Androgens are known to mediate aggressive and defensive behaviour in many vertebrate species. However, high concentrations of androgens might also conflict with the expression of nurturing behaviours and therefore a trade‐off can exist between aggressive and nurturing behaviours during parental care. We explored the role of testosterone in paternal care in bluegill sunfish (Lepomis macrochirus), where males provide both sole defence of the young from predators and sole nurturing behaviour such as fanning of the eggs. At the onset of parental care, we manipulated testosterone levels in males using testosterone propionate implants. We then observed the frequency of nurturing and aggressive behaviours displayed by the males over 6 d of parental care. Testosterone‐implanted fish were more aggressive when presented with a brood predator, performing more bites, opercular flares and lateral displays than control males. Testosterone‐implanted males, however, were not less nurturing than control fish, performing similar levels of fanning and nest‐cleaning behaviours. Thus, our results support a positive relationship between testosterone and paternal aggression but no testosterone‐mediated trade‐off between paternal nurturing and aggression.
Study Species and Site
Bluegill are native to North America and are found in freshwater lakes ranging from Northern Mexico to Southern Canada (Scott & Crossman 1973). We conducted our study in the 900-hectare Lake Opinicon (44°34′N, 76°19′W) in Ontario, Canada, where bluegill have been studied for over 30 yr (e.g. Colgan et al. 1979; Neff & Knapp 2009). In Lake Opinicon, males of the reproductive phenotype known as ‘parentals’ mature at the age of 7 yr and then spawn with females from late May to early July in several bouts (Gross 1982). A bout begins when a group of parentals form a colony and build individual nests in the littoral zone. An established colony can contain up to 300 males (Cargnelli & Neff 2006). Spawning at a colony typically lasts only a single day, after which females leave the colony and only the parental males care for the offspring in their nests. For the first 3 d after spawning, a parental male defends his nest from brood predators, fans his eggs to increase oxygen availability and removes moulding eggs from the nest (Côté & Gross 1993). The eggs then hatch and the parental male focuses on protecting the young from brood predators until the young leave the nest 4–7 d later. A parental male will then return to deeper waters of the lake to replenish his energy supplies before possibly returning to the littoral zone for another breeding bout (Cargnelli & Neff 2006).
The field portion of our study was conducted from June 14 to 21, 2009. Using daily snorkelling surveys, swimmers located nests with spawning parental males and females. The day after spawning, 56 parental males were captured between 1000 and 1700 EST and were brought to a nearby boat, where initial blood samples (approx. 300 µl) were taken from the caudal vein. Blood collection time (measured from the time the fish was caught until the needle was removed from the caudal vein) averaged 117 ± 47 s. A numbered tile was placed at each male’s nest for fish identification, and while the male was on the boat, a nest cover was placed over his nest to protect the eggs. After blood sample collection, males were anesthetised using clove oil. Length and mass were taken, from which we later calculated Fulton’s condition factor [(mass/length3) × 105], which estimates the energetic state of an individual (Neff & Cargnelli 2004). Individuals were implanted in the abdominal cavity with either one placebo silastic implant (P) filled with silicone sealant, one testosterone implant (T1) or two testosterone implants (T2). Males in a fourth group (control, C) were handled but had no surgery. Fourteen males were assigned to each of the four treatments through haphazard collection and subsequent rotation through the treatments. Silastic implants (i. d., 1.47 mm, o.d., 1.96 mm; Konigsburg Instruments, Pasadena, CA, USA) were packed with 8-mm T propionate (Sigma-Aldrich, Oakville, ON, Canada), and each end was sealed with 1-mm silicone sealant. Following implant placement, 50 µl of an antibiotic solution (oxytetracycline; Sigma-Aldrich, St. Louis, MO, USA) was injected into the wound to prevent infection, New Skin (Prestige Brand Holdings, Inc., Irvington, NY, USA) was applied to the wound, and fish were placed in a bucket of lake water for a 5-min recovery period. Males were then returned to their nests where they resumed care within a few minutes.