Primate encephalization and intelligence.

The amount of brain mass exceeding that related to an animal's total body mass is called encephalization. And quantifying encephalization has been argued to be directly related to an animal's level of intelligence. In this study, the legitimacy of the encephalization hypothesis was tested for encephalization slopes 0.28, 0.67, and 0.75 in Snell's equation of simple allometry by determining the intersexual encephalization similarity for humans and 18 other primate species and by comparing the encephalization quotients for humans and six other primate species against the learning ability evaluations for their intelligence. Results suggest that the most accurate means for quantifying the encephalization of humans and other adult primate species requires the use of Lapicque's universal exponent of 0.28 in Snell's equation of simple allometry. Since Lapicque's slope was derived from various vertebrate groups, this equation may potentially be universally applicable for determining relative adult vertebrate encephalization and intelligence. Copyright 2002 Elsevier Science Ltd. All rights reserved.