Have you ever wondered why dogs and humans reproduce very differently? Why their litter is so big, often up to 7-8 pups while humans rarely give birth to more than 1 child at a time?
Well, there is a reason for it, and it has to do with the rate of mortality or death the a species faces at which stage of life.
There are organisms, like humans, elephants, and whales that show similar characteristics of reproduction. They breed occasionally, have longer gestation periods and give birth to one or two offspring at a time. These offspring take time to grow, and do so under the constant care and supervision of their parents. More often than not, they grow into adulthood and grow old, and this is when they are most likely to die a natural death.
Other organisms, like dogs, cats, insects, and fish show the complete opposite tendencies in reproduction. They breed once or twice in their lives, and have a huge number of offspring. These organisms (and their offspring) are generally small, and do not require too much parental care before they reach maturity. All of the offspring rarely live to adulthood, and these organisms show a characteristic tendency of having a very high mortality rate when they are young.
The first kind of organisms are called k-strategists in terms of reproduction, and the second kind of organisms are called r-strategists. The terms “k” and “r” comes from the typical graph that describes the population growth of organisms with respect to the natural resources and natural forces available in their environment.
K-strategist organisms “live” near the carrying capacity, k. This means that the environment they live in is stable, and has limited resources. Their population has reached a specific size, and any uncontrolled growth will result in the death of the entire population. So, they decided (well, nature decided for them) that they are better off focusing their energy on generating a few, healthy, complex offspring that can receive ample care so that they go on to survive till adulthood in a highly competitive world.
R-strategist organisms, on the other hand, “live” near the line representing exponential growth. These organisms are nowhere near the carrying capacity, and can therefore afford to grow their population. In fact, they need to. This is because they often live in unstable environments where the slightest disturbance can wipe out their population. So, they evolved a mechanism where they can spend their precious energy to increase their chances of survival as much as possible. They decided to develop as many offspring as possible, ensure that at least a few will survive their harsh environment. It is no surprise that these offspring are small in size; the need for numbers results in simpler, faster organization of the offspring. They also do not depend on the parents for long, they grow and move out and start reproducing on their own.
This natural tendency is very clearly visible when you look at a survivorship curve, which is a curve that depicts the number of survivors of a particular species at each stage of their life.
The Type I curve, or A curve is typically followed by k-strategist organisms. Their population mortality is low until they reach the end of their lifespan.
The Type III or C curve, is typically followed by r-strategist organisms. They exhibit high mortality at the early stages of their life. However, if they grow to maturity, then their chances of survival drastically increases.
In between, there are some organisms like birds, mice, rabbits, butterflies, etc. that neither fit the k-, or r-strategist type of survival. They lie in a zone where their chances of survival remain the same throughout their lifespan. Such organisms follow the Type II or B curve of survivorship. Within these, there are some organisms (like butterflies and other insects) that lean more towards the A curve, and therefore follow a B1 curve. Likewise, organisms (like rabbits, mice) that lean more towards the C curve, are said to be following a B2 curve.
In the end, all that matters for an organism is the continuation of its species and the transfer of genes to the next generation. This theory had been proposed as an all encompassing theory to understand this need of an organism. However, after the 1970’s, this theory received much scrutiny and criticism. As ecologists tried to validate this theory through empirical evidence, they found that various other factors also played a (greater) role in deciding the survivorship of populations. So is this theory still valid? Probably not anymore.