Steroid link to fish breeding strategies

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Scientists have confirmed that male Bluegills that practice different breeding strategies have different levels of steroid hormones in their blood.

In a paper published in the latest issue of the journal Biology Letters, Rosemary Knapp and Bryan Neff studied the circulating levels of four steroid hormones (testosterone, 11-ketotestosterone, oestadiol and cortisol) among male Bluegills (Lepomis macrochirus) that adopt different reproductive strategies.

Male Bluegills may adopt one of three reproductive strategies, divided into ~parental and ~cuckolder forms.

Parental males mature at about seven years of age, compete with each other for nesting sites, court and spawn with females, and provide sole parental care for the offspring.

Cuckolder males mature precociously (when approximately two years old) and parasitically steal fertilization opportunities from parental males.

When small (two to three years old), cuckolder males (~sneakers ) sneak spawns by streaking through a spawning pair and when large (about four to five years old), they use female mimicry to gain access to nests (~satellites ).

The authors collected fish from a spawning colony, and measured the levels of steroid hormones in the blood.

They found that parental males had significantly higher levels of 11-ketotestosterone than cuckolder males, while cuckolder males had significantly higher levels of testosterone than parental males.

The authors also found that cuckolder males had significantly higher levels of cortisol and oestradiol than parental males, and that there was a strong negative relationship between oestradiol levels and body length (a surrogate for age) among satellite males which mimic females in appearance and behaviour.

The latter suggests that oestradiol dependency of mating behaviour decreases with increasing mating experience for satellite males.

For more information, see the paper: Knapp, R and BD Neff (2007) Steroid hormones in Bluegill, a species with male alternative reproductive tactics including female mimicry. Biology Letters 3, pp. 628"631.