Variation in number of ventral scales in snakes: effects on body size, growth rate and survival in the adder, Vipera berus

Abstract

The relationship between number of ventral scales, correlating with number of body vertebrae, and body size of adders, Vipera berus, was investigated using captive‐born young and wild‐caught adults (snout‐vent length > 400 mm) from six populations in eastern Sweden. Females had significantly more ventral scales, and were larger, than males in all populations. Among adult individuals, snout‐vent length was highly positively correlated with the number of ventral scales when differences due to sex and locality were controlled for. The same pattern was true for newborn individuals when differences due to litter and sex were controlled for. The influence of number of ventral scales on survival and growth rate as possible causes of this positive correlation was examined. Mean number of ventral scales was lower in new‐born snakes than in adults, indicating selection against individuals with a low scale count. Since this selection appears to take place in the early juvenile phase, it is inadequate to explain the relationship between adult body size and number of ventral scales. However, individuals with many ventral scales had significantly higher growth rates than individuals with few ventrals. This suggests that individuals with many ventral scales enjoy a higher growth rate and therefore are able to reach a larger size than their conspecifics of the same age with few ventral scales. This would explain the positive relationship between body size and ventral scale number observed in new‐born and adult snakes.