An overview of the central nervous system of the elephant through a critical appraisal of the literature published in the XIX and XX centuries
Introduction
The African elephant (Loxodonta africana) and the Indian elephant (Elephas maximus) possess the largest brain among land mammals. The development of the central nervous system in mammals, the relationships between brain and body weights, and the thickness of the neocortex have all been studied to evaluate the evolution of the brain in different Orders and between Families of mammals 12, 17, 29, 34, 36, 52, 53, 55, 57, 58, 59, 60, 62, 77, 80, 105, 107, 114, 117, 119, 120, 128, 130. For recent and comprehensive reviews see [120] and [128]. Due to these studies it is widely known that the brains of the two species of elephant rank among the highest for absolute and relative mass, cortical expansion, and complexity, features comparable only to those of some Cetacea and man 29, 77, 128, 130.
However, a close scrutiny of comparative studies of brain evolution reveals that data relative to the brain of the elephant seldom rely on original investigations and rarely (if ever) discriminate between Loxodonta and Elephas. It appears that the values relative to brain size and architecture are mostly taken from few original investigations, or quoted second-handedly from the original studies.
The literature on the brain of the Order Proboscidea is not abundant or easy to access. We started this study with the intent of collecting the largest possible body of literature on the brain of elephants, and to make it available to investigators wishing to study the subject. In the present paper we also briefly outline the main features of the brain of elephants, with special attention to the relevant differences with the general mammalian plan. Furthermore we analyzed the literature to understand the reasons for the relatively scarce development of the investigations, also taking into account the growing interest in other large mammals.
Section snippets
Materials and methods
We started our investigation by use of conventional electronic data banks, and specifically Medline and Current Contents (courtesy of the Universities of Milan and Padua, Italy) checking key words related to elephants.
We then proceeded to search the Zoological Record (printed version, courtesy of the Museum of Natural History of Milan, Italy) for any reference relative to elephants dating to the years preceding the use of electronic data banks in science.
References 1, 2, 3, 4, 5, 6, 7, 8, 9, 10
Brain weight
The elephant brain has a weight of 4.5 to 6.5 kg 2, 6, 14, 86, 119 (Elephas’s brain is up to 5.5 kg, while Loxodonta’s is slightly heavier and larger).
Telencephalon
The most striking features of the elephant forebrain are the complexity and depth of gyri, the development of the parietal and temporal lobes and the large cerebellum (Fig. 1 A–C). Rostrocaudal diameter is compressed, while the lateral one is enlarged [78]. The general outline of the telencephalon is reminiscent of that of Cetacea, and
Discussion
Our data indicate that the literature dedicated to the brain of Proboscidea is scarce. In relatively recent years, the elephant brain has been taken into account only as comparison in brain-body weight ratios and evolutionary studies. Most of the articles dealing with the neuroanatomy of Proboscidea discuss the brain of the elephant in general (n = 26 out of 52), or describe features specific of the Indian elephant brain (n = 17). Direct references to the neuroanatomy of Loxodonta africana are
Acknowledgements
We wish to thank Dr. Luigi Cagnolaro and Dr. Michela Podestà, Director and Curator, respectively, of the Museum of Natural History of Milan, Italy, for their precious help with the specific literature. A special thanks goes to Andrew Berman of New York, NY, USA, for critical revision of the manuscript. A special thanks also goes to Professor Antonio Barasa of the University of Turin, Italy, who kindly allowed us to use a drawing of a pyramidal neuron from one of his preparations of the brain of
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