Of dogs, bells, and tadpoles
/In this blog post, we'll explore how the risk of predation affects the cognitive functioning of prey.
Different populations of the same species of prey often face different levels of risk. For example, one population may coexist with a low number or low diversity of predators compared to another population. Experiments and models have revealed that this background level of risk influences how prey respond to future predation risk (e.g., the risk allocation hypothesis). However, it remains to be seen how experience with risk affects prey cognition (e.g., learning new things and remembering old things).
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The authors make use of a classic system: tadpoles. These critters have been shown by countless researchers to be sensitive to the risk of predation by changes in body shape, life history, and behavior. Grinding up tadpoles produces alarm cues that inform other tadpoles danger is nearby. Combining these alarm cues with the smells released into the water from a salamander allowed the researchers to use classic Pavlovian conditioning to “teach” the tadpoles about a new risk.
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A clever design with a variety of treatments revealed that coming from a high-risk background makes prey less responsive to continued risk (tadpole alarm cues) but more likely to respond to new kind of risk (predaceous salamander cues). Although these effects mostly wore off after ten days, the Pavlovian conditioning only stuck around for tadpoles from a high-risk background.
These results largely confirm hypotheses about how prey should respond to the risk of predation. When living in a scary world (high-risk background), prey cannot afford to freak out at more run of the mill scares. However, these same on-edge prey are better prepared for a new kind of risk than those who have lived a life free of stress (low-risk background). The stress-free prey have the luxury of taking time to evaluate a new risk and see if it is worth responding to, whereas prey living in a scary world take a shortcut and assume the new risk is definitely dangerous. The loss of responses after ten days indicates prey incorporate experience to adjust their assessment of risk: what seems scary at first no longer elicits a response if its scariness is not reinforced somehow. Exactly why prey from a high-risk background would remember a conditioned response longer is not totally clear, but it may be that the conditioning combines with the background to cement the memory more permanently that in the mind of prey from a low-risk background.
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So, what does it all mean? Well, prey are clearly not passive players in the predator-prey game of eat or be eaten. Not only can they sense their predators and respond to them, but their history of sensing predator risk colors their response. Information uncertainty is central here, as prey are constantly gathering information and updating estimates of how risky their world is. Perfect information (knowing all risks exactly) isn't attainable, so all organisms must make assumptions about how best to spend their time. Because prey can't respond to predators all the time (they also have to eat, mate, and do other stuff), they are forced to take risks. Computing when and where to take these risks is complicated, and the prey we see alive today are the offspring of parents who did a good enough job solving these problems. If the ability to do these risk-taking calculations is heritable, then we would expect prey to continue to improve their ability to avoid predators. However, predators evolve as well, keeping life interesting for both prey and scientists who study predator-prey interactions.