Mating and Parental Care
In mammals, males typically have shorter lives than females. This difference is due to behavioural traits which enhance competitive abilities, and hence male reproductive success, but impair survival. ~ Austrian evolutionary biologist Helmut Schaschl et al
Animal mating and parental care practices are life-history variables with knock-on effects across a wide range of conspecific interactions. To hedge bets on survival via greater numbers, the earliest mammals produced large litters. The shift to fewer offspring occurred as the payoff for parental care rose.
Male competition for mating privilege is typical in animal species where a female is the primary caretaker of progeny. Besides sexual dimorphism being the norm, males pay the price in shorter lifespans.
Testes size is indicative of the trade-off in male primates between mating and parenting effort. Male chimpanzees are especially promiscuous. They generally do not provide parental care. Chimp testes are twice as big as humans. In contrast, male gorillas protect their young and sport small testes. Among humans, those with smaller testes are innately more interested in taking care of their children. But – irrespective of testes size – testosterone level drops as a father spends more time with his children.
Parenting is an outcome of social interactions between and within sexes, and so cannot itself evolve. But behaviors that provide information about parentage can evolve and affect mating and parental care.
The heuristic that parental care increases with evolutionary descent, size, and lifespan is full of exceptions. Hooded seal pups are nursed only ~4 days, whereas parental cichlid fish care may last months.