Paradoxical Exception to Island Tameness: Increased Defensiveness in an Insular Population of Rattle

Person, Carl E.

Gren, Eric C.

Hayes, William K.

Department of Earth and Biological Sciences

Loma Linda University

Loma Linda, CA, 92350 USA

King, Julie

Catalina Island Conservancy

Avalon, CA, 90704 USA

Briggs, Erick

Natural Solutions

Yucca Valley, CA, 92286 USA

Island tameness exists among diverse animal taxa on islands worldwide, and results largely from a lack of natural predators. Several insular populations of rattlesnakes lack functional rattles as the consequence of relaxed selection arising from isolation and reduced predation. We therefore hypothesized that Santa Catalina Island, California, USA, populations of the Southern Pacific Rattlesnake (Crotalus helleri) would exhibit a decrement in defensive behavior relative to their mainland counterparts. Contrary to our prediction, rattlesnakes from the island not only lacked tameness compared to mainland snakes, but instead exhibited measurably greater levels of defensiveness. Island snakes attempted to bite 4.7 times more frequently as we endeavored to secure them by hand, and required 2.1-fold more time to be pinned and captured by hand. When controlling for body size, island snakes delivered 2.1-fold greater quantities of venom when induced to bite a beaker after being grasped. The additional venom resulted from 2.1-fold larger boluses of venom ejected from the fangs during a given bite. Relative head size was equal for the two populations, so differences in venom expenditure likely reflected behavioral decisions associated with risk assessment and level of defensiveness. No effect of duration in captivity (2–36 months) existed, which suggests an absence of long-term habituation and that behavioral differences are innate. Breeding Bird Surveys and Christmas Bird Counts indicated reduced population densities of avian predators on Catalina compared to the mainland. However, historical estimates confirmed that populations of foxes and introduced mammalian predators (cats, pigs) and antagonists (herbivorous ungulates) substantially exceeded those on the mainland in recent centuries, and therefore best explain the paradoxically exaggerated defensive behaviors exhibited by Catalina rattlesnakes. These findings augment our meager understanding of anthropogenic effects on the behaviors of island animals, and underscore how these effects might negatively impact human safety.