Insulin-Like Growth Factor I and Prolactin Levels in Chimpanzees (Pan troglodyte) Across the Lifespan

As human and chimpanzee genomes show high homology for IGF1 and PRL, we analyzed sera of 367 healthy chimpanzees obtained during routine physical examinations in a single colony and measured chimpanzee insulin-like growth factor (IGF)-I and prolactin (PRL) levels across the lifespan using standard human immunoassays. Assuming chimpanzee IGF-I levels peak during puberty as in humans, we randomly defined puberty as the age at which most IGF-I levels were equal to or above the 90 th percentile for each sex (males, ages ≥7.00 but <9.20 years; females, ≥5.00 but <8.00 years). IGF-I levels steadily increased at a similar rate in juvenile males and females and peaked in puberty, strongly correlating with age, then slowly decreased faster in adult males than in adult females. As a group, males had higher mean IGF-I level than did females, but comparison by age category showed similar mean IGF-I levels in males and females. PRL levels increased with age in females more than males and levels were twice as high in females than in males. One pubertal male reported to have short stature had lower IGF-I and weight compared with other males in the age group, confirming suspected GH deficiency; a second male of normal height but low IGF-I may have had delayed puberty. Overall, results show differences in IGF-I levels over the lifespan in this cohort of chimpanzees largely mimic those seen in humans, while patterns of PRL changes are less similar.