Summary
Parental care is an essential life-history component of reproduction for many animal species, and it entails a suite of behavioural and physiological investments to enhance offspring survival. These investments can incur costs to the parent, reducing their energetic and physiological condition, future reproductive capabilities and survival. In fishes, relatively few studies have focused on how these physiological costs are mediated. Male smallmouth bass provide parental care for developing offspring until the brood reaches independence. During this energetically demanding life stage, males cease active foraging as they vigorously defend their offspring. Experimental manipulation of cortisol levels (via implantation) and food (via supplemental feeding) in parental males was used to investigate the fitness consequences of parental care. Improving the nutritional condition of nest-guarding males increased their reproductive success by reducing premature nest abandonment. However, supplemental feeding and cortisol treatment had no effect on parental care behaviours. Cortisol treatment reduced plasma lymphocyte numbers, but increased neutrophil and monocyte concentrations, indicating a shift in immune function. Supplemental feeding improved the physiological condition of parental fish by reducing the accumulation of oxidative injury. Specifically, supplemental feeding reduced the formation of 8-hydroxy-2′-deoxyguanosine (8-OHdG) on DNA nucleotides. Increasing the nutritional condition of parental fish can reduce the physiological cost associated with intensive parental activity and improve overall reproductive success, illustrating the importance of nutritional condition as a key modulator of parental fitness.
Methodology
From 17 to 22 May 2014, 55 nest-guarding male smallmouth bass were identified by snorkeling surveys in the littoral regions of interconnected lakes in the Rideau watershed in eastern Ontario, Canada. During this period, smallmouth bass are actively spawning and Ontario Provincial Law strictly prohibits recreational angling for this species. The study sites were rigorously monitored for angler activity, and sampled fish that bore evidence of angling or capture (i.e. visible hook wound in the mouth) were excluded from the study. Upon identification of a nest with an actively guarding male, a brood survey was conducted. The snorkeler visually assessed and recorded brood age and density; density was scored from 1 (low) to 5 (high) based on the method of Suski and Philipp (2004). Each nest was individually marked with a numbered polyvinyl chloride tile, and the guarding males were randomly assigned to one of four treatment groups: control (n=14), control plus food (n=13), cortisol (n=15) and cortisol plus food (n=13).
Cortisol-treated males were given an intraperitoneal injection in the ventral midsection between the pectoral fins and the cloaca of cocoa butter (5 ml kg−1 body mass) containing emulsified hydrocortisone 21-hemisuccinate (Sigma H4881; Sigma-Aldrich, St Louis, MO, USA) (10 mg ml−1 of cocoa butter), using a 16-gauge needle and 10 ml syringe. This method has been used in previous studies to elevate circulating plasma cortisol levels for up to 6 days (Gamperl et al., 1994; Dey et al., 2010). Control fish were handled in the same way as cortisol-treated males but did not receive an intraperitoneal injection of any kind. A sham treatment group was not included due to previous validation studies that have shown that a sham injection of cocoa butter does not induce the semi-chronic cortisol elevation that is characteristic of cortisol injections (O'Connor et al., 2009). Moreover, the control fish serve to some extent as a sham because they have to be captured for initial assessment.