Lesson Explainer: Reproduction as a Life Process Biology

In this explainer, we will learn how to outline the significance of reproduction to organisms and when it occurs in a life cycle and describe two different reproductive strategies, with relevant examples.

Every living organism, whether it is a tiny bacterium or a massive blue whale, carries out several essential life processes. These processes common to all organisms are respiration, excretion, obtaining nutrition, sensitivity to surroundings, and reproduction. While the first four processes are absolutely vital to the survival of an individual, not all individual organisms will reproduce. In fact, most organisms can survive even if the genitalia are removed!

Reproduction is the process by which an organism passes on their genetic material to another generation by having offspring. Offspring are the young of an organism, otherwise known as its children or progeny.

Definition: Offspring (Progeny)

Offspring are the young born of living organisms, produced either by a single organism or, in the case of sexual reproduction, two organisms.

Reproduction functions to ensure the continuity of a species at the population level. Individuals can survive without producing offspring, but a species cannot survive in the long term if all of its members die before they reproduce. Failure to produce offspring on a larger scale can therefore result in species extinction.

Past extinctions, such as those of the dinosaurs, are reflected in the fossil record. Fossils, which are the preserved remains of once living organisms that are usually found buried underground, provide us with evidence of species that have lived in the past and went extinct. When many, many members of the same species are found to have died in a short space of time, this suggests that the species may have failed to reproduce resulting in their extinction. Many species are currently critically endangered, partly due to difficulties in reproduction.

For example, in 2020, there were believed to be less than 80 individual Sumatran rhinoceroses remaining on Earth. They are at a very high risk of extinction, and this is believed by most scientists to result from poaching and fragmentation, or splitting apart, of their habitat. This means that the male and female rhinos rarely encounter each other enough to reproduce. To make matters worse, females that go too long without breeding develop issues with their reproductive organs that further hinder their reproductive capability. This means that the Sumatran rhinoceros, which you can see in the photograph below, may become extinct in a matter of years. Even if the remaining individuals survive, they are unlikely to produce many, if any, offspring.

Sumatran rhinoceros at safari

Figure1

Reproduction usually occurs when an organism reaches sexual maturity after a period of growth, during which organisms focus on their individual survival. This means prioritizing other life processes like obtaining food to supply enough energy for their growth. The duration of this growth period differs greatly between species. For example, while the Sumatran rhinoceros takes between 6 and 10 years to mature, a species called the turquoise killifish can take only 14–15 days to reach sexual maturity!

At a certain stage in life, all organisms will reach maturity. At this point in many organisms, the energy they previously directed toward growth and development is now directed toward a new goal: maintaining species survival. The way they achieve this is through reproduction.

Example 1: Identifying the Least Essential Life Process for Individual Survival

Which feature of living organisms is the least important for individual survival?

  1. Nutrition
  2. Sensitivity
  3. Reproduction
  4. Respiration
  5. Excretion

Answer

There are several life processes that all living organisms carry out, essential to their survival.

In order for an individual to survive, they must be sensitive to the environment surrounding them and to changes within their bodies so that they can respond appropriately. Individuals must also obtain nutrition to build up their bodies and gain molecules that can be metabolized to release energy in cellular respiration. Individual organisms must also be able to excrete waste products, so that wastes that might be toxic do not build up within their body causing damage and possibly death.

Reproduction is another process common to all living species, but it is not essential to an individual’s survival as failure to reproduce does not usually change the life expectancy of an individual. In fact, in some species such as the octopus, reproduction can actually lower an individual’s life span! If all the individuals in a species or population fail to reproduce however, the species can become extinct. As a result, reproduction is a vital life process on a population level.

Of the options provided in the question, we can see that nutrition, sensitivity, respiration, and excretion are all vital to the survival of an individual living organism. Reproduction, though vital for species survival, is the least important to ensure an individual’s survival as it usually does not affect their life span.

Therefore, the feature of living organisms that is the least important for individual survival is reproduction.

There is massive diversity in the ways in which different species reproduce. In this explainer, we are going to focus on how different species may follow one of two major reproductive strategies. A reproductive strategy refers to how an organism mates and also how it raises any offspring that are produced.

Key Term: Reproductive Strategy

The reproductive strategy of an organism refers to the way in which it mates and/or raises offspring.

An organism’s reproductive capacity is the maximum number of offspring it can produce in optimum conditions. Conditions are rarely ideal however, so an organism’s reproductive capacity is seldom reached.

Key Term: Reproductive Capacity

The reproductive capacity of a species is the largest number of offspring it is able to produce via reproduction under optimum conditions.

