能準確分辨獵物數量的能力在動物的攝食決策中極為重要，然而動物的攝食決策取決於當下的損益關係，因此最佳攝食決策必須評估其風險，但風險評估與食慾狀態的交互作用目前仍不清楚。本篇研究利用無脊椎動物中擁有最複雜神經系統的頭足類動物烏賊，藉由不同的實驗設計來探討兩大議題。第一，烏賊是否具有數字感；第二，烏賊是否會因為其食慾狀態的不同而改變其攝食決策。研究中使用蝦子做為刺激用的獵物，當烏賊看見獵物出現時，不需額外的訓練，先天本能的將身體轉向獵物並伸出攻擊腕攻擊獵物。我們利用此視覺主導的攝食行為設計實驗方法，將不同數量的蝦子放入強迫二選一的裝置中(1 vs. 2, 2 vs. 3, 3 vs. 4, and 4 vs. 5)，呈現在烏賊面前。研究結果發現烏賊偏好選擇較多的數量。然而，當裝置中改放一隻活蝦子與兩隻死蝦子時，烏賊偏好選擇數量較少的一隻活蝦子。更重要的是，當裝置中改放一隻大活蝦與兩隻小活蝦時，烏賊在飢餓狀態會偏好選擇一隻大活蝦，但在飽食狀態則偏好選擇兩隻小活蝦。本篇研究結果證實烏賊具有鑑別數字的能力，並發現烏賊在攝食決策中對於數量的選擇會因為獵物的品質與烏賊本身的食慾狀態而改變。本研究結果也證實烏賊在進行攝食決策時會整合內在與外在訊息，來評估獲取食物的成本與風險。

Identifying the amount of prey available is an important part of an animal’s foraging behavior. The risk-sensitive foraging theory predicts that an organism’s foraging decisions with regard to food rewards depend upon its satiation level. However, the precise interaction between optimal risk-tolerance and satiation level remains unclear. In the present study, we examined, firstly, whether cuttlefish, with one of the most highly evolved nervous system among the invertebrates, have number sense, and secondly, whether their valuation of food reward is satiation state-dependent. When food such as live shrimps is present, without training, cuttlefish turn toward the prey and initiate seizure behavior. Using this visual attack behavior as a measure, cuttlefish showed a preference for a larger quantity when faced with two-alternative forced choice tasks (1 vs. 2, 2 vs. 3, 3 vs. 4, and 4 vs. 5). However, cuttlefish preferred the small quantity when the choice was between one live and two dead shrimps. More importantly, when the choice was between one large live shrimp and two small live shrimps (a prey size and quantity trade-off), the cuttlefish chose the large single shrimp when they felt hunger, but chose the two smaller prey when they were satiated. These results demonstrate that cuttlefish are capable of number discrimination and that their choice of prey number depends on the quality of the prey and on their appetite state. The findings also suggest that cuttlefish integrate both internal and external information when making a foraging decision and that the cost of obtaining food is inversely correlated with their satiation level, a phenomenon similar to the observation that metabolic state alters economic decision-making under risk among humans.

誌謝............................................................................................................................... i
摘要..............................................................................................................................iii
Abstract.......................................................................................................................iv
Table of contents.........................................................................................................vi
List of Figures...........................................................................................................viii
Chapter 1 Introduction........................................................................................1
1.1 Cognitive ability in cephalopods..........................................................1
1.2 Number sense and its core systems......................................................1
1.3 Number sense in different animals.......................................................3
1.4 Functions of number sense...................................................................4
1.5 Optimal foraging theory.......................................................................5
1.6 Foraging behavior in cephalopods.......................................................7
1.7 Specific aims of the present study........................................................7
Chapter 2 Materials and Methods......................................................................9
2.1 Subjects................................................................................................9
2.2 Experimental apparatus and procedure................................................9
2.3 Experimental design...........................................................................11
2.4 Data analysis......................................................................................13
Chapter 3 Results...............................................................................................15
3.1 Number discrimination in cuttlefish...................................................15
3.2 Quality over quantity in food choice..................................................17
3.3 Trade-off between prey size and quantity is satiation level dependent...........................................................................................18
Chapter 4 Discussion..........................................................................................20
4.1 Number sense in cuttlefish.................................................................20
4.2 Number sense of cuttlefish compared with other animals.................21
4.3 Optimal foraging decision in cuttlefish..............................................22
4.4 Satiation state dependent valuation in cuttlefish................................24
4.5 Conclusion..........................................................................................25
References...................................................................................................................27
Figures.........................................................................................................................32

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