探討Kryptolebias marmoratus敗者效應之荷爾蒙機制

Abstract

個體在打鬥中落敗後，再度遇到競爭對手時往往會表現得較被動，若是進行打鬥則較易認輸，此現象稱為敗者效應（loser effect），但敗者效應之生理機制至今仍不清楚。許多類固醇荷爾蒙被指出與打鬥行為相關，在一些魚類的研究中發現，相較於獲勝者，落敗者的糖皮質素（glucocorticoids）分泌量顯著較高；雄性素（androgen）分泌量則顯著較低。另外，外加糖皮質素以及抑制雄性素的分泌會降低個體的攻擊性。由這些研究可知，血液中糖皮質素以及雄性素濃度與攻擊行為之間存在著複雜的交互關係。然而，大部分的研究只專注在荷爾蒙狀態是否能預測個體的打鬥行為，以及在打鬥中落敗後個體的荷爾蒙狀態如何變化，卻沒有在個體落敗之後繼續監控其隨後的打鬥行為，以探討個體落敗後荷爾蒙反應與打鬥行為之間的關聯性。因此，這些研究的結果無法確定落敗經驗之行為效應是否由類固醇荷爾蒙居中調控。 本研究以Kryptolebias marmoratus為實驗物種，探討落敗者打鬥行為改變之荷爾蒙機制：強制給予個體三次輸的經驗後，觀察其體內皮質醇和睪固酮分泌量的變化，並探討這些內分泌變化是否可以解釋此個體的行為改變。此外，本研究也更進一步探討輸經驗是否會影響個體再打鬥時體內之內分泌變化。結果顯示，輸的經驗會降低K. marmoratus個體在之後打鬥中參與激烈打鬥的機率、打鬥時間以及獲勝的機率。然而，輸的經驗卻不會改變個體的皮質醇或睪固酮分泌量。這些結果顯示皮質醇和睪固酮並非調控敗者效應的主要生理機制。實驗結果也發現獲得輸經驗的個體，其經驗後睪固酮分泌量越高，打鬥時發生激烈打鬥機率越大。經驗給予後的打鬥勝負則顯著地影響了打鬥後皮質醇分泌量：落敗者打鬥後皮質醇分泌量顯著低於獲勝者。另外，打鬥勝負決定後，獲勝者攻擊對手的次數以及落敗者被攻擊的次數與其打鬥後皮質醇濃度呈現顯著的正相關。顯示皮質醇和睪固酮與打鬥行為有顯著的相關性，但是這些相關性無法解釋敗者效應。論文最後亦針對敗者效應的可能生理機制進行討論。A recent losing experience often causes an individual to behave submissively and to have a higher chance of losing again in a subsequent contest (loser effect). The physiological mechanisms underlying the loser effect are currently unknown. In fish, losers often exhibit higher cortisol and/or lower testosterone levels than winners. Furthermore, experimentally, either elevating cortisol or reducing testosterone levels often decreases individuals’ aggressiveness. These results suggest that cortisol and testosterone could play important roles in mediating the loser effect. Although these hormones have been shown to be associated with social status or aggressiveness, however, most studies focusing on physiological mechanisms have not monitored post-contest behavioral changes. Consequently, observed changes in the neuroendocrine systems after losses can not be mapped on to subsequent changes in contest behavior to understand the parallel physiological and behavioral mechanisms of the experience effects fully. In this study, I gave individuals of Kryptolebias marmoratus three losing experiences to investigate the relationship between changes in behavior and levels of cortisol and testosterone. I also examined how interactions in post–experience contests might further influence an individual’s cortisol and testosterone levels. Losing experiences significantly decreased an individual’s persistence in a subsequent contest as well as its likelihood of escalating or winning the contest. However, losing experiences did not have a significant effect on cortisol or testosterone levels. These results indicate that cortisol and testosterone are not the main physiological modulators of the loser effect in this species. However, cortisol and testosterone were found to correlate with several contest behaviors: in individuals with losing experiences, higher post-experience testosterone was associated with an increased tendency to escalate. Additionally, losers had higher post-contest cortisol levels than winners, which indicated that contest outcome influenced post-contest cortisol levels. Furthermore, both winners and losers' post-contest cortisol levels were positively correlated with the frequency of post-contest attacks. In conclusion, the results of the present study suggest that, although cortisol and testosterone are correlated with contest behavior and outcome, they are not the main physiological modulators of the loser effect. Possible physiological mechanisms of the loser effect in the fish were discussed.