Paternal kin discrimination by sons in male chimpanzees transitioning to adulthood

Abstract

20 21 22 23 24 25 26 27 2 28 Abstract 29 30Although paternal investment explains the evolution of fatherhood from a functional perspective, 31 its evolutionary origins are unclear. Here we investigate whether a building block for paternal 32 investment, father-offspring discrimination, is present in our closest living relatives, 33 chimpanzees. Adolescent and young adult males (12 -21 years old) maintained proximity and 34 groomed with their fathers more frequently than with other males given how often they 35 associated. This discrimination did not likely increase the short-term inclusive fitness of fathers 36 or sons because the absolute time they spent in proximity or grooming did not exceed the time 37 spent in these activities by other dyads. Almost all grooming was done by sons rather than 38 fathers, suggesting that sons are responsible for observed biases in father-son behavior. Father-39 offspring discrimination could partly be explained by young males socializing with males who 40 were more likely to be their father based on their age at the time of the young male's conception.41 Two other cues of paternity, the other male's rank at the time of the young male's infancy and 42 the other male's association frequency with the young male's mother during the young male's 43 infancy/juvenility, failed to predict association-controlled proximity or grooming. Father-son 44 biases persisted even after controlling for characteristics of males that predicted paternity 45 probability, implicating other cues that we did not examine. These results suggest that an 46 important factor for the evolution of highly investing fathers in humans, father-offspring 47 discrimination, may have been present in simpler form in the last common ancestor they shared 48 with chimpanzees. 49 50 3 51 Introduction 52 53In most human societies, fathers contribute to the health and fitness of their offspring.54 Through co-evolution with sexual-division of labor, this has led to the unusual combination of 55 high fertility and low mortality responsible for our ecological dominance of the planet (1-6).56 Although human fatherhood has been well studied from this functional perspective, its evolution 57 (sensu 7) remains puzzling. Phylogenetic inertia plays a strong role in the evolution of social and 58 mating systems, with those of closely related species generally being very similar (8, 9). Yet in 59 humans' closest living relatives, chimpanzees and bonobos, females are highly promiscuous and 60 mate with many adult males during each conception cycle (10-12). Like all complex adaptations, 61 human fatherhood represents the end-product of a multi-step evolutionary sequence in which 62 more elaborate forms succeeded one another (13). For fathers to invest heavily in their offspring, 63 they must first evolve a capacity for kin discrimination, which refers to differences in behavioral 64 responses individuals show toward kin compared to non-kin using conspecific cues correlated 65 with kinship (14). 66Pair bonds play a key role in father-offs...