Mechanisms of honest sexual signalling and life history trade-offs in three cyprinid fishes

Abstract

Anssi Vainikka on väitöskirjassaan selvittänyt vaikuttavatko särjen ja suutarin ornamenttien tuottoon kytketyt hormonit niiden immuunipuolustuskykyyn. Vastoin ennakko-oletuksia, testosteronin tai estradiolin ei havaittu alentavan immuunipuolustuskykyä. Myöskään ornamenttien ja immuunipuolustuskyvyn mittareiden väliltä ei löytynyt seksuaalivalinnan teorioiden ennustamia positiivisia yhteyksiä. Korkea testosteronipitoisuus oli kuitenkin yhteydessä koirassuutarin hyvään kuntoon ja ornamentit kertovatkin koiraan hyvästä elinkyvystä.

Testosterone is known to enhance reproductive success, but has also been proposed to suppress immune defence. Prior to spawning, male roach (Rutilus rutilus L.) produce testosterone-associated breeding tubercles those have been found to signal parasite load and parasite resistance. Tench (Tinca tinca (L.)) is sexually dimorphic throughout the year, and in addition to their large pelvic fins males produce ventral protuberances of unknown function during breeding. The indicator value of male tench ornaments, immunosuppressivity of testosterone, cortisol and oestradiol in roach and tench, and the level of stress during spawning in roach were studied. A study of crucian carp (Carassius carassius (L.)) aimed to link individual’s behavioural decisions to somatic growth, reproductive effort and future survival. In these studies, cortisol but neither testosterone nor oestradiol was found to have immunosuppressive effects. Neither did an immunological challenge decrease the concentration of testosterone. Although testosterone was positively related to the condition factor in tench, it was only weakly associated to the expression of male ornaments. High spawning-time prevalence of epidermal papillomatosis in male roach was suggested to result from sex-differences in spawning stress. Body depth gain in crucian carp was associated to growth in a manner dependent on predator presence, but the behavioural decisions of an individual did not correlate with measures of growth or reproductive effort. I suggest that stress steroids contribute to the honesty of sexual signalling in the studied cyprinids, whereas testosterone in both sexes may act as a condition-dependent hormone having a positive effect on sexual signalling and primary sexual traits, but being metabolically costly at high levels.