打鬥勝敗經驗對攻擊性以及其與相關行為、生理和生活史特徵關聯性之影響

Abstract

動物攻擊性的展現和個體本身的生理、形質及生活史特徵有關。攻擊性較高的個體通常在生理特徵上有較高的基礎代謝率、較高濃度的睪固酮與皮質醇；且伴隨其代謝率，個體在形質特徵上，有著較大的體型與器官組織，在生活史特徵方面，則可能是在能獲得較多食物的情況下，生長較快。動物的攻擊性也會與外在因子有關。像是個體近期有過的獲勝／落敗經驗時，其下次打鬥時有較高／較低的攻擊性（勝／敗者效應）。據以上所述，個體的攻擊性會隨著內在與外在因子改變而展現出適應性。動物攻擊性的展現也被發現會和其他行為有關。像是攻擊性較高的個體比較勇敢、有較高的進食傾向。同時，動物的攻擊性也被發現具有一致性。前人提出生活步調假說，認為動物的行為、生理、形質與生活史特徵為成套地相互有關，且會一同演化。生活步調假說預期，生活步調較快的個體，在行為上會具較高攻擊性和勇敢程度。此假說也預期生理與形質特徵上會有較高的基礎代謝率、較低的壓力反應，及相對較大的器官組織，來支持個體的較高攻擊性與勇敢程度。因此，生活步調假說也被前人認為可被應用來解釋攻擊性和其他行為有關，且有一致性的情況。過去有許多研究呈現勝敗經驗對攻擊性的影響，也有許多研究檢測個體攻擊性和其自身因子之間的關係。然而，尚未有研究探討，倘若攻擊性和個體自身因子有關，當勝敗經驗會改變個體攻擊性時，是否會一併改變這些與攻擊性有關的特徵，或是改變這些特徵間的關聯性。因此，我以北美紅樹林鱂魚(Kryptolebias marmoratus)為研究對象，針對其攻擊性，欲探討打鬥勝敗經驗對攻擊性及其相關的行為、生理與生活史特徵的影響，並進一步探討打鬥勝敗經驗是否會影響個體攻擊性與其生理、生活史特徵的關聯性。北美紅樹林鱂魚在野外與飼養環境皆會展現攻擊性，其攻擊性已被發現會受打鬥勝敗經驗影響。前人研究也顯示，在此魚中，打鬥能力較差的個體，打鬥勝敗經驗對其攻擊性的影響可能比較強。此魚的攻擊性也被發現與其耗氧量、睪固酮與皮質醇濃度呈現正相關。此外，北美紅樹林鱂魚其雌雄同體、自體受精的特性，可自成不同品系，也能在研究中利於控制其遺傳背景。有研究比較有著不同生活步調(性成熟速度)的品系，發現生活步調越快的品系，能量消耗越快，對乾旱環境的耐受度越低。這些研究皆顯示此魚相當適合用來回答本篇論文的研究目的。本篇論文共四章。我於第一章回顧與動物攻擊性展現有關的文獻，包含和個體攻擊性展現有關的自身因子、打鬥勝敗經驗對個體攻擊性的影響以及探討攻擊性展現有所限制的相關理論。在此章節，我也同時介紹北美紅樹林鱂魚其攻擊性的相關研究，並陳述本篇論文的目的及欲回答的問題。在第二章，我先探討個體攻擊性隨打鬥勝敗經驗改變的狀況是否會因個體本身的狀況而有所不同。在此研究中，實驗個體被給予指定的勝敗經驗後，測量其攻擊性於打鬥經驗處理前後的變化量。同時也檢測個體本身的狀況，包含打鬥能力、打鬥經驗處理前後的攻擊性、勇敢程度與耗氧量。此章主要結果顯示，打鬥能力較好的個體，在落敗經驗處理後，其攻擊性顯著下降。另外，本章結果也顯示，不論於打鬥經驗處理前或後，攻擊性越高的個體，其勇敢程度與耗氧量也都越高，代表此三個特徵在不同時間測量都有相關性。初步瞭解打鬥勝敗經驗對攻擊性的影響之後，我於第三章探討打鬥勝敗經驗是否影響個體攻擊性和行為（勇敢程度與進食行為）、生理（耗氧量、睪固酮與皮質醇）、形質（身體質量與鰓、肝臟與肌肉組織重量）與生活史特徵（產蛋量與生長速率），以及這些特徵之間的關係。同時，本章也檢測個體攻擊性與相關特徵之間的關聯性是否符合生活步調假說的預期。本章主要結果顯示，打鬥勝敗經驗僅影響攻擊性，不影響其他行為（勇敢程度與進食行為）、生理（耗氧量、睪固酮與皮質醇）與形質（身體質量），也不影響攻擊性與行為、生理、形質與生活史特徵間的關聯性。在本章中，特徵間關聯性的結果顯示，攻擊性越高的個體有較少的肌肉量且生長較慢。這樣的關係不符合生活步調假說的預期，且可能和能量有限的情況下，須對攻擊性與生長有所取捨有關。綜合第二章與第三章的結果，本篇論文顯示，打鬥勝敗經驗雖會影響攻擊性，但是並不改變攻擊性與及其相關特徵間的關聯性。針對此結論，我於第四章提出建議，未來可以透過研究與打鬥能力、能量代謝相關的生理路徑或機制，進而瞭解攻擊性與和其相關特徵之間的關聯性如何構成。Animal aggression has been found to be correlated with physiological, morphological and life-history traits. For example, individuals with higher levels of aggression have higher basal metabolic rates and higher levels of testosterone and cortisol. Due to the higher basal metabolic rates, individuals often have larger body sizes and organ and tissue sizes. And more aggressive individuals can grow faster because they have a greater ability to occupy more food. Animal aggression has also been found to be affected by environmental factors. For example, the winning/losing outcomes of recent fights have been shown to increase/decrease the aggression of an individual (winner/loser effects). Based on these studies, individuals’ aggression can be modulated by their intrinsic and extrinsic factors. Animal aggression has also been shown to be associated with other behaviours. For example, more aggressive individuals are bolder and (or) have a higher feeding tendency. And animal aggression has been shown to be a consistent behavioural trait. Therefore, the aggression of individuals is probably limited in some contexts. The pace-of-life syndrome (POLS) hypothesis has been mentioned to propose that individuals’ behaviour, physiology, morphology and life-history traits are related to each other and co-evolve. This hypothesis predicts that individuals with faster paces of life have higher levels of aggression and boldness, higher basal metabolic rates, lower responsiveness to stress and larger organs and tissues. Therefore, the POLS hypothesis can be used to explain that the aggression of individuals is related to other behaviours and is a consistent trait. Previous studies have investigated the influence of contest experiences on animal aggression. And several studies have detected the relationships between individuals’ aggression and intrinsic traits. However, there is no research yet to investigate whether contest experiences affect aggression-related traits when they change the aggression of individuals; and whether contest experiences affect the relationships between individuals’ aggression and