The endemic bovid Myotragus, from the Plio-Pleistocene deposits of Majorca, underwent a significant reduction in relative brain size, especially affecting the vision and locomotor centres. These important modifications allow the hypothesis that, under altered conditions in predator free insular environments, functional demands on neural performance are reduced, allowing a reduction in brain structures and associated sense organs. This is suggested, for example, by the small orbits and reduced visual brain structures of Myotragus balearicus Bate, 1909. These changes, which parallel those shown by domesticated animals, have been interpreted as an adaptive response to insular environmental conditions, in particular to a lack of interspecific competitors and predators. Results obtained from analysis of the smallest Cretan deer (Candiacervus ropalophorus de Vos, 1984, and Candiacervus spp. II) are not as outstanding as it is in Myotragus, and apparently run counter to the fact that dwarfed insular artiodactyls living in environments without any predation pressure would be characterised by a proportional decrease in brain volume as a rule. Brain/body mass proportion of the smallest Candiacervus species provides evidence that this endemic deer underwent only minor changes in the relative size of brain after its geographic isolation. On the other hand, orbit and foramen magnum size are proportionally reduced. Accordingly, different evolutionary process likely affected brain and sense organs changes of these endemic artiodactyls. Other factors, nature of available niches, inter-specific competition, maybe island size and time from isolation, would have played a role in the possible differential evolution of insular ruminants in addition to the change in adaptive strategy for a more efficient energy use under the free-predators environmental conditions of the insular ecosystem. © 2007 Elsevier Ltd and INQUA.