Video Transcript
In this video, we will learn how to outline the significance of reproduction to organisms and when it occurs in a life cycle. We will describe two different reproductive strategies with plenty of relevant examples in different living organisms. These reproductive strategies range from organisms with shorter lifespans which tend to produce a large number of small offspring, like this mouse, to organisms which live for longer and invest a lot of energy into producing a few high- quality offspring, like us humans.
Every living organism, whether it is a tiny bacterium or a massive blue whale, carry out several essential life processes that are common to all living things. We can remember these processes with a mnemonic such as MRS GREN. M stands for movement, describing how all living things move, whether quickly like a cheetah or incredibly slowly like a plant. The first R stands for respiration, showing how organisms release energy. The S shows that all living things are sensitive to their surroundings. The G stands for growth, the second R for reproduction, the E for excretion, describing how all living things must remove waste from theirselves, and the N for nutrition, describing how all organisms must obtain some kind of nutrition in order to survive.
While most of these processes are absolutely vital to the survival of living organisms, not all individuals will reproduce. Reproduction is the process by which an organism passes on their genetic material to another generation by having offspring. Offspring, sometimes known collectively as an organism’s progeny or their children, are the young that are born of living organisms. But if reproduction is not essential to an individual’s survival, why do organisms need to reproduce at all? Reproduction functions to ensure the continuity of a species at the population level, as a species will become extinct if all of its members die before they can reproduce.
Let’s see what this means with an example. This is a Sumatran rhinoceros. In 2020, they were believed to be only 80 individuals of these rhinos remaining on Earth, which put them at a very high risk of extinction. Most scientists believe that the fragmentation or splitting apart of the rhinos’ habitats for purposes such as building roads means that the remaining rhinos are unlikely to meet a mate and therefore very unlikely to produce many, if any, offspring. If these few remaining members of the species do not reproduce, the Sumatran rhino could become extinct in just a few years.
Now that we know why reproduction is so important to species survival, let’s discover more about the process itself. One question we need to answer is, when does reproduction actually occur? The simple answer is after a period of growth, following which an organism will reach sexual maturity. During this growth period, the most important thing to this little organism is its own survival. It needs sufficient food to supply the energy needed for this growth. So it must prioritize learning how to obtain this food. It might also need to learn how to avoid predators during this time period.
The duration of this growth period differs greatly between different species. For example, this emperor penguin chick may take between three to eight years to reach sexual maturity, while this African turquoise killifish can reach sexual maturity in a mere 14 days. Generally, the longer an organism takes to reach sexual maturity, the longer its lifespan will be.
A lifespan is a period of time between an organism’s birth and its death. While the average lifespan of an emperor penguin is 20 years, the average lifespan of this fast maturing killifish is only four to nine months. When an organism reaches sexual maturity, they tend to redirect a significant portion of the energy that they previously invested into growth into a new purpose, reproduction. Whether they are aware of it or not, this reproduction functions to pass on their genes and maintain their species survival.
There is a huge diversity in how different species reproduce, which is known as their reproductive strategy. The reproductive strategy of an organism includes how it mates, the total number of offspring produced or quantity, and the size of each individual offspring, which is sometimes referred to as offspring quality, and how much energy a parent invests into raising its offspring during their growth period. The maximum number of offspring that an organism can produce in optimum conditions is known as its reproductive capacity.
An organism’s lifespan, time to maturity, and the number of individuals in its population and how easily they can access each other will all affect an organism’s reproductive capacity. These factors, the risk of predation, and other external environmental pressures may also affect their reproductive capacity and, therefore, the reproductive strategy that they employ. Let’s look at some examples of different organisms’ reproductive strategies so we can understand this better.
All organisms have a limited amount of energy that they’re able to invest in their offspring. And they can either expend this energy into producing few large offspring, sometimes termed the quality over quantity approach, or invest smaller portions of energy into many offspring, which will be comparatively smaller in size, sometimes termed the quantity over quality approach.
We can visualize these strategies on a scale from organisms with typically shorter lifespans which produce a lot of small offspring to typically longer-lived organisms that produce just a few large offspring. For example, frogs, which tend to produce between 500 and 2000 offspring per successful fertilization, usually have a fairly short lifespan of around six years. Chimpanzees have an average of one child per successful fertilization, much like humans, and have a much longer lifespan than the frog of around 33 years.
Comparatively to an adult’s body size, frogs also produce tiny eggs that have very little parental care invested into them once they have hatched, while chimpanzee young are fairly large compared to adults but are relatively helpless, so require a large amount of parental care and investment into raising them or they will die before reaching maturity. Let’s explore why certain species might adopt these different strategies, starting with the quantity over quality extreme of the scale. Frogs are amphibians, which means they live partly in water and partly on land. Organisms that solely live in water, otherwise known as aquatic organisms, tend to exhibit this quantity-over-quality reproductive strategy even more strongly.
Let’s keep a checklist of what we already know about the typical features of organisms which tend to follow this reproductive strategy so we can add to it as we learn more. Water is a harsh and challenging environment to live in. Resources such as food are rare. It’s difficult to hide from predators, and strong currents are able to move resources and organisms vast distances away from each other. Therefore, many organisms that live in aquatic environments are very short-lived. These conditions and the short lifespans of many aquatic organisms means that they must produce large numbers of offspring as so few of them are likely to survive. The number of organisms in a species which dies over a period of time is known as its mortality rate, and this tends to be very high in these sorts of species.
