競爭壓力對尼泊爾埋葬蟲繁殖資源選擇之影響

Abstract

不論是在社會科學或是生態學中，如何在風險中進行決策，一直是個備受關注的研究主題。風險敏感性理論(Risk-sensitivity theory)認為，生物能夠調整對於風險的偏好，以適應多變的環境。其中，能量預算法則(the energy budget rule) 預測生物在危急時刻會採取冒險的態度(risk-prone)，在一般狀況下則會迴避風險(risk-aversion)。關於風險敏感性，目前僅止於討論環境因子與食物資源的變化量如何影響生物在覓食上的決策，鮮少考慮到生物因子，例如種間競爭，對於決策的影響，因此本研究以尼泊爾埋葬蟲(Nicrophorus nepalensis)作為研究物種，來探討種間競爭對於風險敏感性與選擇繁殖資源的影響。我們以雙翅目的蠅類 (Order: Diptera)所帶來的種間競爭，作為造成埋葬蟲繁殖失敗的風險，並沿著海拔梯度進行操作實驗，以取得競爭壓力的變化，進而瞭解一對埋葬蟲在不同程度的風險下，如何選擇繁殖資源的大小。本研究結果發現，埋葬蟲在不同的競爭壓力下確實做出了不同的選擇。在無競爭壓力的狀況下，選擇結果與雌蟲的身體指數(體重除以背板寬)有關，體型大的雌蟲會偏好選擇大屍體，而小個體則偏好小屍體。小個體的雌蟲選擇小屍體應是為了迴避來自種內競爭的風險，而我們也進行了室內實驗，確認了雌蟲的身體指數確實與屍體的使用權有關，體型大的雌蟲較能取得屍體的使用權。另外在有蠅類的競爭的狀況下，選擇結果與海拔相關，在海拔較低處，蠅類的競爭較為強烈，埋葬蟲會好選擇大的屍體，在海拔較高處則無明顯偏好。這可能是因為大屍體的腐爛速度明顯地較小屍體來的慢，因此埋葬蟲在競爭壓力下選擇大屍體是為了確保有足夠的資源來進行繁殖。尼泊爾埋葬蟲面對不同程度的競爭壓力確實具有風險敏感性，還能依據自身的屍體狀況來做出決定，以適應不同環境中的風險，我們認為這對於瞭解在自然環境中，如何在風險中進行決策有重要的意義。藉由了解社會生物對於風險的權衡，或許能提供同為社會生物的我們在風險決策上有更多思考的方向。

Decision making has been an important issue of interest in both social science and biology. Risk-sensitivity theory argues that organisms will maximize their fitness by adjusting their preference towards risk in changing environments. The energy budget rule predicts that decision-makers will tend to be risk-prone when they have urgent needs of food, and be risk-averse in general status. Most studies regarding risk-sensitivity have focused on how environmental factors and variable food resources affect decision making. However, the influence of biological factors--such as pressure of inter-specific competition--on risk-sensitive decision-making has rarely been considered. Here, we study how inter-specific competition affects risk-sensitive decision-making in reproductive resource in burying beetles (Nicrophorus nepalensis). Specifically, we used competitive pressure from blow flies (Order: Diptera) as a risk factor to see how varying risk along an elevational gradient influences the decision making over choosing large- or small- reproductive resources in one pair of burying beetles. We found that burying beetles made differential decisions while facing varying degrees of competitive pressures. With no competition, female burying beetles chose carcass depending on female body index. Females with higher body index tended to choose bigger carcasses and vice versa, which might be because small females choose small carcass to avoid competition from large same-specific individuals. Further, we did laboratory experiment, and found that female with higher body index had the right to use resource. When competition appeared, in lower elevation--the competition was strong-- burying beetles tended to choose bigger carcasses, whereas, in higher altitude, no preference was observed. Burying beetles chose large carcass under high competitive pressure presumably because large carcass decomposed slower than small carcass. In short, our results showed that burying beetles employ risk-sensitive decision making and making decisions based on their own body condition to cope with environments with different risks, which, we believe, might have important implications for understanding the nature of the decision making under risks. Through understanding how social organisms weigh risks, might be able to provide us more perspective in decision making under risks.