Macphail, EM (1996) Cognitive function in mammals - the evolutionary perspective. Cognitive Brain Research, Vol.3, No.3-4, Pp.279-290.
“My argument throughout this paper will be that there is remarkably little evidence of species differences in intelligence” (p. 280)
Macphail, E. M., & Bolhuis, J. J. (2001). The evolution of intelligence: adaptive specializations versus general process. Biological Reviews, 76(3), 341-364.
“The evidence reviewed above supports Darwin's supposition that there are no qualitative between-species differences in the cognitive capacities of animals.” (p. 361)