Proximate mediators of reproductive behaviors in vertebrates have a long history of study. In fishes, relatively few studies have focused on hormonal control of parental care, despite a comprehensive background on the general physiology of fishes, and the frequent occurrence of parental care behaviors. Studies on this taxon have repeatedly found that the relationships between androgens and paternal care do not follow the predictions made in the avian and mammalian literature. Glucocorticoids may also have a role in mediating parental behaviors, possibly through their role as regulators of metabolism. As such, we investigated the role of 11-ketotestosterone (11-KT) and cortisol in mediating parental effort of male smallmouth bass (Micropterus dolomieu) by manipulating hormone titers in wild fish. In smallmouth bass, males spawn annually with a single female and defend a single brood for up to 30 days. Treatment of parental fish with cyproterone acetate (CYA; an androgen receptor antagonist) resulted in a decrease in nest defense in response to a simulated brood predator; however, no changes in nest success, nest tending or biochemical indicators of nutritional status were detected. Treatment with exogenous cortisol did not change parental behavior, but did increase the rate of nest failure, possibly owing to the energetic cost of chronically elevated cortisol concentrations. We discuss these findings in the context of resource-driven trade-offs and highlight life history as an important factor controlling parental effort in species with costly parental care behaviors.
All fish were sampled under an Ontario Ministry of Natural Resources Scientific Collection Permit issued to S.J.C. and handled in accordance with the guidelines of the Canadian Council on Animal Care as administered by Carleton University (B09-12). From May 7 to 9, 2009, 50 male smallmouth bass (M. dolomieu) guarding nests with fresh eggs (0–1 day old) were identified by a snorkeling survey in the littoral zone of Sand Lake (N 44 33.418, W 76 14.558), in eastern Ontario, Canada. Angling for bass is prohibited by Ontario provincial law at this time, and we saw no evidence that experimental fish were targeted (i.e., no illegal angling observed) or caught (no evidence of hook wounds) by recreational anglers. The snorkeler recorded nest location and egg score, a visual assessment of brood size ranging from 1 (low) to 5 (high) (Philipp et al., 1997). Nests were marked with numbered polyvinyl chloride (PVC) tiles for identification and fish were assigned to one of three treatment groups: control, CYA or cortisol.
Cortisol-treated fish (n = 15) received an intraperitoneal injection of cocoa butter (5 mL kg−1 body weight) impregnated with 10 mg mL−1 hydrocortisone 21-hemisuccinate (Sigma H4881; Sigma-Aldrich Inc., St. Louis, MO). This widely used method elevates circulating cortisol concentrations for 5–6 days (Gamperl et al., 1994). Similarly, CYA-treated fish (n = 15) received an intraperitoneal injection of cocoa butter (2 mL kg−1 body weight) impregnated with 10 mg mL−1 cyproterone acetate (Sigma C3412; Sigma-Aldrich Inc., St. Louis, MO). Previous studies have administered CYA via daily intraperitoneal injections (e.g., Chowdhury and Joy, 2001), aquarium water (e.g., Sharpe et al., 2004) or food (e.g.,Navarro-Martin et al., 2009), methods that were not possible in the current study. Consequently, the CYA dose used in the current study was estimated from mammalian studies (e.g., Neumann and Kalmus, 1991). Control fish were handled in the same manner as treated fish (see below), but did not receive an intraperitoneal injection. Based on the arguments put forward by DiBattista et al. (2005) concerning variable cortisol responses in sham-treated animals, a sham treatment group (i.e., fish injected with cocoa butter alone) was not included in the experimental design.