Abstract
It is currently widely accepted that the complexity of a species’ social life is a major determinant of its brain complexity, as predicted by the social brain hypothesis. However, it remains a challenge to explain what social complexity exactly is and what the best corresponding measures of brain anatomy are. Absolute and relative size of the brain and of the neocortex have often been used as a proxy to predict cognitive performance. Here, we apply the logic of the social brain hypothesis to marine cleaning mutualism involving the genus Labroides. These wrasses remove ectoparasites from ‘client’ reef fish. Conflict occurs as wrasse prefer client mucus over ectoparasites, where mucus feeding constitutes cheating. As a result of this conflict, cleaner wrasse show remarkable Machiavellian-like behaviour. Using own data as well as available data from the literature, we investigated whether the general brain anatomy of Labroides provides any indication that their Machiavellian behaviour is associated with a more complex brain. Neither data set provided evidence for an increased encephalisation index compared to other wrasse species. Published data on relative sizes of brain parts in 25 species of the order Perciformes suggests that only the diencephalon is relatively enlarged in Labroides dimidiatus. This part contains various nuclei of the social decision making network. In conclusion, gross brain anatomy yields little evidence for the hypothesis that strategic behaviour in cleaning selects for larger brains, while future research should focus on more detailed aspects like the sizes of specific nuclei as well as their cryoarchitectonic structure and connectivity.
Highlights
There is enormous variation among vertebrate species concerning both absolute brain size and brain size relative to body size [1]
We had asked whether apparently sophisticated strategies during cleaning interactions are correlated with an increase in relative brain size within the wrasse family and/or with an increase of specific brain areas within the order of perciformes
The main result of putative relevance is that L. dimidiatus has a large diencephalon relative to the rest of the brain compared to other perciformes
Summary
There is enormous variation among vertebrate species concerning both absolute brain size and brain size relative to body size [1]. Brain evolution reflects the degree of complexity of the social environment for any given species. This line of argument has been developed further by [2], with a focus on the variety of potential socio-cognitive adaptations through which an individual may exploit the potential benefits of its social world, as well as dealing with the hostile aspects of it. Such adaptations are for example perspective-taking and knowledge of social hierarchy, social play, social curiosity, social learning and teaching, and cultural transmission, as well as socially influenced flexibility, problem solving and innovation. The term ‘Machiavellian intelligence’ refers to the possible use of the various cognitive adaptations in the context of coalition formation against third parties as well as for the manipulation and deception of others
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