Prey evolved adaptation to snake venom

    An important aspect of snake venom evolution is the predator-prey interactions that lead to venom resistance and increased venom potency. These interactions lead to an evolutionary “arms race” that is further complicated by differing selection pressures among subpopulations of prey and predators. A recent paper [1] examines the variations of venom resistance among two subpopulations of Douglas squirrels and two subpopulations of Beechey squirrels to the venom of northern Pacific rattlesnakes, southern Pacific rattlesnakes, and western diamondback rattlesnakes. One of the Douglas squirrel subpopulations came from an area with a high density of northern Pacific rattlesnakes while the other population had an extremely low density of rattlesnake predators. The two Douglas squirrel subpopulations are estimated to have diverged about 3,000–15,000 years ago. The two subpopulations of Beechey squirrels both have high densities of rattlesnake predators with one subpopulation coming into contact with only the northern Pacific rattlesnake, while the other comes into contact with both the northern and southern Pacific rattlesnake. The venom of the southern Pacific rattlesnake was likely used as a baseline of venom resistance for the Douglas squirrel subpopulations, as the range of this rattlesnake does not overlap with the Douglas squirrel subpopulations studied. Likewise, the venom of the western diamondback rattlesnake was likely used as a baseline of venom resistance for the Beechey squirrel subpopulations, as the range of this rattlesnake does not overlap with the Beechey squirrel subpopulations studied.

    

The Northern Pacific Rattlesnake (left,courtesy of the Fresno Chaffee Zoo) and the Southern Pacific Rattlesnake (right, courtesy of USGS).

    

    To test the potency of the rattlesnake venoms, Biardi et al. (2006) measured the metalloprotease activity (which breaks down the lining of blood vessels), fibrinolysis (which prevents coagulation by breaking down fibrin clots), and hemolysis (which breaks open red blood cells), in addition to a radioimmunoassay (RIA). Plasma from the Douglas squirrel subpopulations was assayed against northern and southern Pacific rattlesnake venom, the Beechey squirrel subpopulations were tested against all three rattlesnake venoms. When the venom of the northern Pacific rattlesnake was assayed with the plasma of the Douglas squirrel subpopulation with a high density of northern Pacific rattlesnakes, the venom showed decreased collagen hydrolysis (the assay used to measure metalloprotease activity) and decreased hemolysis compared to the venom-plasma assay of the other Douglas squirrel subpopulation. However, the Douglas squirrel subpopulation with an extremely low density of northern Pacific rattlesnakes was better able to inactivate the collagen hydrolysis and fibrinolysis activity of the southern Pacific rattlesnake venom as compared to the other subpopulation. There were no statistically significant differences between the Beechey squirrel subpopulations in any of the venom assays. The results for the Douglas squirrel subpopulations suggest that increased resistance against the venom of a specific rattlesnake species comes at the cost of decreased general venom resistance. It is also interesting to note the level of resistance in the Douglas squirrel subpopulation with extremely low rattlesnake density to both of the venoms assayed, even though there is little selective pressure for this resistance to remain.


Western Diamondback Rattlesnake (Courtesy of Wikimedia Commons)


 Resources cited in this section:

1. Biardi, J.E., Chien D.C., & Coss, R.G. California ground squirrel (Spermophilus beecheyi) defenses against rattlesnake venom digestive and hemostatic toxins. J. Chem. Ecol. 32, 137-154 (2006).