R-selected species: These are the small animals that have larger litters. The letter ‘R’ symbolizes higher reproductive rate. The adult members of these species usually invest relatively less time and effort in taking care to upkeep their offspring. The young ones of these species grow relatively faster than those of the K-selected species. The cycle repeats quite frequently, allowing them to become hugely populated within a short span of time. E.g: Rodents, like mice, rats.
K-selected species: These are the larger animals that have smaller litters. Unlike the ones that fall under the R-selected species, the K-selected ones are much bigger in size and have smaller litters. These animals usually spend more hours a day to care for their young ones, because the babies are not capable of caring themselves until they grow much older. They usually born blind and remain so for many weeks, which is why they bank on their mother for food and protections for comparative a longer span of time. The grow rate of these species relatively faster than those enlisted under the R-selected groups. The cycle is comparatively much slower… these animals are likely to have babies as frequent as the R-selected species, which is why these animals do not get over-populated.
When it comes to the litter size of mammals, one most obvious expression of reproductive activity is the number of babies born in a single litter. This varies among the species and even with species. The variation of the litter size, as a whole, arises from genetic and environmental impact. The difference among the eutherian mammals in the individual weight and number of their young at the birth are huge. Let us compare the two extremities among species. While mice give birth to young ones each weighing 1 grams, a whale gives birth to a baby that may weigh 1 tonne right after the birth. Now I have a couple of questions:
- Firstly, why does a single pattern of reproduction has such a huge variation?
- Secondly, if there’s such a huge variation, why isn’t it even more?