Sexual Color Dimorphism in Crotalus lepidus klauberi Gloyd (Reptilia, Serpentes, Viperidae)

Abstract

Specimens of Crotalus lepidus klauberi collected in the southwestern United States and northern Mexico have exhibited sexual color dimorphism. The basic body color is green for males and gray for females with both sexes possessing black or dark brown crossbands. This dichromatism has probably evolved for background color-matching through natural selection influenced by predator pressure. The sexual basis of the dimorphism in body color may be the result of a sex-limited character that acts to balance the percentage of color morphs in the population or it may function in intraspecific sex recognition if C. I. klauberi, a diurnally active snake, has redeveloped color vision. Boulenger (1913:233) described sexual color polymorphism for Vipera berus stating that this characteristic was contrary to the rule for snakes. Noble (1937:687, 688) commented that sexual color differences are rare among snakes but noted that V. berus, V. aspis and Coronella are sexually dichromic. Angel (1950:240) cited the occurrence of sexual dichromatism in five species of snakes in the eastern hemisphere: Ablabes baliodirus, Psammodynastes pulverulentus, Trimeresurus puniceus, V. aspis and V. berus. Werler and Smith (1952:570) described sexual dichromatism in Manolepis putnami, heretofore the only snake in the western hemisphere accurately reported to exhibit this character. Crotalus horridus has been reported to generally display sexual dimorphism in body color (Klauber, 1956:656-657), but Schaefer (1969) has shown that the color phases of C. horridus are independent of sex. This paper describes sexual color dimorphism in Crotalus lepidus klauberi Gloyd. Wright and Wright (1957:970-971) first noted color differences between a male and female C. I. klauberi. Specimens of C. I. klauberi (15 males, 11 females) collected from the following geographic areas and examined by us in life have exhibited sexual dimorphism in body coloration: New Mexico (10), Chihuahua (15) and Sonora (1). The collecting localities (Fig. 1) for our specimens encompass a large portion of the northern part of the range of C. I. klauberi (Klauber, 1956; Conant, 1975). We did not have the opportunity to examine material from the southern or extreme western portions of the range, but four specimens from Arizona have been reported to exhibit sexual dichromatism consistent with our observations (Wright and Wright, 1957; J. W. Vincent, pers. comm.). Apparently C. I. lepidus, C. I. maculosus and C. /. morulus normally do not exhibit this character, but it does occur in some populations of C. I. lepidus X klauberi (H. S. Harris, Jr., pers. comm.). Sex was determined by the presence or absence of hemipenes, caudal scale counts and relative tail length measurements. These counts and measurements (Table 1) are consistent with data published by Gloyd (1940:113) and Klauber (1956:124) for sexual size and scutellation dimorphism in C. I. klauberi. Dichromatism occurs only in the dorsal body coloration between the distinct dark crossbands which are either black or dark brown in both sexes. The basic body coloration is green for males and gray for females (PI. 1). There are slight variations (i.e., bluish-green, bluish-gray and pinkish-gray) from these basic colors with juveniles of both sexes generally