Lesson Explainer: Reproduction as a Life Process Biology • Third Year of Secondary School

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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

Organisms with a long life span tend to produce fewer offspring than those with a short life span, but parents invest more energy into each individual offspring so they are typically larger and require more parental care after birth.

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.