The time an organism takes to mature, its overall life span, the number of individuals in the population, and how easily they can access each other will all affect an individual’s reproductive capacity. An organism’s life span is the period of time between their birth and death. Generally, organisms with a longer growth period to reach maturity will also have a longer life span.

Definition: Life Span

An organism’s life span is the period of time between its birth and its death.

These factors, in addition to the prevalence of risks like predation and other environmental pressures, may affect an organism’s reproductive strategy. Let’s look at some examples of different species to see why these fascinating reproductive adaptations might have evolved.

Two major groupings of reproductive strategies are represented at each end of the scale in Figure 2 below. At one end of the scale are species with typically long life spans. These long-lived species invest heavily in the quality of their offspring over quantity. At the other end of the scale are typically short-lived species that invest in high quantities of offspring, sacrificing their quality.

Figure 2 shows us a scale of reproductive strategies. At one extreme are the organisms that tend toward “quantity over quality” like the frog. At the other end of the scale are the organisms that tend toward “quality over quantity” like the chimpanzee, or even humans! We can see the typical number of offspring that each individual in the four species shown will produce in a single successful fertilization event. We can also see the average life span in the wild of an individual belonging to each species.

You can see from Figure 2 that generally, as the average life span of an individual increases, the number of offspring they tend to produce decreases, so there is an inverse relationship between these two factors.

All organisms only have a certain amount of energy that they can afford to put into developing and caring for their offspring. They have two options toward which they can lean to expend this limited energy. They may expend their energy on producing small numbers of large, high-quality offspring like the chimpanzee. Alternatively, they may spend it on producing huge numbers of smaller, low-quality offspring per successful fertilization, each with less energy investment like the frog.

Let’s see how these two strategies work in more detail and examine some more exceptions to the rule, starting with the group with a short life span.

Aquatic organisms show an incredible diversity of reproductive strategies. Water is a harsh and challenging environment to live in: resources such as food are rare, it is difficult to hide from predators, and strong currents are able to move resources and organisms vast distances away from each other. Therefore, many species that live in aquatic environments are short lived.

These conditions also mean that short-lived aquatic organisms tend to be similar in producing very large numbers of offspring. Each individual that reproduces usually has a very high reproductive capacity, as so few of their offspring are likely to survive in aquatic environments. The number of organisms in a species, or any other defined population, that die over a certain period of time is referred to as this species’s mortality rate.

Key Term: Mortality Rate

Mortality rate is the number of deaths in a defined population per unit time.

The high mortality rate in aquatic organisms’ offspring might be due to failure to find food, an appropriate place in which they can develop, or the fact that predators often seek out breeding areas and consume so many offspring. If an individual were to produce just a few offspring, none of them would be likely to survive. Therefore, they usually produce vast quantities of very small offspring, bargaining on the chance that a few of them will be successful and survive until maturity, but limiting the energy spent on the many that will not.

For example, the female sea urchin can release up to 500 million eggs in her lifetime! Sea urchins, and many other aquatic species, breed through a process called spawning, during which there is a mass release of egg cells and sperm cells. Group spawning is when many individuals spawn at once. This can reduce the risk of each organism having its offspring consumed by a predator. It also increases the chance of successful fertilization. Some sea urchins synchronize this spawning using cues from the moon, so thousands of urchins will spawn at once! Just one male sea urchin can release a billion sperm during spawning. You can see a male sea urchin releasing sperm, the white cloudy substance, in the picture below.

The rare sight of a sea urchin spawning at night

Figure3

Some terrestrial species, those which primarily live on land, show similar reproductive strategies to aquatic organisms. For example, weeds, rodents, and insects all produce large quantities of offspring with little investment into each individual.

Parasites are organisms that live on or in a host organism and get their food from or at the expense of its host. For example, fleas and human body lice feed on the blood of their host to obtain their nutrition. Most parasites also produce a lot of offspring as they have such high mortality rates due to this lifestyle and need to adapt quickly to coevolve with their host.

These species not only develop quickly but also generally have shorter overall life spans. The mayfly, which you can see swarming to mate in the image below, is an extreme example. An individual mayfly typically lives for only a single day, but can produce up to 1‎ ‎000 offspring in its short life.

Mayfly Swarming at Night

Figure4

Typically, species that produce large quantities of offspring do not invest much parental care into raising their offspring as there are simply too many of them.

Example 2: Describing the Importance of Reproduction

Which statement about reproduction in organisms is true?

