Like many teleost fishes, bluegill (Lepomis macrochirus) are characterized by sole male parental care of offspring. In addition, bluegill parental males experience cuckoldry by specialized parasitic male morphs. This cuckoldry has previously been shown to influence the expression of parental care behavior. To better understand some of the proximate mechanisms mediating parental behavior, we examined the relationships between circulating steroid hormones, paternity, and parental behavior during the egg and fry stages of care in parentals that spawned during the first third of the breeding season. During the egg stage of care, we found that males with higher paternity had lower levels of testosterone, but there was no relationship between paternity and either 11-ketotestosterone or cortisol. There also was no relationship between the hormones and care behavior comprising fanning of the eggs, nest rim circles, chases of brood predators, or pecking at the eggs (indicative of egg cannibalism), except for a negative relationship between cortisol and pecking behavior. During the fry stage of care, we conversely found that males with higher paternity had higher levels of testosterone and 11-ketotestosterone. There also was a negative relationship between the concentrations of these two androgens and the defensive behavior of males when exposed to a potential brood predator (a pumpkinseed, Lepomis gibbosus). We discuss these results in relation to previous work in fishes and other vertebrate taxa. Overall, our data suggest a complex relationship between circulating steroid hormone levels, paternity and parental behavior.
Parental care behavior
During the 2005 breeding season, daily surveys were conducted of our study site on Lake Opinicon to locate spawning activity. We selected four colonies in which to study parental care behavior. The colonies spawned between the 5th and 11th of June, which falls within the first third of the breeding season (see Magee and Neff, 2006). The day after spawning, nest position within the colony (central versus peripheral) and egg scores were assigned as a rank between 1 (= eggs covering 20% of nest) and 5 (= eggs covering 100% of nest). The scores have been shown to be highly correlated with the actual number of eggs: score 1, 27–4889 eggs; score 2, 4666–28,806; score 3, 27,072–53,221; score 4, 49,369–86,552; and score 5, 82,063–112,810 (R = 0.96, p < 0.001, n = 32; Claussen, 1991). For two of the colonies, we took behavioral measures during the first day of the egg stage of care (n = 40 parentals). These measurements comprised the number of (i) egg fanning motions using either the pectoral or caudal fins, (ii) nest rim circles, (iii) chases of potential brood predators, and (iv) pecking motions used to cannibalize eggs (see Coleman and Fischer, 1991, Neff, 2003b for descriptions of the behaviors). Nests were first marked with a numbered tag and behavior of each parental was recorded for 15 min between 1300 and 1600 EST.
Directly after the behavioral observation, the parental male was collected using a dip net and quickly taken to a nearby boat where approximately 500 μL of blood was collected from the caudal peduncle using a heparinized syringe. Blood collection time, from the moment the fish was caught, averaged 84 s (range: 51–159 s; bleed time did not correlate with concentrations of any of the hormones: p > 0.45 for all). All blood samples were kept on ice until they were returned to the lab (within 2 h), where they were centrifuged to separate the blood plasma. Plasma was then stored at − 20 °C until the hormone assays were conducted.