Immune configuration in hatchling snakes is affected by incubation moisture, and is linked to subsequent growth and survival in the field

We incubated the eggs of field-caught keelback snakes (Tropidonophis mairii) on wet versus dry substrates to explore impacts of incubation conditions on white blood cell (WBC) concentrations and differential WBC counts of hatchlings. In a second, independent study young snakes were released into the field, allowing us to explore fitness correlates of WBC profiles. Dry incubation reduced embryonic survival and hatchling body size, thus decoupling egg size from hatchling size. Incubation conditions also altered WBC profiles. Lymphocyte and azurophil counts were related to hatchling body size but not to initial egg mass, whereas heterophil counts were related to both of these traits. The egg-size effect on heterophil counts may reflect a maternal effect on offspring immune configuration. The ratio of heterophils to lymphocytes (an index of stress) was higher in hatchlings from eggs incubated on dry substrates. Snakes with higher counts of lymphocytes at hatching were more likely to be recaptured as adults (H:L; an index of survival), whereas snakes with higher basophil counts exhibited more rapid growth. In summary, our experiments show that incubation moisture levels influence the immune configuration of hatchling snakes, and that variation among individuals in WBC counts at hatching is a significant predictor of an individual's fitness after it is released into the wild. The demonstrated link between incubation conditions and offspring fitness is likely to impose strong selection on maternal nest-site choice.