) from which venom could be recovered. Spiders modulated venom expenditure at two levels. First, spiders chose whether to use venom, frequently delivering dry bites (>50% of interactions) independent of venom depletion. Second, spiders metered venom quantity, delivering 1.8-fold more venom per bite when pinched on the body versus a leg, and 2.3-fold more venom when successive target presentations were separated by a lengthy (5 min) compared to a brief (5 s) interval. These findings suggest that spiders make decisions about silk and venom use for defence. Because these weapons are metabolically costly to replace, they represent limited commodities that should be used judiciously.