aggression-related traits. Therefore, in this dissertation, the mangrove killifish, Kryptolebias marmoratus, was used as the study organism to investigate the effect of contest experience on aggression and its associations with related behavioural, physiological and life-history traits. The aggression of K. marmoratus has been shown to be affected by contest experiences. And the individuals of K. marmoratus with worse competitive ability are more likely to show stronger winner and loser effects. It has also been found that this fish's aggression is positively related to its oxygen consumption rate, testosterone and cortisol. Kryptolebias marmoratus is a hermaphroditic and self-fertilizing fish that therefore generates multiple lineages. The lineage with a faster life pace (laying eggs earlier) has been shown to have a higher energy expenditure and a lower survival rate outside of the water. Based on these results, K. marmoratus is suitable for this dissertation. This dissertation has four chapters. Chapter 1 reviews the literature on animal aggression. In this chapter, I described factors associated with animal aggression and introduced the theories that explain the consistency of animal aggression. In the same chapter, I also described studies relating to K. marmoratus’ aggression. And I described in more detail the objectives of this dissertation. Chapter 2 first examines whether the individuals’ intrinsic condition affects theeffect of winning-losing experiences on aggression. In this chapter, I measured the change in the fish’s aggression after giving assigned contest experiences. And I measured the competitive ability of the fish and their aggression, boldness and oxygen consumption before and after contest experience treatments. The main results of this chapter showed that only losing experiences affected the fish’s aggression. And the effect of the contest experience on the fish’s aggression was only dependent on their competitive ability. Individuals with better competitive ability showed greater decreases after receiving losing experiences. Furthermore, in this chapter, the fish’s aggression, boldness and oxygen consumption rate were correlated before and after contest experience treatments, which meant that these relationships were stable over time. After understanding the effect of contest experience, Chapter 3 investigates whether contest experiences affect the individuals’ aggression and aggression-related behaviours (boldness and feeding tendency), physiology (oxygen consumption rate and levels of testosterone and cortisol), morphology (body condition and weights of gill, liver and muscle) and life-history traits (number of eggs and growth rate) and affect the relationships between these traits. And this chapter also detects whether these relationships are consistent with the predictions of the POLS hypothesis. The main results of this chapter showed that contest experiences only influenced the fish’s aggression: individuals with losing experiences decreased their aggression. But contest experiences did not affect the relationships between the fish’s aggression and aggression-related traits. The results of this chapter also showed that the more aggressive fish had fewer muscles and grew slower in this chapter, which was not consistent with the predictions of the POLS hypothesis. Due to the limited energy budget, I inferred a trade-off between the fish’s aggression and growth. Combining the results of Chapter 2 and Chapter 3, Chapter 4 concludes that contest experiences only influenced the fish’s aggression but did not change its relationships between aggression-related traits. Based on this conclusion, to establish a better understanding of the influence of contest experiences on the correlations between aggression and related behavioural, morphological and life-history traits, Chapter 4 also proposes future work on investigating possible pathways and mechanisms.