Living with the constant threat of being eaten is thankfully not something many of us have to deal with, but it is commonplace for many animals. Studies have shown that risky, predator-laden environments tend to lead to physiological stress, but also to prey that are better suited to dealing with their adversaries. In particular, alterations in anatomical traits involved in locomotion and defense arise in populations under the chronic threat of predation. Such phenotypic plasticity is usually quite conspicuous, and as a result has been well studied in a variety of systems, usually aquatic. But what if morphological change isn't the only way to deal with the constant stress of being eaten? Perhaps more nuanced shifts in behavioral strategy are an equally good means of defense or evasion, but have remained under scientists' radars exactly because of their subtlety. This is just the point made in a recent paper by Dror Hawlena, working with Oswald Schmitz and their colleagues at Yale University.
Hawlena and collaborators used grasshoppers as a model for studying subtle biomechanical tactics for enhanced escape performance. This is a good system because earlier work from Schmitz's lab demonstrated that chronic exposure to predatory spiders leads to a physiological stress response in these grasshoppers, but no predator-induced shifts in morphology. Do they exhibit a more highly tuned escape response, despite exhibiting no obvious anatomical changes? To answer this the researchers designed 14 small field plots (0.25 m2 area×1 m high) and placed six third-instar grasshopper nymphs in each. In seven of the plots, an adult predatory spider was also introduced a day later, although each was rendered harmless with glue holding its chelicerae together. After several months of living in their mesocosms, grasshoppers from spider plots and their controls (no spider ever introduced) were placed into an outdoor, flat arena and stimulated to jump until exhausted. The team recorded jump distances. The animals were then taken into the lab where high-speed video recordings of at least two jumps from each individual were used to calculate variables such as takeoff speed and angle.
In the outdoor experiments grasshoppers raised with spiders jumped, on average, about 50% farther than control animals. Reviewing high-speed videos of lab jumps revealed that the grasshoppers raised in a riskier environment took off at speeds significantly higher than those of animals raised without spiders. Moreover, the former, more skittish animals also appeared to alter the way in which they took off, delaying their onset of motion and improving the tibia's leverage in the 10 ms before takeoff.
Most studies of predator-induced phenotypic plasticity emphasize morphological changes in prey that make them less likely to be eaten, either because of improved defense mechanisms or because of enhanced escape capacity. However, this work demonstrates that species that lack such anatomical alterations can use behavioral or biomechanical shifts to compensate instead. In studying grasshopper jumping in a more ecologically relevant context, Hawlena and colleagues have not only shown that predation threat can improve performance but also provided biomechanists with specific variables related to tibia leverage that deserve more attention in future studies of jumping in these and similar animals.