This research was selected as the featuring paper of the week at the Frontiers homepage: http://www.frontiersin.org/

This research was covered in the following media　
(*=online edition, Ev.= Evening edition)
◆3 August: NHK ‘News Hot Kansai’, MBS ‘Voice’, Jiji Press*, Kyodo News*
◆4 August: Asahi newspaper, Yomiuri newspaper, Mainichi newspaper, Nihon Keizai Shinbun newspaper (Ev.), Sankei newspaper (Ev.)
◆5 August: Osaka Nichi Nichi newspaper
◆5 August: Wall Street Journal:
http://blogs.wsj.com/japanrealtime/2015/08/05/japan-researchers-find-fish-can-think-logically/

and an additional number of local media.

Outline

The research group of Professor Masanori Kohda of the Graduate School of Science reported an extraordinary finding, confirming that some kind of fish can think logically and deduct that if A>B and B>C, then A>C.

Up until now conventional wisdom said that lower vertebrate animals, the category to which fish belong, only respond to stimuli and have only simple learning abilities, but the research results of Professor Kohda have now overturned this idea. The results were reported in the online edition of the Swiss biological scientific journal ‘Frontiers in Ecology and Evolution’ on 3 August 2015.

【Journal Name】
　Frontiers in Ecology and Evolution
【Article Title】
　The use of multiple sources of social information in contest behaviour:
　testing the social cognitive abilities of a cichlid fish
【Authors】
　Takashi Hotta, Tomohiro Takeyama, Dik Heg, Satoshi Awata,
　Alexander Jordan, Masanori Kohda
 【URL】
　http://journal.frontiersin.org/article/10.3389/fevo.2015.00085/full

Research

The fact that mammals, and especially primates, have the ability to think logically and deduct that if A>B and B>C, then A>C, is widely known. In 2004 an article appeared in Nature magazine, describing a verification method (※1) used for the pinyon jay (a bird species related to the crow). That same research method has since then been often used for birds, but similar research on fish based on this method had not yet taken place.  

The research group of Professor Kohda applied the verification method as described in the article in Nature magazine to the cichlid fish Julidochromis transcriptus (see photo below) and confirmed that this fish species has the ability to identify each other, that it has a power hierarchy and that the weaker individuals show submissive behavior towards stronger individuals (※2).  　

In the reported experiment, 3 males of the same size were used. First, 2 were placed in an aquarium and brought into confrontation with each other. Fish B wins, fish C. loses. (Fish C. that lost becomes the object of the experiment.) At this point, fish C. knows it is weaker than fish B. Next, fish A and fish B are brought into confrontation, witnessed by fish C. Fish A. wins, so fish C. has seen that fish A is stronger than fish B. Fish A. and C. are not directly brought into confrontation, but we can assume that if fish C. understands that fish A. is stronger than fish B, who was stronger than fish C. itself (in other words A＞C), fish C. would show submissive behavior towards fish A. When fish A and C were then put in the tank and confronted with each other, 11 out of 12 C-fish indeed showed submissive behavior, giving a significant statistical result. Other factors that may cause submissive behavior were ruled out during the experiment, thus proving that fish can logically conclude that if A>B and B>C, then A>C.

Notes:
※1 "Pinyon jays use transitive inference to predict social dominance"
　　Nature 430 (August 12, 2004), pp. 778-781; doi:10.1038/nature02723
※2 “The effect of body size on mating system and parental roles in a biparental cichlid fish (Julidochromis transcriptus): a preliminary laboratory experiment” J. Ethology, 24: 125-132. Doi:10.1007/s10164-005-0171-5