Researchers have found that chimpanzees can be categorized on the basis of five personality types: reactivity/undependability, dominance, openness, extraversion and agreeableness. There is also a sixth personality type - the methodical - but this requires additional research to be proven.

"Understanding chimpanzee personality has important theoretical and practical implications. From an academic standpoint, the findings can inform investigations into the evolution of personality. From a practical standpoint, caretakers of chimpanzees living in zoos or elsewhere can now tailor individualized care based on each animal's personality thereby improving animal welfare," said Hani Freeman, postdoctoral fellow, Study and Conservation of Apes at Lincoln Park Zoo, according to a news release.

Researchers said that although assessing chimpanzee's personality isn't entirely new, it was previously done using tools that didn't give good results. Some personality tests were actually designed for other species, so they lacked traits that are unique in chimpanzees.

Also, there are some personality tests designed for chimpanzees that account only for specific traits, and don't give accurate descriptions of an individual's personality. "This approach also has limitations as it impedes comparisons with findings in other studies and other species, which is essential if you want to use research on chimpanzees to better understand the evolution of human personality traits," said Freeman.

In the present study, researchers developed a personality scoring scale based on available tools for personality assessment. The new scale has 41 behavioral descriptors including boldness, jealousy, friendliness and stinginess, among others.

The study included 99 chimpanzees aged between 8 to 48 years from Michale E. Keeling Center for Comparative Medicine and Research, UT MD Anderson Cancer Center in Bastrop, Texas. About 17 of their caretakers used the scale to assess their personality.

Researchers also used behavioral data on these chimpanzees to validate their findings. The team found that personality was associated with the way in which different chimpanzees behaved. Also, the caretakers who rated these animals made independent assessments of their personalities by agreeing in their individual judgment of their personality, and didn't project traits on other chimpanzees.

The study is published in the American Journal of Primatology.