Body size evolution in insular speckled rattlesnakes (Viperidae: Crotalus mitchellii)

Abstract

Background: Speckled rattlesnakes (Crotalus mitchellii) inhabit multiple islands off the coast of Baja California, Mexico. Two of the 14 known insular populations have been recognized as subspecies based primarily on body size divergence from putative mainland ancestral populations; however, a survey of body size variation from other islands occupied by these snakes has not been previously reported. We examined body size variation between island and mainland speckled rattlesnakes, and the relationship between body size and various island physical variables among 12 island populations. We also examined relative head size among giant, dwarfed, and mainland speckled rattlesnakes to determine whether allometric differences conformed to predictions of gape size (and indirectly body size) evolving in response to shifts in prey size.

Methodology/principal findings: Insular speckled rattlesnakes show considerable variation in body size when compared to mainland source subspecies. In addition to previously known instances of gigantism on Angel de la Guarda and dwarfism on El Muerto, various degrees of body size decrease have occurred frequently in this taxon, with dwarfed rattlesnakes occurring mostly on small, recently isolated, land-bridge islands. Regression models using the Akaike information criterion (AIC) showed that mean SVL of insular populations was most strongly correlated with island area, suggesting the influence of selection for different body size optima for islands of different size. Allometric differences in head size of giant and dwarf rattlesnakes revealed patterns consistent with shifts to larger and smaller prey, respectively.

Conclusions/significance: Our data provide the first example of a clear relationship between body size and island area in a squamate reptile species; among vertebrates this pattern has been previously documented in few insular mammals. This finding suggests that selection for body size is influenced by changes in community dynamics that are related to graded differences in area over what are otherwise similar bioclimatic conditions. We hypothesize that in this system shifts to larger prey, episodic saturation and depression of primary prey density, and predator release may have led to insular gigantism, and that shifts to smaller prey and increased reproductive efficiency in the presence of intense intraspecific competition may have led to insular dwarfism.

Figure 1. Photos in life of a typical adult speckled rattlesnake from Isla El Muerto (A) and a typical adult speckled rattlesnake from Isla Ángel de la Guarda (B) (both males).

(C) Preserved specimens from A (right) and B (left) photographed to scale, showing size difference.

Figure 2. Map of the Baja California peninsular region depicting locations of islands inhabited by speckled rattlesnakes in the Sea of Cortés and Pacific Ocean.

The solid line bisecting the peninsula indicates the political boundary between Baja California and Baja California Sur and the approximate boundary between the mainland subspecies Crotalus mitchellii pyrrhus to the north (A) and C. m. mitchellii to the south (B).

Figure 3. Scatterplot of log SVL means for island populations of speckled rattlesnakes as a function of log island area.

The R² value for this model (including only area as a predictor variable) equals 0.84.