Darkening coat colour reveals life history and life expectancy of male Thornicroft's giraffes

Abstract

Much of the information available about the life history of the giraffe, Giraffa camelopardalis, is derived from captive studies or short-term field studies. The coat colour of male giraffes, especially the blotches, darkens with age, but no studies have systematically mapped the colour transition with chronological age based on long-term data. We examine the value of using darkening coat colour as a biomarker of male age. We analyzed 33 years of data from 36 male Thornicroft's giraffes, G. c. thornicroftii, living in Zambia in order to document key milestones in male development. We found that the change in male pelage colouration takes an average of 1.8 years and that males are completely covered with coal-black blotches at an average age of 9.4 years. Using lifetime data on male deaths and disappearances, combined with cross-sectional records on coat-colour transformation, we conclude that the average age of death among male giraffes is about 16 years old. The maximum lifespan of male giraffes is about 22 years compared with a maximum lifespan of about 28 years for female giraffes. We conclude that the possible proximate mechanisms and adaptive significance of male coat-colour changes should be studied in more detail.