“Destruction of spider webs and rescue of ensnared nestmates by a granivorous desert ant (Veromessor pergandei)”
Christina L. Kwapich and Bert Hölldobler (Sep 2019)
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Ants destroy spider webs and rescue trapped sisters that call for help
Ant colonies benefit when nestmates get tangled
Few prey species seek out and destroy the traps designed to capture them, and only a handful rescue group members in distress. That is why Kwapich and Hölldobler were surprised to discover that desert seed harvesting ants systematically dismantle spider webs constructed along their foraging routes, and retrieve sisters ensnared in spider silk. Animals that perform rescue behavior typically live in small groups with high-value individuals, but Veromessor pergandei form enormous societies that deploy up to 30,000 foragers each morning.
To determine why colonies rescue seemingly disposable workers, the authors calculated the costs and benefits of web removal. They found that the seeds carried by foraging ants become tangled in undetected webs, reducing the total number of foraging trips individuals can take per day. By accounting for the length of a foraging career and number of trips per day, they estimated that unchecked spider predation could cost colonies 65,000 seeds per year. This is a high price to pay, because colonies need to gather enough resources to rear 600 new sisters each day.
Many ant species clear debris from their foraging routes, but Kwapich and Hölldobler showed that V. pergandei foragers ignore novel objects, and even lack an innate ability to detect spider silk. Instead, ensnared ants release a chemical alarm signal, which stimulates a subset of large-bodied nestmates to remove surrounding webbing. Frozen ‘dummies’ marked with the same alarm compound were also rescued, and freed from their silk bindings. In essence, colonies only benefit from the removal of webs when workers are captured in them. Other seed harvesting ant species arrest foraging or change their foraging patterns in response to spiders. The authors propose that foraging on a single route during a limited temperature window, coupled with the necessary scale seed harvesting, led V. pergandei to its unusual defensive strategy.
Prey species rarely seek-out and dismantle traps constructed by their predators. In the current study, we report an instance of targeted trap destruction by an invertebrate, and a novel context for rescue behavior. We found that foragers of the granivorous desert ant, Veromessor pergandei, identify and cooperatively dismantle spider webs (Araneae: Theridiidae, Steatoda spp. and Asagena sp.) During group foraging, workers ensnared in webs are recovered by sisters, who transport them to the nest and groom away their silk bindings. The presence of an ensnared nestmate and chemical alarm signal significantly increased the probability of web removal and nestmate retrieval. A subset of larger-bodied foragers participated in web removal, and 6.3% became tangled or were captured by spiders. Most animals that perform rescue behavior live in small groups, but V. pergandei colonies include tens of thousands of short-lived workers. To maintain their size, large colonies must collect enough seeds to produce 650 new ants each day. We hypothesize that the removal of spider webs allows for an unimpeded income of seeds on a single foraging path, during a brief daily temperature window. Despite the cost to individuals, webs are only recognized and removed when workers are captured in them.