  1. An organism cannot survive without reproductive organs.
  2. A failure of one individual to reproduce will lead to the extinction of the whole species.
  3. Reproduction is essential for the continuity of a species.
  4. Terrestrial organisms generally produce more offspring than aquatic organisms.

Answer

Reproduction is a process by which an organism produces offspring. Reproduction is common to all living species but can take on a range of vastly different forms depending on the species and its environmental pressures.

In spite of its commonality across living organisms, an individual organism does not need to reproduce in order to survive. In fact, whether or not they reproduce usually has little to no effect on the duration of an individual’s life span.

If one individual fails to or chooses not to reproduce, it is unlikely to impact the species as a whole, as the majority of other individuals can successfully produce offspring to continue the species.

If most, or all, individuals in a species fail to reproduce however, that species can become extinct. Therefore, though it is not essential for an individual’s survival, reproduction is essential for the continuity of a species.

Let’s review our answer choices to see which is true.

As we have discussed, an individual organism does not need to reproduce to survive. This means that individuals can usually survive without their reproductive organs, as their primary purpose is to aid reproduction, so the statement that says otherwise is false.

We also know that the failure in reproduction of one individual in a population will not result in extinction, as there will be plenty of other organisms who do produce offspring to continue the species. The statement that says otherwise is therefore also false.

We know, however, that if many or all organisms in a species fail to reproduce, it can cause species extinction. Therefore, the statement saying that reproduction is essential for the continuity of the species is true.

While there are several species that are exceptions, in general, it is more likely that an aquatic organism that lives in water will produce a lot of offspring than a terrestrial organism that lives mainly on land. This is due to the harsh nature of most aquatic environments: scarce in resources, rich in predators, and vast in size. As each individual will find it harder to survive, many aquatic species resolve this issue by producing large numbers of offspring, bargaining that a few of them will survive against the challenging odds. The statement claiming that terrestrial organisms generally produce more offspring than aquatic organisms is also false, as though there are some species of terrestrial organisms that follow this reproductive strategy, it is far more common in aquatic environments.

Therefore, the true statement is that reproduction is essential for the continuity of a species.

Let’s take a look at some examples of organisms that take the opposite approach in their reproductive strategy by producing a few, high-quality offspring.

These organisms, such as elephants and primates, tend to have longer life spans and also take longer to reach maturity. Unlike species with a short life span that produce vast quantities of offspring, species with a long life span also tend to invest a lot of time and energy into raising their offspring.

Parental care increases the chance of their offspring’s survival until maturity, at which point the offspring themselves can reproduce. In species that only produce a few offspring per individual, a parent usually tries to ensure that at least one will survive to maturity, so the offspring tend to have lower mortality rates than those in species that produce large numbers of offspring.

You can see an example of an African elephant mother caring for her calf in the image below. She works hard to provide her offspring with milk and protects it from predators. She also teaches it how to protect itself and find food and water by the time it reaches maturity between the ages of 10–11 years. Many female elephants even remain with their mothers for their entire life, forming a cohesive maternal herd.

African Elephant (Loxodonta africana)

Figure5

Not all species that are long lived will follow these trends however.

Tortoises are some of the longest-living terrestrial species on Earth. Depending on the species of tortoise, a single female can produce between 2–30 eggs multiple times in one year. Over a lifetime, some tortoises can therefore produce many offspring.

Three giant Española tortoises were the only males of the 15 remaining individuals in their subspecies, one of whom, Diego, was around 100 years old. Diego, who you can see in the image below, helped avoid the extinction of this subspecies by fathering more than 800 offspring! In spite of living for a very long time and having a slow maturation time of around 25 years, tortoises show no parental care toward their offspring, which can be vast in numbers and have a fairly low mortality rate.

the Espanola tortoises

Figure6

Example 3: Identifying the Features of Different Reproductive Strategies

Which statement about reproduction is not true?

  1. Animals with long life spans usually reproduce more than once in their lifetime.
  2. Organisms with short life spans usually produce large numbers of offspring.
  3. Mammals decrease the mortality rate of their offspring through parental care.
  4. Organisms with long life spans are usually larger in size than those with short life spans.
  5. Insects usually delay their reproduction and invest a lot of energy in producing every single progeny.

Answer

The reproductive strategy of an organism refers to the way in which it mates, and if, and how, it expends energy into raising its offspring. The question is asking us to determine which of the features is not typical of the patterns that we observe in different reproductive strategies.

There are two main groups of reproductive strategies that form a scale. At one end of the scale are species that prioritize producing a large number of offspring and compromise by each individual offspring’s size being smaller. At the other end are species that tend to produce fewer, larger offspring. There are, of course, species and individuals that are exceptions to these rules, but in general, these two ends of the scale tend to display different features.

