Back when I was in college in the 1970s, I took Principles of Evolutionary Biology for my science requirement. I very much enjoyed the class and I'm a little surprised at how much I remember. One concept which was relatively newly coined that made it into the textbooks quickly was the idea of r and K strategies for reproduction, first introduced in 1967 by Robert MacArthur and E.O. Wilson.
The idea is that different species have different strategies of reproduction. The letter r and K are from a differential equation defining the growth of populations, where r stands for the rate of growth and K stands for the maximum capacity of the ecological environment. The idea is that some species, usually smaller organisms with short lifespans, breed like crazy and do not expend much energy in raising their young, which is called an r strategy, sometimes referred to as the opportunistic strategy. K strategy is to have relatively few offspring and to invest time and energy in the raising and nurturing of the young. This is also known as the equilibrium strategy, and is often seen in larger organisms with long lifespans.
If I had to guess, I think the theory got a lot of early acceptance because there was a differential equation involved. It's the mathematician in me showing a little pride, but physics envy is a real phenomenon in the sciences. Physics uses math very well and is considered the hardest of the "hard sciences", and other sciences are given somewhat less respect because they can't point to nice equations that predict the outcomes of phenomenon.
While the ideas of r and K strategies started off as separate categories, soon enough people started talking about the r and K continuum. Take, for example, ants and other social insects. They are small and have short lifespans, so they are probably r strategy species. But compared to other bugs, they look like K strategists, because they limit their reproductive rate by having only one egg laying female, the queen, and by investing time and energy to ensure the young make it to adulthood by having a guarded nursery. As a result, ants do not breed as fast as beetles, bees do not breed like flies and termites do not breed like cockroaches. It may seem like there's a hell of a lot of them when we get an infestation in our homes, but for their size and lifespan, they fall on the K side of the spectrum.
Warm blooded creatures, mammals and birds, have little choice but to invest time in raising the young, because they are usually born unable to feed themselves and incapable of moving around the way the adults can, either moving quickly or flying, in the case of birds and bats. Even so, small mammals are usually thought of as r strategy species, breeding quickly with relatively little care shown to the raising of the young. A counterexample are meerkats, small but social critters. If you've watched Meerkat Manor, you know that the clan invests a lot of time in the raising of the young, and the hierarchy of dominance means a dominant female will attack any less dominant female who makes the mistake of getting pregnant when the dominant female is pregnant.
One of the controversies regarding the theory comes from the idea that humans are on the r and K continuum. In places and times when infant mortality is very high, many families will decide to have as many children as possible in hopes that some will survive to adulthood. In safer environments, couples will often have fewer children and invest more time in their children's care. The controversy usually arises when people claim these differences are racially based, with Africans being "natural r strategists" and Europeans and Asians being "natural K strategists".
While larger size and long lifespan are usually signs of K strategy species, this is not a hard and fast rule. Sea turtles live a long time and grow to be one of the largest creatures in their ecosystem, but as we know, they lay massive numbers of eggs in the sand, cover the eggs up and leave, investing no time in the raising of the young. For sea turtles, the question of infant mortality is not one of how many make it to one year old, but how many make it to a few minutes old by surviving the trip across the beach to the relative safety of the ocean.
Another set of large creatures with long life spans that use r strategy for the most part are sharks. For some species, though there is no time invested raising the young, which would argue an r strategy, an egg sack like the one shown at left will only produce one or two sharks, which looks more like a K strategy. What happens is the egg sack has many, many eggs, and as the sharks hatch, they learn to do what sharks do, eat anything and everything they can sink their teeth into, which in this small environment means their brothers and sisters. After a nice healthy first meal of cannibalistic fratricide, a single small but bad ass shark emerges from the egg sack, ready to prey on creatures from different species.
This is called The Boy Named Sue strategy.
Okay, I made the name up, but not 'splainin'. That really is a description of life in a shark egg sack.