We investigated whether circulating glucocorticoids and androgens are correlated with reproductive investment in smallmouth bass (Micropterus dolomieu), a teleost fish with sole paternal care. Circulating cortisol and androgens prior to and 25 min following a standardized 3 min emersion stressor were quantified for non-reproductive and parental fish across the parental care period. To experimentally investigate the influence of reproductive investment on endocrine parameters, we manipulated brood size (reduced, enlarged, sham-treated, or unmanipulated) 24 h prior to sampling parental fish. We predicted that fish guarding offspring would exhibit increased androgens and baseline cortisol levels, and an attenuated cortisol response to the stressor when compared with non-reproductive individuals. We further predicted that these effects would scale with reproductive investment. As predicted, parental care-providing fish exhibited lower post-stress plasma cortisol concentrations than non-reproductive fish. This difference was strongest early during parental care. However, no differences in baseline or post-stress cortisol concentrations were detected among parents guarding offspring with varying brood sizes. There was, however, a trend for parental fish to exhibit an increased cortisol response following brood manipulation, regardless of the direction of change in brood size, a response that likely reflected disturbance. No differences were found in baseline cortisol concentrations. Circulating androgens were found to be highest during early parental care, and no differences were found among parents guarding manipulated broods. Collectively, these findings demonstrate that the endocrine stress response is affected by reproductive status, but the response in this model species does not appear to be scaled according to reproductive investment as predicted by life-history theory.
During May and June 2008, male smallmouth bass guarding nests at various stages of development were identified by snorkeling on Charleston Lake, a public lake in eastern Ontario that is part of the Gananoque River system (44°32′N, 75°59′W). For all males, an egg or fry score was assigned. An egg score is a standard and highly repeatable measure of the relative number of eggs within a nest, and ranges from 1 (low, <500) to 5 (high, >4000; , ). Fry scores are based on a similar relative scale. For standardization, only males with an egg or fry score of 3 or 4 were included. For the purposes of this study, offspring development stages were as outlined in . The ‘egg’ stage denotes freshly laid and fertilized eggs. ‘Egg-sac fry’ appear approximately 1 week post-fertilization, when the yolk sac has been partially absorbed, and the larvae have developed tails and are able to wriggle within the nest. At this stage, the larvae are still feeding endogenously, and are unable to swim. The ‘free-swimming fry’ stage occurs approximately 3 weeks post-fertilization, when the offspring have almost entirely absorbed the yolk sac, and have gained the ability to swim. Larvae at this stage subsist on a combination of endogenous and exogenous feeding, and can be found in the water column above the nest. The parental male is still required to defend the brood, since free-swimming fry have not yet fully developed predator avoidance tactics .
On May 22, 25, and 26, 2008 (water temperature 14–16 °C), 25 parental male smallmouth bass (350–466 mm) guarding nests with fresh eggs (0–1 day old) were identified. On June 2 and 5, 2008 (water temperature 17–18 °C), a different group of 23 males (339–493 mm) guarding nests that contained egg-sac fry was identified. Finally, on June 12 and 13, 2008 (water temperature 20–21 °C), a final group of 23 males (273–465 mm) guarding free-swimming fry was identified. Individual fish were sampled at only one stage. Once a nest was identified, the parental male was captured using standard rod-and-reel angling and a rubber mesh landing net, and immediately placed in a foam-lined trough filled with fresh lake water, a procedure that took less than 30 s from hooking the fish. A blood sample (∼1 mL) was withdrawn by caudal puncture, using lithium-heparinized 3 mL vacutainers (B.D. Vacutainer, Franklin Lakes, NJ) and 21 gauge, 38 mm needles. Fish were then subjected to a standardized stressor consisting of 3 min of air exposure (a period of time that is sufficient to elevate circulating cortisol; ). During air exposure, fish were placed on a damp foam mat within a 35 × 60 cm plastic tub covered by a loosely-fitted lid to shield the fish from direct sun. After the stressor, fish were placed in individual 50 L coolers filled with fresh lake water for a 25 min recovery period, before a second blood sample was withdrawn as described above. Total length (TL) was measured and the fish was released.
To provide a comparison group of non-nesting control fish, 53 mature fish of reproductive size (273–502 mm) were captured in May and June 2009, from several lakes in eastern Ontario across a range of water temperatures that corresponded to the water temperatures during parental care (see Table 1). Identifying the sex of smallmouth bass that are non-reproductive is not reliable without dissecting the animals, so these non-reproductive fish were likely a mixture of males and females. Upon capture, all fish were subjected to the sampling regime and standardized stressor described above.