Besides the distinction between fast and slow lives based on absolute body size, the lives of certain species may be fast or slow for a given body size. Habitats with instable food supplies engender fast lives with high reproductive output, as does a relatively high mortality rate.
Populations that inordinately die off adaptively trend to shorter gestation, smaller neonates (offspring), larger litters, and faster development (weaning and reaching maturity). Species populations with a high death rate tend to reproduce earlier.
Social dynamics can selectively influence infant development. Infanticide is exemplary.
Infanticide is the result of an evolutionary arms race, where males compete with each other for reproduction and try to influence females in mating with them. In species where it happens more often, it can certainly influence the nature of the social relationships between males, as well as between males and females. ~ Canadian primatologist Pascale Sicotte
Infanticide occurs in several mammal species, including lions, bears, rodents, and primates. Typically, an adult male kills infants sired by another so that he can mate with the mother and have her raise his offspring instead.
The evolutionary response to potential infanticide in colobus monkeys is for a mom to accelerate her offspring’s development by investing more energy in early growth. This only happens when a female lives in a group where infanticide appears a threat.
Monkey moms are forewarned by new males entering the group, and by their aggressive behaviors toward younglings. Efforts are most pronounced by mothers with the youngest offspring which are most at risk.