Quantitative Genetics of Scale Counts in the Garter Snake Thamnophis sirtalis

This study addresses the quantitative genetic basis of phenotypic variation and covariation for a series of meristic traits in the garter snake Thamnophis sirtalis fitchi (six head scale counts: loreals, supraand infralabials, preand postoculars, temporals; three body scale counts: ventrals, subcaudals, dorsal scale rows at midbody; two derived traits: umbilical scar size and position). Each trait was scored on approximately 540 offspring and their 47 dams captured in the wild while gravid. Correlations of the meristic traits with body mass at birth, dam's snout-vent length and body mass, litter size, and number of days each dam was held under laboratory conditions prior to giving birth were removed by computing residuals from multiple regression equations. Narrow-sense heritabilities (estimated by restricted maximum likelihood) of residuals were high for temporal scale counts (0.59), moderately large for ventral (0.29) and subcaudal scale counts (0.41), and low (in the range 0-0.12) for the other five traits. Probably as a consequence of the low statistical power of significance testing under restricted maximum likelihood, only the heritability for temporal scales was significantly different from zero. Phenotypic (rp = 0.25) and genetic (rg = 0.67) correlations between ventrals and subcaudals were positive and significant. Phenotypic correlations between the head scale counts were generally low; however, the genetic correlations were larger, suggesting relatively tight integration at the genetic level. Phenotypic correlations between the head and body scale counts were generally low, but several genetic correlations were large (e.g., rg = -0.59 for ventrals and infralabials, rg = 0.59 for subcaudals and supralabials). These data indicate that scale counts from different regions of the body are not evolutionarily independent characters, despite their different spatial and temporal relationships during development. Overall, genetic correlations were not strongly correlated with either phenotypic (r = 0.42) or environmental correlations (r = 0.16).