Acta zoologica cracoviensia, 50A(1-2): 9-13, Kraków, 31 May, 2007 The causes of intraspecific variation in sexual dimorphism in the common grass snake populations, Natrix natrix LINNAEUS, 1758 (Serpentes, Colubridae): Data from the South Western Poland Bartosz BORCZYK Received: 31 Oct., 2006 Accepted: 27 Nov., 2006 BORCZYK B. 2007. The causes of intraspecific variation in sexual dimorphism in the com- mon grass snake populations, Natrix natrix LINNAEUS, 1758 (Serpentes, Colubridae): Data from the South Western Poland. Acta zoologica cracoviensia, 50A(1-2): 9-13. Abstract. Sexual dimorphism is widespread in animals, including snakes, and has impor- tant implication in both ecology and behaviour. I studied a grass snake (Natrix natrix) population from “Stawy Milickie” nature reserve. Mean snout-vent length (SVL) for fe- males was significantly greater than SVL for males, but males had proportionally longer tails. However, relative tail length (TL) in males decreased with increasing SVL whereas in females it was constant. Larger tails in males have frequently been associated with in- creasing mating success (e.g. tail wrestling behaviour between males). However, it is pos- sible this is less important in the population that I studied; instead, male snakes can allocate more energy to body growth. Key words. allometry, ecology, Natricinae, sexual size dimorphism, snakes, tail length. Bartosz BORCZYK, Laboratory of Vertebrate Zoology, Institute of Zoology, University of Wroc³aw, Sienkiewicz Street 21, 50-335 Wroc³aw, Poland. E-mail: borczyk@biol.uni.wroc.pl I. INTRODUCTION Sexual dimorphism in snakes has been frequently studied, especially in terms of body size, body mass, shape and other morphological traits (e.g. KMINIAK &KALUZ 1983; FERICHE et al. 1993; SHINE 1993). In snakes, females generally reach larger size than the males (SHINE 1993). However, the reverse trend is observed in species in which the males fight (SHINE 1978, 1993, 1994). Others factors leading to differences in body size between the sexes include: (i) Size dependence of repro- ductive costs (larger females can store more and/or larger eggs). (ii) Reaching maturity and produc- ing offspring at an earlier age (and at smaller size) may be favored if the probability of reaching maturity is at all low. If the mortality rate is high, then there may be an advantage for an individual to mature at an earlier age. Thus, one possible explanation of female-biased sexual dimorphism is higher mortality in males (BROWN &WEATHERHEAD 1999). (iii) Because larger snakes typically eat larger prey (ARNOLD 1993) size differences between the sexes may result in reduced competi- tion for food between them (SHINE 1993). The ontogenetic development of such dimorphism has been studied less often. For example, in many snake species, males have proportionally longer tails than females (SHINE 1993) and this di-