Biol. Rev. (2008) doi:10.1111/j.1469-185X.2008.00049.x A claim in search of evidence: reply to Manger’s thermogenesis hypothesis of cetacean brain structure Lori Marino 1 *, Camilla Butti 2,3 , Richard C. Connor 4 , R. Ewan Fordyce 5 , Louis M. Herman 6 , Patrick R. Hof 2 , Louis Lefebvre 7 , David Lusseau 8 , Brenda McCowan 9 , Esther A. Nimchinsky 10 , Adam A. Pack 11 , Joy S. Reidenberg 12 , Diana Reiss 13 , Luke Rendell 14 , Mark D. Uhen 15 , Estelle Van der Gucht 2 , and Hal Whitehead 8 1 Neuroscience and Behavioural Biology Program, Emory University, Atlanta, GA, USA 2 Department of Neuroscience, Mount Sinai School of Medicine, New York, NY, USA 3 Department of Experimental Veterinary Science, University of Padova, Italy 4 Department of Biology, University of Massachusetts, Dartmouth, North Dartmouth, MA, USA 5 Department of Geology, University of Otago, Dunedin, New Zealand 6 Department of Psychology, University of Hawaii at Manoa, HI, USA 7 Department of Biology, McGill University, Qu ebec, Canada 8 Department of Biology, Dalhousie University, Nova Scotia, Canada 9 Department of Population Health and Reproduction, School of Veterinary Medicine, University of California, Davis, CA, USA 10 Department of Radiology, Mount Sinai School of Medicine, New York, NY 11 The Dolphin Institute, Honolulu, HI, USA 12 Center for Anatomy and Functional Morphology, Mount Sinai School of Medicine, New York, NY, USA 13 Department of Psychology, Hunter College, CUNY, USA 14 Center for Social and Cognitive Evolution, Sea Mammal Research Unit, School of Biology, University of St Andrews, Fife, UK 15 Alabama Museum of Natural History, Tuscaloosa, AL, USA (Received 9 October 2007; revised 12 June 2008; accepted 25 June 2008) ABSTRACT In a recent publication in Biological Reviews, Manger (2006) made the controversial claim that the large brains of cetaceans evolved to generate heat during oceanic cooling in the Oligocene epoch and not, as is the currently accepted view, as a basis for an increase in cognitive or information-processing capabilities in response to ecological or social pressures. Manger further argued that dolphins and other cetaceans are considerably less intelligent than generally thought. In this review we challenge Manger’s arguments and provide abundant evidence that modern cetacean brains are large in order to support complex cognitive abilities driven by social and ecological forces. Key words: brain, brain size, marine mammals, thermogenesis, intelligence, cognition, encephalisation, dolphin, cetacean, temperature. CONTENTS I. Introduction ...................................................................................................................................... 2 II. Claim 1: cetacean brains enlarged in a ‘‘punctuated’’ evolutionary event when oceanic temperatures dropped during the Eocene-Oligocene transition ..................................................... 2 (1) The real test of the thermogenesis hypothesis is in the Eocene. .............................................. 3 (2) Homeostatic inconsistencies in Manger’s thermogenesis hypothesis ........................................ 4 * Address for correspondence: E-mail: lmarino@emory.edu Biological Reviews Ó 2008 The Authors Journal Compilation Ó 2008 Cambridge Philosophical Society