Paternal kin discrimination by sons in male chimpanzees transitioning to adulthood

Although paternal investment explains the evolution of fatherhood from a functional perspective, its evolutionary origins are unclear. Here we investigate whether a building block for paternal investment, father-offspring discrimination, is present in our closest living relatives, chimpanzees. Adolescent and young adult males (12 - 21 years old) maintained proximity and groomed with their fathers more frequently than with other males given how often they associated. This discrimination did not likely increase the short-term inclusive fitness of fathers or sons because the absolute time they spent in proximity or grooming did not exceed the time spent in these activities by other dyads. Almost all grooming was done by sons rather than fathers, suggesting that sons are responsible for observed biases in father-son behavior. Father-offspring discrimination could partly be explained by young males socializing with males who were more likely to be their father based on their age at the time of the young males conception. Two other cues of paternity, the other males rank at the time of the young males infancy and the other males association frequency with the young males mother during the young males infancy/juvenility, failed to predict association-controlled proximity or grooming. Father-son biases persisted even after controlling for characteristics of males that predicted paternity probability, implicating other cues that we did not examine. These results suggest that an important factor for the evolution of highly investing fathers in humans, father-offspring discrimination, may have been present in simpler form in the last common ancestor they shared with chimpanzees.