Lower Total Leukocyte Count

Humans and great ape species differ in total leukocyte (white blood cell) counts. All great ape species tested in captivity have significantly higher leukocyte counts than humans. Of all great apes, the bonobos show the highest leukocyte counts, followed by chimpanzees, orangutans and gorillas. This increase seems to include all types of leukocytes. The mechanism and significance of this difference are unknown.  One study ruled out the possibility that it is due to the stress caused by anesthesia used at some centers before blood drawing.