The study of behavioral ecology may broadly be divided into two areas. The behavioral ecology of survival
examines the role of behaviors that enhance survival through such means as optimization of feeding and predator avoidance, for example. The behavioral ecology of reproduction
examines, in effect, behaviors that enhance reproductive output. In fact, the distinction is an arbitrary one. Anything that increases an animal's survival is likely to produce greater mating opportunities for that animal. Anything that increases mating opportunities, fecundity or offspring survival is likely to impact upon the animal's survival in some way.
In essence, when we talk about fitness we are ultimately concerned with reproduction. An animal does have to survive to reproduce - but if it survives and doesn't reproduce, then it is likely to be an evolutionary failure. Whatever genetic combination caused it not to breed would be, in most instances, strongly selected against. That is, animals with alternative genes that permitted them to breed would pass on those genes whereas genes that inhibited breeding would not be passed on and would disappear from the population.
Darwin’s triumph was to explain evolution through this process of Natural Selection whereby attributes that enable individuals to outcompete each other become more prevalent in succeeding generations. Such selection should seemingly encourage animals to behave selfishly and not for the good of the species or the group in which they live. But it seemed like some animals did appear to behave altruistically and this was where Bill Hamilton's notion of Kin Selection
triumphed as an explanation.