Reproduction is a normal part of life, but why do animals reproduce, and why have some species evolved different methods of reproduction?
Reproduction ensures the continuation of the species. Animals will only reproduce with the fittest of the species and choose not to mate with animals with the disease or in periods of drought or lack of food. Some animals require a mate to reproduce, while other species can reproduce alone.
The method of child-rearing also differs between species. Most mammals will provide food for their young and protect them from predators, but many reptilian species do not care for their young. They are capable of hunting immediately after birth and do not require any parental care.
So, what is the benefit of expending extra energy caring for offspring that are so vulnerable after birth? Is there a long-term payoff compared to egg hatchers, or is the reproductive process simply a by-product of their anatomy and environment?
The Purpose of Reproduction
The purpose of reproduction – creating new individuals of a species – ensures the continuation of the species. Biologist Richard Dawkins noted in his book The Selfish Gene that nature is designed to keep replicating genes.
Reproduction is much more than simple duplication, with many species showing competitive behaviors during courtship. Animals who require a mate for reproduction will not just choose any member of the opposite sex.
In most species, the male will win his right to mate with a female, either by fighting other males or by courting the female with gifts or physical displays such as dancing and calling.
This competition ensures that only the fittest animals pass on their genes so that future generations will be healthier and stronger.
In times of hardship, such as drought, lack of food, or disease, females may choose not to breed, as conditions are not suitable for rearing young. They may decide to wait until the following breeding season in the hopes that conditions are more favorable. This suggests that the purpose of reproduction is to ensure the survival of healthy individuals, or animals would continue to reproduce even in harsh environments.
The reproductive behavior of animals and the manner of birth differs between classes. For example, mammals typically give birth to live young, whereas most invertebrates lay clutches of eggs. For mammals, some invertebrates, and most reptiles, fertilization (egg and sperm uniting) happens internally, but fertilization for most fish species happens externally. The female will lay her eggs, and the male will follow behind her, releasing sperm into the water over the eggs.
Sexual reproduction is considered “normal” reproduction, but in the animal kingdom, there are many forms of reproduction, each with its own benefits. Some species are asexual, meaning they are able to reproduce on their own, and there are also animals that are able to change sex depending on the availability of males or females in their environment.
Mammals reproduce sexually, meaning an individual is either male with testes or female with ovaries. Mating is the act of the male transferring sperm to the female to fertilize the eggs released into her uterus. If the egg is successfully fertilized, it will implant into the wall of the uterus and begin developing.
Mammals also give birth to live young, meaning the offspring are born via the birth canal and are well developed. Mammalian young are often vulnerable for the first few weeks or months as their eyesight and muscles strengthen. There is a heavy expenditure from the parents towards caring for the young. Typically, the mother does most of the nursing, but in some species, the male will also help to find food and protect the den or nest.
Some animals will have just one or two young and focus all their energy on ensuring they survive, whereas other species will have larger broods. In these situations, it is common for the weakest of the litter to die as the parents will prioritize the stronger offspring. The idea here is that a greater number of offspring survive, and the weaker animals are not able to pass on their genes, which would weaken future generations.
In contrast, reptiles lay eggs, and the young will develop inside the egg. When they hatch, they are able to function much better than a mammalian infant and can usually survive without their parents.
Green sea turtles are a perfect example. Females will develop a clutch of eggs, fertilized by a male during sexual reproduction. The female will then travel to a nesting sight, sometimes thousands of miles, to a sandy beach with a warm climate. In Florida and other warm southern states, nesting season begins in March.
Females crawl up the beach from the sea, searching for the perfect spot to lay their eggs. Females usually return to the same beach where they hatched. Experts believe that sea turtles can use the Earth’s magnetic field for navigation.
Before egg laying can begin, the female must dig a deep hole, which she does with her large front flippers. She will then lay over the hole so the eggs roll into the center. A clutch can have as many as 60 eggs. The female then uses her flippers to cover the clutch of eggs with sand before returning to the ocean.
The temperature of the eggs is vital to their development. Eggs incubated below 82°F will develop as males. If the eggs have incubated above 86°F, the hatchlings will be female. Clutches incubated between these temperatures will develop a mix of both male and female hatchlings. Sea turtle hatchlings can fend for themselves immediately after hatching and will head straight to the water.
Leeches are hermaphrodites, meaning they possess both male and female reproductive organs. They still need to swap genetic material with another individual, but they are able to fertilize their own eggs.
Earthworms have a unique method of reproduction. Since they spend almost all of their time underground, finding a mate can be difficult. When they do come across another worm, they give a gamete and receive a gamete. Gametes are sperm (spermatozoa) and eggs (ova). Earthworms have both male and female genitalia, so being able to swap genetic material with another worm means each worm is able to produce its own offspring, increasing the number of babies born.
Water fleas are able to reproduce without fertilization, using a process called parthenogenesis. Offspring are basically a copy of their mother (most animals that use parthenogenesis are female). Water fleas, found mostly in freshwater, will produce eggs that are held in a dorsal brood patch until they are ready to hatch. They do not require fertilization to develop.
In a process called fission, some invertebrates, such as flatworms, are able to separate their bodies and regrow the missing parts on both sections, effectively creating two individuals from one. Budding is another formal of asexual reproduction common in lower classes of invertebrates such as corals and hydras. A bud or growth from the main body will form, develop into an adult, and separate, becoming a second individual.
Fragmentation is a common form of reproduction for annelids, echinoderms, and turbellarians. Fragmentation is when several parts break off from one individual. This may occur due to injury from predators or a purposeful action to reproduce. A new individual may form from a separated arm. In some fisheries, workers may try to prevent sea stars from eating oysters and clams by cutting them in half, but the sea stars can regenerate the missing part of their body and become two individuals.
Asexual reproduction is beneficial for animals who tend to live a mostly solitary life. It is most common in lower forms of invertebrates due to their less developed anatomy and the speed at which their bodies develop into adulthood.
Reproductive efforts on the part of the parents are most common in mammalian species. They expend a lot of time and energy to nurse and protect their young. Mammalian young are vulnerable and are born with much development still to do, whereas reptiles can hatch fully developed but simply a smaller size than their parents.
The advantage of reproduction via eggs is that the parents do not need to care for their young. They are developed enough that they can survive alone immediately after hatching. Gestation for livebearers is longer than the time it takes for eggs to develop, but the parents must spend some time caring for their young as most mammalian young are born blind.
Biocyclopedia. (2012). Why Do So Many Animals Reproduce Sexually Rather Than Asexually? Retrieved from General Zoology / Continuity and Evolution of Animal Life: https://biocyclopedia.com/index/general_zoology/why_do_so_many_animals_reproduce_sexually_rather_than_asexually.php
Kent, G. (2018, June 1). Animal reproductive system. Retrieved from Encyclopedia Britannica: https://www.britannica.com/science/animal-reproductive-system
Molnar, C., & Gair, J. (2015). 13.1 How Animals Reproduce. In C. Molnar, & J. Gair, Concepts of Biology – 1st Canadian Edition. Pressbooks.
Shah, R. (2021, January 3). Why Animals Reproduce: The Selfish Genetic Reason. Retrieved from Shortform: https://www.shortform.com/blog/why-do-animals-reproduce/
Thomas, B. (2015, May 28). Why Do Animals Use Sexual Reproduction? Retrieved from Institute For Creation Research: https://www.icr.org/article/why-do-animals-use-sexual-reproduction/