Parental Care

Introduction

Parental care in largemouth bass involves a suite of behaviors that are critical to ensure the success of a reproductive attempt. In this study, we found that exogenous supraphysiological elevations of plasma cortisol in parental males resulted in physiological changes consistent with a change in energy mobilization and carbohydrate catabolism. Observationally, we noted some evidence of immune suppression. We found that, ultimately, supraphysiological elevations of plasma cortisol were associated with a decrease in nest success, with fewer cortisol treated males guarding the offspring until the swim-up fry stage when compared with the control or sham-treated groups. We did not observe a change in androgen, in important reproductive hormone s, or in parental care behavior before nest abandonment. We also did not observe a difference in behavior of cortisol-treated fish between fish that ultimately abandoned and cortisol-treated fish that were successful. These results indicate that while supraphysiological cortisol elevation during parental care decreases nest success, this is not a direct affect of cortisol on reproductive hormones or on behavior (e.g., reduction in vigilance resulting in depredation and subsequent abandonment). Instead, the decrease in nest success is likely associated with longer-term secondary and tertiary affects of cortisol on other functions, such as metabolism or immune function.

Discussion

The use of cortisol in coconut oil to raise plasma cortisol created some confounding factors associated with elevated cortisol as a result of the capture and handling stress of administering injections. The use of a sham treatment group does not entirely control for these factors because the sham treatment can be a Stressor that can raise plasma cortisol, perhaps chronically. Indeed, the sham treatment fish in the laboratory verification study showed plasma cortisol levels approaching those of the exhaustively exercised fish. We do not believe that this methodological limitation reduces the validity of the study, but it does require acknowledgment. Our cortisol-treated fish were subjected to handling stress and received an additional treatment of a cortisol impregnated vehicle, while the sham treatment fish were subjected to handling stress and injection with the vehicle alone (Campbell, 1150). The control and reference control groups were untreated and unhandled in any way. Our results represent the comprehensive effects of handling stress and extended cortisol elevation during parental care, rather than an isolated effect of elevated plasma cortisol.

The field setting for this study precluded a pilot study on the effectiveness of the cortisol dose, with water temperatures dictating a simultaneous laboratory study. As a result, the doses selected based on literature from rainbow trout resulted in plasma cortisol concentrations an order of magnitude higher than desired in largemouth bass. This could be a result of inter specific differences, or it could be a result of warmer water temperatures and therefore a softer coconut oil implant and faster release of cortisol; studies in rainbow trout are generally conducted at water temperatures of ~I2°C, while water temperatures during parental care in largemouth bass can exceed 21''C. Despite the pharmacological dose, we observed a typical glucose response to elevated plasma cortisol, and ...