Encephalization of The Koala, Phascolarctos cinereus.

Abstract

Brain size relative to body size is considered to be an indicator of variously defined 'braininess' (=encephalization). Indices of encephalization are based on the ratio of the animal's actual brain size to its expected brain size calculated from an allometric equation derived from a brain size I body weight relationship in a series of taxa Freshly collected data on brain and body weights of 27 adult koalas (Phascolarctos cinereus) from South Australia are analysed here. Sex- averaged brain weight in this sample is 19.2 g and body weight is 8.0 kg. General equations for mammals produce encephalization values for the koala well below the mammalian average: EQ=38.9% ac- cording to Jerison's equation (1973), EQ= 49.7% applying Eisenberg's equation (1981) and EQ= 35.3% using Martin's equation (1990). When a 'basal' insectivore line is used, the koala appears to be progressive: IP =155.9% according to Stephan's equation (1972) and ICC= 131.7% using Martin's equation (1990). Use of 'basal' marsupial lines also indicates progressive encephalization of the koala: Pl=l l6.5% according to Pirlot's equation (1981}, E=108.4% following Nelson and Stephan's equation (1982) and E=107.9% using Haight and Nelson's equation (1987). These new results are clearly higher than the indices for the koala reported earlier by other authors (Nelson and Stephan 1982; Haight and Nelson 1987). It follows that choice of samples and equations influences conclusions regarding encephalization of a species.