When the Second World War ended, the two superpowers developed their nuclear weapon programs in an “arms race.” This idea of one force putting pressure on another to change, improve, or adapt can be seen in nature as well. Predator-prey interactions shape the behavior and morphology of both sides so that both prey defenses and predatory strategies co-evolve over time. There are three basic steps to prey consumption: predator’s ability to locate, capture, and handle the prey. This may seem simple but organisms have evolved incredible ways to effectively subdue species that are comparable in size.
Segovia et al. investigated this phenomenon with the delicate-bodied recluse spider and its heavy-bodied prey, the harvestman. They tested three hypotheses specifically: spiders use chemical cues left by prey to determine their location, vibratory cues are essential in the predatory process, and the web allows adequate handling of the prey so vulnerable regions could be bitten.
In their first experiment, they let the spiders roam in an arena with chemicals on one half and no chemical cues on the other. They found that the spiders spent the same amount of time on both sides. To test vibratory cues, the spiders were forced to hunt on substrates that either enhanced or inhibited vibrations, filter paper and granite, respectively. Again, the substrates had no effect on the number of harvestmen eaten or the latency for attack. In their final experiment, they categorized spiders into web or non-web groups to see if webs in their environment aided in the capture of prey. To their surprise, webs also did not make a difference in the capture times of harvestmen.
In the end, the researchers had to reject all of their hypotheses since their observations contradicted their predictions. Instead, they came up with alternative explanations for what they saw. A flaw in their design was looking at one mode of communication at a time (vibration, chemicals). Thus, it is possible that multimodal signaling is taking place, such as detection of air displacement, in addition to the ones they studied. Another interesting observation they noticed was that all spiders selectively attacked the joints and distal portions of the harvestmen’s legs. By accurately biting in the Harvestmens’ Achilles heel, the spiders can inflict more pain and venom onto their opponents. It is therefore quite likely that recluse spiders have developed a fighting strategy to take down stronger prey.
(Photograph by Nicholas Ta from http://commons.wikimedia.org/wiki/File:Brown_recluse_spider_in_June_2010.jpg)
Reference:
Segovia JMG, Del-Claro K, Willemart RH. 2015. Delicate fangs, smart killing: the predation strategy of the recluse spider. Animal Behaviour 101:169-177.
