體型與資源擁有權差異如何共同影響動物的打鬥決策

Abstract

許多研究顯示動物的打鬥決策受到許多內在與外在因子的影響。例如動物的自身狀況，一般發現體型越大的個體，其攻擊性和獲勝機率往往越高，因此體型可以作為個體實際打鬥能力的指標之一。而資源擁有權也會增加個體的攻擊性，進而提升其獲勝機率。由於擁有資源的個體在族群中經常是打鬥能力較好的個體，因此資源擁有權也可以提供打鬥能力的資訊，藉此評估彼此的打鬥能力。然而，體型和資源擁有權差異對動物打鬥決策之相對重要性並不明確。本研究以北美紅樹林鱂魚（Kryptolebias marmoratus）作為實驗動物探討體型和資源擁有權如何共同影響打鬥雙方的行為決策。我預期隨著雙方體型差異越大，資源對個體的攻擊性與打鬥的結果之影響會越小。本研究使用3×2之二因子實驗設計（共6種處理），包含三種體型差異處理（體長差異：0~1毫米、1~2毫米、2~3毫米），和兩種資源差異處理（提供體型較小者遮蔽物、不提供任何一方遮蔽物）。此物種在與對方發生打鬥時，可能會出現肢體攻擊行為，甚至會出現激烈的互動，如激烈打鬥（被攻擊的一方進行反擊）或互咬角力（兩隻個體的上下顎互相咬住，利用身體的力量甩動）。實驗結果顯示：（1）資源擁有者在打鬥開始時於遮蔽物內會傾向率先向對方進行肢體攻擊且增加自身的攻擊性；（2）互咬角力發生率受體型和資源擁有權差異之交互作用的影響，當雙方體長差異至2~3毫米時，在遮蔽物內的資源擁有者越容易與對手發生互咬角力；（3）體型和資源擁有權差異分別影響不同激烈程度打鬥之結果，資源擁有權影響低激烈程度打鬥（未發生個體互相攻擊之打鬥）之勝敗結果，資源擁有者的獲勝機率較無遮蔽物且體型較對手小者高。體型差異則影響高激烈程度打鬥（發生個體互相攻擊之打鬥）之勝敗結果，當雙方體型差異越大時，體型較小的個體越容易落敗。整體而言，體型和資源擁有權差異皆會影響北美紅樹林鱂魚之打鬥行為。資源擁有權和體型差異分別影響低、高激烈程度打鬥之結果。資源擁有權會增加資源擁有者的攻擊性與攻擊意願，使其在對手未反擊的情況下獲勝。然而當對手回擊且對手的體型較自身大時，資源擁有權對勝負結果的影響降低，導致即使個體擁有資源，在面對體型較自己大的對手時依然落敗。Numerous studies have shown that animal’s contest strategies are influenced by many intrinsic and extrinsic factors. For example, larger individual often has higher aggressiveness and winning probabilities. Therefore, the body size could be used as one of the indicators of an individual's fighting ability. Animals’ aggressiveness and winning probabilities are also known to be increased by resource ownership. Because individuals with resources often have better fighting ability than the ones without resources, resource ownership can also provide an individual information about its relative fighting ability. It remains unclear the relative importance of body size and resource ownership to animal contest decisions. In this study, I used mangrove killifish, Kryptolebias marmoratus, to investigate how the differences in body size and resource ownership jointly affect contest decisions. I predicted that the greater the difference in body size, the less influence resource ownership has on the individual's aggressiveness and contest outcome. In this study, pairs of K. marmoratus were divided into three size-difference treatments (∆, in mm 0<∆≤1, 1<∆≤2, 2<∆≤3) and two resource ownership treatments (the smaller contestant was given a shelter, neither contestants was given a shelter). When this species fights with each other, there may be physical attacks, and even fierce interactions, such as escalation (mutual attacks between the two contestants) or mouth wrestling (two individuals interlock their jaws and engaging in mutual pulling). The results of this study showed that: (1) the tendency of an owner to attack its opponent and owner’s aggressiveness was increased when owner was inside the shelter at the contest start; (2) the probability of mouth wrestling was affected by the interaction of differences in body size and resource ownership. When the difference in body size is larger, owner in the shelter was more likely to wrestle with its opponent; (3) resource ownership influenced an individual’s winning probability in less intense (non-escalated) contests. Owner has higher winning probability than smaller individual without shelter. While size differences influenced an individual’s winning probability in more intense (escalated) contests. The size difference was larger, the smaller individual was more likely to lose. These results show that both size differences and resource ownership influenced the fighting behavior of K. marmoratus. Resource ownership increased resource owners’ aggressiveness and chance to win in the non-escalated contests. However, resource owners would still lose the contests when the opponents was much larger and the contests were escalated into mutual attacks.