Rattlesnakes represent the quintessential ambush predator. They can withstand months of starvation whilst they patiently await the arrival of prey within striking distance. Only then can a snake launch an attack to envenomate, subdue and consume its meal. Whilst studying this dramatic act, it is no wonder that most researchers have directed their attention to the traits of the impressive reptilian predators. But a pair of studies, recently published by Rulon Clark's group at the San Diego State University, USA, focused instead on the adaptations that their prey, specifically kangaroo rats, use to evade death.
Like the Australian marsupials from which their name is derived, kangaroo rats have huge, powerful legs to propel them across the sandy terrain. In the first study, led by Grace Freymiller, the team investigated how the legs of the kangaroo rats can be used to escape or for self-defence (doi:10.1093/biolinnean/blz027). To study the event, the team tracked snakes, which they had rigged earlier with small radio transmitters, in an Arizona desert. When they found a rattlesnake lying in ambush, the group set up high-speed video cameras and infrared lighting so that they could film after nightfall, when the kangaroo rats become active. To lure the kangaroo rats to the arena, the research team sprinkled a few sunflower seeds – a tempting treat – in the vicinity of the snakes.
Freymiller and colleagues filmed a breath-taking series of interactions, in which the agile rodents displayed a plethora of kicks, flips and twists to deter and escape a rattlesnake strike. The kangaroo rats made near-vertical leaps 40 cm into the air (an impressive feat corresponding to 6 times the animal’s body length). Unlike the case with most other rodents, the jumps were skilfully controlled to direct their path away from the snake. Most strikingly of all, some of the kangaroo rats targeted powerful kicks at the attacking rattlesnakes. The authors therefore believe that the huge hindlimbs may have evolved first as a tool to evade predators.
The prevailing, predator-centric dogma has assumed that the success of an attack primarily depends on the performance of the hunter, in this example the velocity or accuracy of a snake's strike. However, the team now realised that the prey may also be capable of life-saving self-defence mechanisms. In their second paper, led by Malachi Whitford, the group sought to quantify how effective the kangaroo rats’ acrobatics were (doi:10.1111/1365-2435.13318).
The team noted how often the snakes hit their target and how often the kangaroo rats survived the encounter. Furthermore, they recorded whether or not the snake released the rodent from its bite of its own volition, or whether the kangaroo rats actively removed the fangs. Of the 32 strikes that Whitford and colleagues analysed, in just over half (17) the snake missed the kangaroo rat. Usually, this was because the rat dodged the strike, although in one case a snake comically collided with a branch. Of the 15 hits, seven strikes resulted in fatalities, whereas eight kangaroo rats survived the encounter. In only three cases did the snake voluntarily release its fangs, all of which resulted in the rodent's death. However, when the kangaroo rat was able to prematurely dislodge the viper's fangs, it increased its chances of survival to 67% (8/12).
These harmonious companion studies show us that the adaptations of prey animals can be just as extraordinary as those of their predators. After all, they are fighting for their lives, whilst the predator is merely hoping for its next meal. Kangaroo rats are able to effectively defend themselves and, to some extent, hold their fate in their own hands (or feet).
