Arms Race Analogy

In the environment, there exist many biological interactions including predator-prey interactions. Some of these interactions can become coevolution interactions. In other words, two species reciprocally affect each other evolutionarily. Brodie & Brodie (1990) used the “arms race” analogy in respect to a coevolution interaction between Thamnophis sirtalis and Taricha granulosa. The arm’s race analogy indicates the predator will increase their resistance to the prey’s defense mechanism. The prey will follow by increasing its defense mechanism and the circle will continue. With the knowledge that the Newt (T. granulosa) secretes lethal tetrodotoxin (TTX) through its glands, Brodie & Brodie (1990) observed common garter snakes eating the newts and surviving. This caused them to wonder if there was a coevolved interaction occurring between the species. Because the arms race analogy is under scrutiny by other scientists, Brodie & Brodie decided to test for the validity of the concept. After collecting garter snakes, Brodie &
Brodie decided on three test groups: Th. sirtalis which live in the same area as the newts (sympatric), Th. sirtalis that live in areas without newts (allopatric), and Th. ordinoides, which do not eat newts. Since the TTX targets the motor ability of the garter snakes, Brodie & Brodie (1990) tested the sprint speed of the snakes before and after a 0.1 mL injection of TTX. Using a 2 meter track, they performed six trials to have a baseline speed before injecting TTX. The first two trials were test runs and the last four were averaged for the final baseline speed of each individual snake. Brodie & Brodie injected a TTX and saline solution into the skeletal muscles of the snakes and then re-ran them down the racetrack. They recorded the times and compared each group to each other. The data was analyzed using the regression method. Brodie & Brodie (1990) discovered sympatric Th. sirtalis required high doses of TTX to become immobilized while the other two groups were paralyzed. Th. ordinoides could not move after the injection of 0.1 mL TTX-saline solution. The allopatric group could barely move after the injection of TTX. Brode & Brodie (1990) attributed their findings to possible genetic reasons. They believed the snakes which cohabitated with the newts possessed a genetic resistance to the toxicity of the TTX and passed this down through generations. They also found significant data supporting the arms race analogy and decided that as the snake increased resistance to the TTX, the newt would increase the toxicity of TTX levels. Brodie & Brodie (1990) also found the garter snakes that lived in areas with less toxic newts had less resistance to the TTX. Therefore, as the TTX toxicity levels increased so did the garter snake resistance. There were a few instances in which Brodie & Brodie (1990) found high resistant snakes living in areas with low toxicity newts, and this discovery struck them as odd. Brodie & Brodie’s (1990) discovery of high resistant snakes with not very toxic newts raised a question.
Were there ever sympatric Th. sirtalis with low resistance living in areas with highly toxic T. granulosa? This would be something to look into because they did find the opposite instance. However, why were there high resistant snakes residing in areas with low toxicity newts? Because the analogy has a pattern of increasing resistance followed by increased defense, there has to be a fall-off and starting over at some point. In these areas where there were high resistant predators and low defensive prey, it is possible the arms race was either just beginning or starting over because it had maxed out. Brodie & Brodie (1990) could further their research by testing how high the toxicity levels of the newt can go in order to see if an arms race was truly occurring because they felt that there is a possible advantage to increased
toxicity.