If an organism that belongs to a species with a high mortality rate produced just a few offspring, none of them would be likely to survive. But by having a high reproductive capacity, this increases the likelihood that at least a few of them will survive to sexual maturity. As these organisms only have a limited amount of energy that they can spend on producing these many offspring, they must all be fairly small and grow quickly to reproduce themselves before their death.
Parasites are another example of species which exhibit these extreme reproductive strategies. You may recall that a parasite must live on or in another host organism and gain their food at their host’s expense. For example, fleas feed on the blood of host animals such as dogs. This provides a meal for the flea but an annoying itch and even possibly disease for the poor host. Due to this lifestyle, parasites have a high mortality rate as the host will do whatever it can to kill the parasite. Most fleas therefore produce many offspring to counter their high mortality rates and ensure the survival of their species. Within just three weeks, a single flea can infest the dog with more than 1000 bloodsucking offspring. And they have simply too many offspring to be able to afford any sort of energy investment into parental care.
Let’s take a look at some examples of organisms which take the opposite approach in their reproductive strategy by producing a few large, high-quality offspring. Many organisms with long lifespans, such as the African elephant pictured here, tend to adopt the strategy of producing a few large and high-quality offspring. Their offspring take along 10 to 11 years to reach sexual maturity. So let’s get our list of features back and add this to it. During this growth period, they receive a lot of parental care so that the parent can ensure that at least one of their offspring will survive until they are able to reproduce themselves.
The mother will feed her calf milk and teach it how to find safe food and water while protecting it from predators. African elephant offspring therefore tend to have a fairly low mortality rate. And they follow the trend that longer lifespan organisms produce few large and high-quality offspring with a lengthy average lifespan of 60 to 70 years. It is important to remember that as with many trends that we observe in reproductive biology, there are exceptions to these patterns. An example of a long-lived organism which instead favors quantity over quality is the giant Espanola tortoise of the Galapagos Islands.
Let’s keep a checklist of the tortoise’s reproductive features so we can see where these differences lie. Tortoises are some of the longest-living land-dwelling organisms on our planet, with an average lifespan of over 100 years. They also have quite a long growth period, only reaching sexual maturity around the age of 17 years. The offspring they produce are fairly small relative to an adult tortoise. And in spite of the trends we described earlier, these long-lived organisms can produce between four to 10 eggs every single year.
So over their long lifetime, a single Espanola tortoise can have many, many offspring. The tortoises tend to show no parental care whatsoever to their offspring. But even so, the tortoises have a fairly low mortality rate as they have no natural predators. The reproductive features in orange are typical of long-lived organisms, and those in pink show where the tortoise differs from these trends.
Let’s review how much we’ve learned about the typical reproductive strategies of different organisms by having a go at a practice question.
What features and reproductive strategies are common in species that have a short lifespan? Select two answers. Large numbers of offspring, large size, parental care, early maturity, or high chance of offspring survival.
Let’s define some of the key terms in the question before we identify the characteristic features of short-lived organisms. 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. There are two main groups of reproductive strategies which form a scale from prioritizing quantity of offspring over their quality at one end, which tends to be exhibited by short-lived species, and prioritizing quality of offspring over their quantity at the other end in typically longer-lived species. There are of course species and individuals which are exceptions to these rules. But in general, these two ends of the scale tend to display different features.
Let’s explore some of the typical features of the short-lifespan species as this is the group the question concerns. Organisms with short lifespans tend to live in harsh environments such as aquatic organisms, like this fish that lives in water. Fish offspring have high mortality rates, which means that very few of them are likely to survive. To try and resolve this issue, these type of organisms tend to produce a large number of offspring. But as they only have a limited amount of energy available to expend into producing these offspring, each individual offspring tends to be very small. They invest very little energy into producing and caring for each individual offspring as only a few are likely to survive, and there are simply too many to look after.
Organisms with shorter lifespans tend to reach maturity at an earlier age. This is also partly due to the usually challenging environmental pressures, such as high levels of predation. Reaching maturity faster means that there’s a shorter period of growth during which the individuals are vulnerable before they are able to reproduce themselves. Therefore, out of the options we’re provided with by the question, the features and reproductive strategies common in species that have a short lifespan are early maturity and large numbers of offspring.
Let’s review what we’ve learned about reproduction as a life process by reviewing the key points that we’ve addressed in this video. The purpose of reproduction, a trait common to all living species, is to ensure species survival, and a failure to reproduce can result in extinction. 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 typically long-lived organisms, such as elephants and primates, to shorter-lived species, such as fish and insects.
Long-lived organisms tend to have few offspring, while shorter-lived organisms tend to have many. The typical size of the offspring produced by long-lived organisms is large while from short-lived organisms is fairly small. Long-lived organisms tend to take a long time to mature, while shorter-lived organisms have a very short growth period and mature early. And longer-lived organisms also tend to show more parental care to the few larger offspring that they produce to ensure the survival of at least one until they themselves reach sexual maturity. Shorter-lived organisms simply produce too many offspring to be able to provide them with parental care.