Organisms that produce many small offspring tend to live in harsh environments, such as in water, where few of their offspring are likely to survive and they therefore have high mortality rates.

Insects are another example of species with high mortality rates that produce a lot of small offspring with very little energy investment into each individual offspring. They invest less energy into producing each individual offspring, as only a few are likely to survive. The parents also rarely exhibit any care toward their offspring once they have been fertilized, as there are simply too many to keep track of and care for. It is in the best interest of the parents in these sorts of species to protect themselves so they can reproduce again in the future, rather than expending their energy on raising young that may fall prey to environmental pressures regardless.

Organisms that strategize in this way often have shorter life spans and reach maturity faster. This is also due to usually challenging environmental pressures, such as high levels of predation. Reaching maturity faster means that there is a shorter period of growth during which the individuals are vulnerable, and they are able to reproduce in the same method of their parents more quickly. These organisms also tend to reach a smaller overall size once they reach maturity.

The organisms that can be grouped at the other end of the scale, for example, mammals such as humans, are much larger, live for longer, and produce larger offspring. As each individual offspring tends to be bigger, the parents cannot produce as many offspring in each successful fertilization. Instead, the parents tend to invest more energy into raising each individual through parental care. This reduces the mortality rate of these species, so at least one of the few offspring produced will be likely to reach sexual maturity themselves. These species may reproduce more than once during their lifetime but will still produce far fewer offspring than organisms that adopt a different reproductive strategy.

Using this information, we can deduce that organisms with a long life span such as mammals can reproduce more than once and produce a few large offspring to whom they show parental care. We have also discovered that organisms with shorter life spans such as insects produce large numbers of small offspring with little energy investment into each individual.

Therefore, the statement that is incorrect is that insects usually delay their reproduction and invest a lot of energy in producing every single progeny.

Let’s summarize the typical characteristics of these long-lived and short-lived organisms in Table 1 below.

Example 4: Identifying the Species with the Highest Offspring Mortality Rate

Which of these animal species (A to E) is most likely to produce offspring with a high mortality rate?

Answer

Mortality rate is the number of deaths in a certain area, in a period of time, or from a particular cause. The term can be used to refer to the likelihood that an individual’s offspring will not survive to adulthood to then reproduce itself.

Typically, species that produce offspring with high mortality rates have short life spans, show early maturity, and produce a large number of small, low-quality offspring. If many of the offspring are unlikely to survive until maturity, it benefits a species to reduce the time it takes for maturity to be reached. It also benefits them to produce lots of offspring into which little energy has been invested in the hopes that a few of them will survive, but less energy is wasted on the many that will not.

Aquatic species, those which primarily live in and lay their eggs in water such as the frogs labeled D in the image, are most likely to follow a reproductive strategy like this. Water is full of predators, and eggs can easily be washed away or trampled. Therefore, frogs produce a lot of eggs, some of which develop into fast swimming tadpoles, and even fewer of which will eventually develop into adults that are able to exit water to escape some sources of danger.

Terrestrial species, those which primarily live on land such as the horse (A) and the sheep (E) in the image, are more likely to follow a reproductive strategy aiming to invest a lot of energy into producing and caring for just a few, larger and higher-quality offspring. These offspring can have a longer growth period and later maturity as their parents are helping their survival in their early years, keeping offspring mortality rates low.

There are, of course, exceptions to these rules. Ducks, labeled in the image with a B, are aquatic organisms, as are whales labeled with a C. Both ducks and whales produce just a few offspring, and the parents in these species tend to expend a fairly large amount of energy into producing their large offspring and caring for them after they are born. This helps to keep the mortality rate of the offspring they produce low.

Therefore, the animal species that is most likely to produce offspring with a high mortality rate is the frog, D.

Let’s recap some of the key points we have covered in this explainer.

Key Points

  • Reproduction is a key life process carried out by all living organisms that ensures species survival at a population level.
  • Failure to reproduce can result in extinction, which can be reflected in the fossil record.
  • Reproduction usually follows a period of growth, though the duration of this growth period differs between species.
  • A reproductive strategy describes how organisms mate and care for their young. There are two main reproductive strategies that form a scale from quality over quantity and quantity over quality.
  • Species that show preference to quality over quantity tend to be long lived and produce a small number of large, high-quality offspring to whom they tend to show parental care.
  • Species that show preference to quantity over quality tend to be short lived and produce a large number of small, low-quality offspring to whom they tend to show no parental care.

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