Relationship of blood corticosterone, immunoglobulin and haematological values in young crocodiles (Crocodylus porosus) to water temperature, clutch of origin and body weight.

Objective: To examine whether sub-optimal temperature induced stress and immunosuppression in farmed saltwater crocodile (Crocodylus porosus) hatchlings.

Design: A clinico-pathological study.

Animals: A total of 140 hatchlings were used.

Procedure: Body weight and length, plasma corticosterone and immunoglobulin concentrations and total and differential white blood cell counts were measured in 140 hatchlings from five clutches divided between five water temperature treatment groups. Initially all groups were housed at 32 degrees C for 10 weeks, then two groups (L, LC) were changed to low temperature (28 degrees C) and two groups (H, HC) to high temperature (36 degrees C), while one group (C) remained at 32 degrees C. The LC and HC groups were maintained at these temperatures for 10 days, after which the water temperature of both groups was returned to 32 degrees C. Blood samples were collected twice (at 6 and 9 weeks of age) before the initial temperature change, and at 10 days and 4 weeks after the initial temperature change (at 11.5 and 14 weeks of age).

Results: Except for an increase in plasma corticosterone in the HC group and a decrease in the L group when the temperature change was first introduced, changes in plasma corticosterone were not significant. There were no significant changes in immunoglobulin concentrations. There were, however, significant decreases in the total white cell and lymphocyte counts in the LC group after the temperature was decreased to 28 degrees C, and an increase in these counts after water temperature was returned to 32 degrees C. Clutch of origin had significant effects on body weight and length gains, and there were negative relationships between body weight and corticosterone concentrations and between body weight and immunoglobulin concentrations.

Conclusions: As haematological changes indicative of stress were not associated with significant changes in serum corticosterone, immunosuppression in young crocodiles may be independent of the hypothalamic-pituitary-adrenal cortical axis.