第四章 討論
第五節 其他生理機制與勝者效應
較 24 小時後高，7 小時後則與前兩者無顯著差異。可能是因為經驗 處理在 0 小時後對實驗個體造成額外的壓力或代謝需求尚未恢復，使 皮質醇濃度較高；而 24 小時後，因為皮質醇本身的負回饋機制使其 濃度下降。然而，確切的原因尚待後人探討。
出乎預期的，雖然前人研究指出睪固酮與皮質醇間存有互相抑制 的關係 (Carp; Consten et al., 2001; Trout; Pottinger et al., 1996)，本研 究卻發現睪固酮與皮質醇彼此存有顯著的正相關。腎上腺雖能同時分 泌皮質醇及睪固酮，但後者分泌量極少 (相較於性腺分泌量)，因此，
可能有其他的生理途徑同時激活著 K. marmoratus 睪固酮及皮質醇的 分泌途徑，造成本研究的結果。
第五節 其他生理機制與勝者效應
獲勝經驗會造成 K. marmoratus 打鬥行為上的改變 (勝者效應)，
這些改變並非透過睪固酮或皮質醇濃度的變化來調控，但可能由其他 與打鬥行為相關的荷爾蒙所調控。睪丸硬甾酮 (11-ketotestosterone)，
為硬骨魚類特有的雄性素，經酵素轉換睪固酮而產生 (Villars, 1983)，
被認為與魚類打鬥或領域行為有密切關係 (Elofsson et al., 2000;
Hirschenhauser et al., 2004; Rodgers et al., 2006; Parikh et al., 2006)。有
研究發現非洲慈鯛魚雖有顯著的勝者效應，但若在個體得到獲勝經驗 後施打睪丸硬甾酮拮抗劑 (cyproterone acetate) 降低睪丸硬甾酮的影 響，勝者效應則不再顯著；研究者推論，獲勝經驗是透過睪丸硬甾酮 調控非洲慈鯛魚的勝者效應 (Oliveira et al., 2009)。雌雄同體的 K.
marmoratus 個體同時具有濾泡及睪丸組織，兩者都可分泌睪丸硬甾酮 (Minamimoto et al., 2006)，且睪丸硬甾酮被證實與其打鬥行為有關 (Earley & Hsu, 2008)。綜合這些結果，顯示睪丸硬甾酮是調控 K.
marmoratus 勝者效應的可能荷爾蒙之一。另一個可能調控的荷爾蒙為 雌二醇 (17ß-estrodial)，由雌激素 (estrogen) 或睪固酮經酵素轉換產 生，能增進或抑制動物的打鬥行為 (Nelson, 2006)。K. marmoratus 的 濾泡及睪丸組織皆會製造雌二醇，其正常濃度甚至較睪丸硬甾酮為高 (Minamimoto et al., 2006)。因此，雌二醇也可能是調控其勝者效應的 荷爾蒙之一。此外，雄稀二酮 (androstenedione) 及脫氫表雄甾酮 (dehydroepiandrosterone)，為製造睪固酮、雌二醇的上游原料，且有 許 多 研 究 發 現 其 與 打 鬥 行 為 有 密 切 相 關 ( N e l s o n e t a l . , 2006; Soma, 2006)，亦可能成為勝者效應內在的調控荷爾蒙。
此外，由於荷爾蒙分子必須結合在對應的受器 (receptor) 上才能 引發反應，故荷爾蒙受器的數量及型式會影響打鬥行為，舉例來說，
雄性素受器 (androgen receptor) 基因的表現量與動物表現的攻擊性
呈正相關；而雌性素受器 (estrogen receptor, ER) 中，ERα與提升雄鼠 攻擊性有關，ERβ 則與抑制攻擊性有關 (Nelson, 2006)。據此，獲勝 經驗也可能透過受器的數量及形式來調控勝者效應。
除了荷爾蒙外，神經傳導物質 (neurotransmitter) 也影響著打鬥 行為。關於血清素 (serotonin, 5-HT) 與攻擊行為的回顧研究發現，脊 椎動物的血清素與其代謝物 (5-Hydroxyindoleacetic acid, 5-HIAA) 的比例 (5-HT/5-HIAA ) 越高，個體的攻擊性就越低，但此關係在無 脊椎動物中則相反 (Huber & Delago, 1998)。公蜥蜴成為劣勢後，腻 中的 5-HT/5-HIAA 比例明顯下降 (Summers et al., 1998)。北極紅點鮭 的優勢個體，其下視丘中血清素的濃度顯著高於劣勢者 (Elofsson et al, 2000)。這些結果顯示，血清素對打鬥行為有顯著的影響，雖然此 影響可能隨著物種而不同。體抑素 (somatostatin) 也會影響打鬥行為，
有研究顯示：若阻止體抑素結合其受器，非洲慈鯛魚的領域行為會顯 著增加；若增加體抑素受器的結合率，其領域行為則降低 (Trainor &
H o f m a n n , 2 0 0 6 ) 。 上 述 會 影 響 打 鬥 行 為 的 神 經 傳 導 物 質 ， 都有可能調控著勝者效應。
學 習 亦 是 勝 者 效 應 的 可 能 機 制 。 關 於 帕 夫 洛 夫 制 約 反 應 (Pavlovian conditioning) 與打鬥行為的研究顯示：三星攀鱸 (blue gouramis, Trichogaster trichopterus) 能藉著學習連結入侵者出現及其
出現前的訊號 (signal)，在入侵者出現時表現地更具攻擊性，提高自 己防禦領域的成功機率 (Hollis, 1984)。此外，有研究者給予三星攀鱸 個體兩種不同制約條件的訓練 ( CS+：閃燈號 A + 對手或 CS-：閃燈 號 B + 無對手)，接著給予不同個體不同的燈號，再讓接受不同燈號 的實驗個體打鬥，結果顯示，B 燈號使實驗個體的攻擊性降低，而 A 燈號則使之提高 (Hollis, 1999)。這些結果顯示，學習能改變動物在打 鬥時展現的攻擊性，因此，勝者效應也可能透過學習而完成。舉例來 說，加州小鼠在自己領域內成功擊退入侵者後，能在自己領域內展現 顯 著 的 勝者 效應， 若 將 其移 至 不熟 悉 的 領域 ，勝者 效 應 則消 失 (Fuxjager et al., 2009)；研究者推論，雄鼠將自己的領域與擊退入侵者 作出了學習連結，因而僅能在熟悉的領域中提高擊退入侵者的機率。
在本研究中，由於 K. marmoratus 實驗個體在打鬥缸中擊退 STL 三次，
實驗個體可能透過學習，將打鬥缸 (或隔板抽開) 視為對手即將出現 的訊號，或者將打鬥缸中的對手連結為較弱的對手，因而提升了攻擊 性並引發顯著的勝者效應。有研究認為，這類與地域有關的學習連結，
應在端腻 (telencephalon) 內形成 (Vargas et al., 2000)，而可能透過腻 內的多巴胺 (dopamine) 調控 (Farrell & Wilczynski, 2006)。然而，目 前沒有任何研究探討 K. marmoratus 的學習與打鬥行為，及其 內在的生理機制，未來可朝此方向進行研究。
總而言之，本研究顯示獲勝經驗並非透過體內血液中睪固酮或皮 質醇濃度的變化來引發 K. marmoratus 的勝者效應。上述之其他生理 機制與勝者效應間的關係則有待未來研究加以驗證。
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