Butch knew from his earlier experiments that TTX defends newts against predators, so he came up with another hypothesis for their overproduction of TTX. Maybe there was some predator out there that could withstand large amounts of TTX and that had caused the newts to evolve excessive levels of TTX. That predator, Butch reasoned, must be resistant to the effects of TTX and can eat the newts while avoiding suffering the effects of the toxin. Butch hypothesized that newts produce so much toxin because a predator had evolved resistance to TTX.
As luck would have it, another biologist in Oregon observed something quite surprising: common garter snakes were eating newts despite their toxicity! After hearing about newt-eating garter snakes, Butch hypothesized that the snakes had evolved resistance to the newt’s toxin. To test this idea he offered newts to garter snakes in the lab. The snakes gobbled up several of these otherwise deadly animals in a row! While just the tail tip of one newt could kill a full grown human, the 1/2 pound garter snakes slurped down whole newts easily. Butch concluded that garter snakes have indeed evolved resistance to the deadly defenses of the dangerous newts.
Butch suspected that the snakes had evolved this resistance in response to newt toxicity and newt toxicity, in turn, may have evolved in response to snake resistance — in other words, the two species may have been evolving in response to each other.