Reproduction is a normal part of life, but why do animals reproduce, and why have some species evolved different reproduction methods?
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 produce 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 can hunt immediately after birth and do not require any parental care.
So, what is the benefit of expending extra energy caring for vulnerable offspring 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?
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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 by fighting other males or 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 more robust.
In times of hardship, such as drought, lack of food, or disease, females may choose not to breed, as conditions are unsuitable for rearing young. They may wait until the following breeding season to hope for more favorable conditions. 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 occurs 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 there are many forms in the animal kingdom, each with its benefits. Some species are asexual, meaning they can reproduce on their own, and some animals can change sex depending on the availability of males or females in their environment.
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Sexual Reproduction
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 uterus wall 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 the male will also help find food and protect the den or nest in some species.
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. It is common for the weakest to die in these situations as the parents prioritize the stronger offspring. The idea here is that a more significant number of offspring survive, and the weaker animals cannot pass on their genes, which would weaken future generations.
In contrast, reptiles lay eggs, and the young will develop inside the egg. They can function much better than a mammalian infant when they hatch and 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.
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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 create a mix of both male and female hatchlings. Sea turtle hatchlings can fend for themselves immediately after hatching and head straight to the water.
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Asexual Reproduction
Leeches are hermaphrodites, meaning they possess both male and female reproductive organs. They still need to swap genetic material with another individual, but they can fertilize their 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 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 can produce its offspring, increasing the number of babies born.
Water fleas can reproduce without fertilization, using a process called parthenogenesis. Offspring are a copy of their mother (most animals that use parthenogenesis are female). Water fleas, found primarily on freshwater, will produce eggs held in a dorsal brood patch until they are ready to hatch. They do not require fertilization to develop.
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In a process called fission, some invertebrates, such as flatworms, can separate their bodies and regrow the missing parts in 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 live a mostly solitary life. It is most common in lower forms of invertebrates due to their less developed anatomy and the speed 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 on nursing and protecting their young. Mammalian young are vulnerable and are born with much development, 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 it takes for eggs to grow, but the parents must spend some time caring for their young, as most mammalian young are born blind.
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References
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/
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Bryan Harding is a member of the American Society of Mammalogists and a member of the American Birding Association. Bryan is especially fond of mammals and has studied and worked with them around the world. Bryan serves as owner, writer, and publisher of North American Nature.