Quantitative Analysis of Neocortical Gyrencephaly in African Elephants (Loxodonta africana) and Six Species of Cetaceans: Comparison With Other Mammals Paul R. Manger, 1 * Michelle Prowse, 1 Mark Haagensen, 2 and Jason Hemingway 1 1 School of Anatomical Sciences, Faculty of Health Sciences, University of the Witwatersrand, Parktown, Johannesburg, Republic of South Africa 2 Department of Radiology, University of the Witwatersrand—Donald Gordon Medical Centre, Parktown, Johannesburg, Republic of South Africa ABSTRACT This study provides quantitative data on the extent of gyrencephaly in the large-brained African elephant and several species of cetaceans (from smaller to larger brained) in comparison with other mammals. Across three mammalian orders (primates, carnivores, and artiodactyls), the species with the larger brains are more gyrencephalic with each order, exhibiting a spe- cific negative allometry. The African elephant, with a 5- kg brain, has a gyrencephalic index (GI) of 3.89, which, though highly gyrencephalic, is not more so than would be predicted for a mammal with a 5-kg brain. The ceta- ceans had an average GI of 5.43, are the most gyrence- phalic mammals studied to date, and are more gyrencephalic than one would predict based on com- parison with other mammals. No relationship between brain mass and GI was evident in the cetaceans as seen in other mammals, with all cetaceans showing similar GIs irrespective of brain mass (range of GI 5.23–5.70, range of brain mass 577–5617 g). This is yet another parameter indicating cetaceans to be neu- roanatomical outliers. Two species of pinnipeds studied had GIs that were well above those seen for terrestrial carnivores, and the aquatic manatee was close to lis- sencephalic. Thus, all three groups of marine mammals showed unusual extents of cortical gyrencephaly, indi- cating a morphological alteration of the telencephalon associated with the return to the marine environment. The analysis suggests that cortical thickness and neuro- nal density are important factors in determining the extent of gyrencephaly across mammalian species. J. Comp. Neurol. 520:2430–2439, 2012. V C 2012 Wiley Periodicals, Inc. INDEXING TERMS: cerebral hemisphere; neocortex; Afrotheria; primates; Cetacea; Proboscidea The vast majority of mammalian brains are small, with a lissencephalic cerebral cortex. In spite of this, there are several mammalian groups with larger brains, such as pri- mates, carnivores, and artiodactyls, that exhibit varying degrees of gyrencephaly. The examination of gyral and sulcal morphology and patterns has a long history; how- ever, a comprehensive review of the literature indicates that, whereas larger brains appear to be more gyrence- phalic, there are order-specific features in the pattern of gyrification (Welker, 1990). Zilles et al. (1989) provided the first comprehensive quantitative analysis of gyrence- phaly and, through the determination of a gyrencephalic index (GI), demonstrated that in primates larger brains are indeed more gyrencephalic than smaller brains (with the presence of a negative allometry). A more recent study (Pillay and Manger, 2007) confirmed the findings of Zilles et al. (1989) for primates and extended this by dem- onstrating that there are order-specific patterns to the quantifiable parameters of gyrencephaly such that each mammalian order examined demonstrated a unique and Grant sponsor: South African National Research Foundation; Grant number: FA2005033100004 (to P.R.M.). *CORRESPONDENCE TO: Paul R. Manger, School of Anatomical Sciences, Faculty of Health Sciences, University of the Witwatersrand, 7 York Road, Parktown, 2193 Johannesburg, Republic of South Africa. E-mail: paul.manger@wits.ac.za V C 2012 Wiley Periodicals, Inc. Received November 7, 2011; Revised December 10, 2011; Accepted January 7, 2012 DOI 10.1002/cne.23046 Published online January 11, 2012 in Wiley Online Library (wileyonlinelibrary.com) 2430 The Journal of Comparative Neurology | Research in Systems Neuroscience 520:2430–2439 (2012) RESEARCH ARTICLE