Reproductive System
Most ectotherms are r-strategists because they rely on external sources of heat. They have the disadvantage of being vulnerable to fluctuations in external temperature, which means that the risk of losing offspring to random environmental factors is high. Therefore they are r-strategists, hoping that at least one will survive and ensure the survival of that specie. Due to that they do not invest a high amount of parental care into their offspring, since overtime it would not pay off.
Most endotherms are k-strategists because there is a decreased vulnerability to fluctuations in external temperature, making them more stable to the environment. This means that they will less easily lose offspring to random environmental factors, which means that they do not require a high amount of offspring to ensure the survival of the species, as there is a higher ratio of offspring surviving to offspring produced compared to ectotherms. Therefore it makes sense that they would invest more time into a single child if they know that its chance of survival is quite high. External fertilization is going to be more common in r-strategists rather than k-strategists because there is a smaller chance that sperm and egg are going to meet externally than internally since there is a lot more space where the sperm can escape to other than the egg. Therefore animals that fertilize externally usually utilize a strategy where they produce a high amount of offspring and therefore low parental care to ensure that enough sperm and egg will meet over time so that the specie survives. In order of development and complexity: 1. Fish are ectotherms and therefore r-strategists. Most lay eggs and fertilization happens externally. Most are oviparous (they lay eggs), although some can be ovoviviparous and viviparous. 2. In amphibians fertilization occurs externally and most amphibians are r-strategists. Its reproduction has more in common with that of fish than reptiles, birds or mammals since they evolved from lobe-finned fish. 3. Most reptiles provide no care and are therefore r-strategists, although crocodilians do provide some post-egg care. Reptiles are the first amniotes, meaning that they produce young in means of an amniotic egg that provides a safe and self-contained aquatic environment for the developing embryo. Most reptiles reproduce sexually although some are known to reproduce asexually. 4. Most birds are k-strategists. They are amniotes and oviparous (they lay eggs), and nearly all mate for life, meaning that they reproduce with the same partner until they die. All birds reproduce sexually, thereby producing a new organism each time. 5. Most mammals are k-strategists. They are amniotes, mostly viviparous and fertilise internally. Although most birds show monogamy (mate for life), only a very small percentage of mammals do the same (including humans). In all mammals milk nourishes the newborn. All mammals reproduce sexually. Only a small percentage of mammals are monogamous. It is often speculated why, but reasons could be that it usually takes much longer for one new born of a mammal to become an adult and also requires a lot of care, which means that if they were partners for life they would not reproduce as much. Therefore the male usually leaves the female after the offspring have been born and selects a new female to mate with which means that potentially double the amount of new offspring can be produced at the same time. From this one could infer that it would be in terms of reproduction an advantage if there was a higher ratio of females to males as it only requires one male, but several females to produce a large amount of offspring. |