Abstract
Reproductive success is the ultimate measure of individual quality; however, it is difficult to determine in free-living animals. Therefore, indirect measures that are related to reproduction are generally employed. In snakes, males typically possess longer tails than females and this sexual size dimorphism in tail length (TL) has generally been attributed to the importance of the tail in mating and reproduction. Thus, intra-sexual differences in tail length, specifically within males, were hypothesized to reflect individual quality. We used a body condition index (BCI) as a measure of quality in snakes and predicted that tail length would be correlated with BCI in males. We tested our prediction by determining BCI in the free-ranging adult male and female crowned leafnose snake (Lytorhynchus diadema), a colubrid species that inhabits mainly desert sand dunes. The relative TL was correlated positively and significantly to BCI in males (F1,131 = 11.05; r2adj = 0.07; P < 0.01) but not in females, thus supporting our prediction. This is the first time that the relationship between TL and body condition was tested in a free-ranging species. In addition, sexual size dimorphism of TL increased intra-specifically with body size, which was also found in interspecific analyses following Rensch’s rule.
Highlights
Sexual size dimorphism (SSD) occurs in many species of invertebrate and vertebrate taxa[1,2]
Average snout-vent length (SVL) of males (n = 133), 33.5 (±0.31) cm, was significantly longer than that of females (n = 107), 32.0 (±0.32) cm (F1,238 = 10.12, P < 0.01), the difference amounting to 4.5%; but the variance between sexes was similar (P = 0.348)
The regression of tail length (TL) on SVL was significant in males (TL = 0.158 SVL + 0.573; r2 = 0.48; P < 0.01) and in females (TL = 0.118 SVL + 1.512; r2 = 0.44; P < 0.01), and explained more than 40% of the variation in TL in both sexes (Fig. 2); the slope of the regression equation in males was higher than that of females (Sb1-b2 = 0.0190; t = 2.51; df = 236; P < 0.01)
Summary
Sexual size dimorphism (SSD) occurs in many species of invertebrate and vertebrate taxa[1,2]. Interspecific analyses of tail and body lengths, as well as clutch mass in colubrid snakes, supported both the morphological constraint hypothesis and the female reproductive hypothesis These analyses indicated a male-biased TL dimorphism in taxa having relatively short tails[14]. If TL of females is at the optimum size for hunting and locomotion[15], the additional length in males should serve a reproductive function, either to house a longer hemipenis or to increase mating opportunities[18], and could be considered a sexual selected trait[20,21]. BCI could be estimated using a non-invasive measure that is independent of body size and, in snakes, has been shown to reflect fat reserves[38] This parameter has been described as being “intimately related to an animal’s health, quality or vigour and has been widely claimed to be an important determinant of fitness”[30]. We used BCI as a measure of individual quality, which we regarded as being related to reproductive success
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