Snakes in the Grass

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by Michael Hamil­ton, Direc­tor, Blue Oak Ranch Reserve, and
Rulon Clark, Asso­ciate Pro­fes­sor, San Diego State University

One would think that in the oak wood­lands, the most impor­tant preda­tors would be mam­mals such as moun­tain lion, coy­ote and bob­cat. But new field research is dis­cov­er­ing that the most impor­tant preda­tor may be rat­tlesnakes in terms of over­all bio­mass. Rulon Clark, an assis­tant pro­fes­sor at Cal­i­for­nia State Uni­ver­sity at San Diego, and his team of eight grad­u­ate and under­grad­u­ate stu­dents, are study­ing rat­tlesnakes and their prey at one of the newest Uni­ver­sity of Cal­i­for­nia Nat­ural Reserves, Blue Oak Ranch Reserve. The 3,300 acre reserve is sit­u­ated on the west slope of Mount Hamil­ton, only 10 miles from San Jose.

Clark and his stu­dents have doc­u­mented den­si­ties of rat­tlesnakes on the reserve that far exceed any they have pre­vi­ously encoun­tered through­out Cal­i­for­nia, includ­ing the remote Mojave Desert. Rat­tlesnakes have likely been left alone by humans at Blue Oak Ranch Reserve for many decades. Given the local abun­dance of wild prey, the snakes have reached pop­u­la­tion lev­els that may have been typ­i­cal prior to cen­turies of per­se­cu­tion by ranch­ers and hunters, accord­ing to Clark.

The rat­tlesnake researchers, dubbed “Team Cro­talus,” have just com­pleted nearly two months of detailed stud­ies of the inter­ac­tions between the north­ern Pacific rat­tlesnake, Cro­talus ore­ganus, and its pre­ferred prey species the Cal­i­for­nia ground squir­rel, Sper­mophilus beecheyi. When con­fronted by preda­tors, many ani­mals engage in lengthy, con­spic­u­ous inter­ac­tions involv­ing stereo­typed sig­nals and dis­plays. These antipreda­tor sig­nals have been stud­ied mainly as warn­ing sig­nals directed toward con­specifics, even though they may also serve to com­mu­ni­cate with preda­tors. Stud­ies of how these sig­nals affect preda­tors have been rare because pre­da­tion is infre­quent and dif­fi­cult to observe in the field.

Team Cro­talus has begun assem­bling a one-of-a-kind data­base of nat­ural antipreda­tor sig­nal­ing inter­ac­tions and preda­tor responses. To do so, they are using a high tech assort­ment of tools. These range from radio teleme­try trans­mit­ters sur­gi­cally implanted in the rat­tlesnakes, to minia­ture, battery-powered web­cams. The web­cams trans­mit live video of snake/squirrel inter­ac­tions sev­eral miles away to the reserve head­quar­ters for  record­ing and obser­va­tion. The team also con­fronts snakes with a mechan­i­cal, taxi­der­mied rodent affec­tion­ately named “Robosquir­rel.” The Robosquir­rel is pro­grammed to make antipreda­tor sounds and move­ments to snakes in exper­i­men­tal encoun­ters. The bouts allow the sci­en­tists to test pre­dic­tions of how preda­tors and prey com­mu­ni­cate in con­trolled experiments.

Team Cro­talus research has focused on the behav­ior of rat­tlesnakes con­fronted by two prey species, ground squir­rels and kan­ga­roo rats. These two dis­tantly related rodents have evolved sophis­ti­cated anti­s­nake behav­ior inde­pen­dently. Com­par­ing the two rodents’ inter­ac­tions will allow the sci­en­tists to exam­ine the role of var­i­ous eco­log­i­cal and organ­is­mic fac­tors that shape predator-prey sig­nal­ing inter­ac­tions. Their unique approach com­bines these meth­ods in stud­ies that simul­ta­ne­ously con­sider both prey sig­nal­ing behav­ior and preda­tor responses in an exper­i­men­tal con­text. This sys­tem promises to pro­vide novel insights into such areas as hon­esty in ani­mal com­mu­ni­ca­tion, antag­o­nis­tic coevo­lu­tion, and the role of ani­mal sen­sory sys­tems in shap­ing sig­nal­ing behavior.