J. Ethol. 17 : 79-86, 1999 Behavioral Rules and Tail-up Display in Extra- and lntra-pair Interactions of the Butterflyfish, Chaetodon lunulatus. Shinji YABUTA Department of Zoology, Graduate School of Science, Kyoto University, Sakyo-ku, Kyoto, 606-8502 Japan (e-mail:shinji @ zoo.zool.kyoto-u.ac.jp ) Abstract - The butterflyfish, Chaetodon lunulatus, forms monogamous pair bonds. Each pair defends a feeding territory against conspecifics. The tail-up or lateral display of the butterflyfish is performed not only towards non-partners in territorial disputes (extra- pair interaction), but also towards partners (intra-pair interaction). In order to eXplain this phenomenon, I investigated the two interaction types, and found that a simple model explains both interactions very well. The model consists of four behavioral rules and two provisos for applying the rules. Rule 1: Perform the tail-up display when approached by another individual. Rule 2: Perform the tail-up display when another individual performs the tail-up display. Rule 3: Attack another individual that neither approaches nor per- forms the tail-up display. Rule 4: Swim with your partner. Proviso 1: Apply Rules 1 and 2 when you do not intend to run away. Proviso 2: Apply Rules 1, 2, and 3 when you do not recognize another individual as your partner. When Rules 1-4 and Provisos 1-2 are applied, the display can prevent improper attacks between partners caused by imperfect partner recognition. In agonistic interactions with conspecific oppo- nents, many animals use one or more displays, which are called threat or agonistic displays. On the other hand, the same behavioral patterns found in the displays are also used in non-agonistic interactions, such as a pair formation ceremony or in greeting a mating partner in birds and fishes (e.g. Baerends & Baerends, 1950; Tinbergen, 1959; Barlow, 1962; Chiszar, 1978; Yanagisawa, 1982). This phenomenon has been explained from the motivational viewpoint. Since animals that encounter their partners can be in the same moti- vational state (conflicting of inconsistent tenden- cies) as when encountering their rivals, they per- form the same behavioral pattern in both types of social interaction (Tinbergen, 1959; Hinde, 1970). This argument indicates that a common causal mechanism may work under both types of interac- tion. However, we know little about such a mech- anism. In this paper, I propose a model relating to a causal mechanism in social interactions of the but- terflyfish, C. lunulatus. This was formerly assigned to a Pacific subspecies of C. trifasciatus but has recently been recognized as a distinct species (Kuiter, 1995). They form heterosexual pair bonds (Reese, 1975; 1991). Each pair defends a feeding territory against conspecific opponents (Reese, 1975; Sutton, 1985). Mating is restricted to within pairs (Yabuta, 1997). The tail-up or lateral display is performed not only towards conspecific opponents in territorial disputes (extra-pair inter- action), but also in non-agonistic interactions with the pair partner (intra-pair interaction) (Sutton, 1985). The behavioral sequences of both extra- and intra-pair interactions were investigated. Methods