Have you ever wondered why dogs and humans reproduce very differently? Why a dog 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. This has to do with the rate of mortality (death) the a species faces at different stages of life.
The r/k selection theory
The r/k selection theory proposes to explain the reproduction strategies of species with respect to their environmental characteristics.
Ecologists of the 1970’s categorized species into two categories: those that lived in stable environments and those that lived in unstable environments with many environmental stresses threatening the population.
This can be visualized on the population growth curve (see below). The population of species living in stable environments, with limited resources, has a definite size near carrying capacity k. The others, living in unstable environments, need to ensure some of their numbers survive the dangers of their environment. Resources are not a constraint. Their populations rise and fall exponentially, depending on environmental factors.
Ecologists noticed that the two groups of species reproduced differently, to fit their environmental characteristics. The r/k selection theory explains whether a species chooses to be a k-strategist or an r-strategist.
K-strategists “live” near the carrying capacity k on the population growth curve, under stable environment conditions. They have 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 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.
Characteristics of k-strategists
K strategist species show similar characteristics of reproduction like:
- Occasional breeding
- Longer gestation periods and give birth to one or two offspring at a time
- Offspring take time to grow, and do so under the constant care and supervision of their parents.
- Low infant mortality: More often than not, they grow into adulthood and grow old, and this is when they are most likely to die a natural death.
Common examples of k-strategists species include humans, lions and whales.
R-strategists “live” near the line of exponential growth r. 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, R-strategist species 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, ensuring 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.
Characteristics of r-strategists
R-strategists species show the complete opposite tendencies in reproduction, as compared to k-strategists. Their characteristics include:
- Breeding only once or twice in their lives
- Having a huge number of offspring.
- Small size of both offspring and adults
- Minimal parental care before reaching reproductive maturity
- High infant mortality. All of the offspring rarely live to adulthood
Examples of r-strategist species are dogs, cats, insects, and fish.
This natural tendency is 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.
While this theory is elegant, ecologists have not been able to empirically validate it in nature. Other factors also control survivorship of species, and the r/k selection theory is now obsolete.
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 role in deciding the survivorship of populations. So is this theory still valid? Not anymore.