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Life history strategies

How organisms allocate energy to maximize the number of offspring they leave behind.

Summary

  • The life history of a species is the pattern of survival and reproduction events typical for a member of the species (essentially, its lifecycle).
  • Life history patterns evolve by natural selection, and they represent an "optimization" of tradeoffs between growth, survival, and reproduction.
  • One tradeoff is between number of offspring produced and the amount of energy (both physical resources and parental care) put into each offspring.
  • Timing of first reproduction is another tradeoff. Early reproduction lowers the chance of dying without offspring, but later reproduction may allow organisms to have more or healthier offspring or to provide better care.
  • Members of some species reproduce only once (semelparity), while members of other species can reproduce multiple times (iteroparity).

What is a "life history"?

What does your life history look like? In the world of ecology, that question doesn't refer to the many challenges and successes you've experienced, or to the friendships you've made along the way. (Not that those aren't good too!)
Instead, when we're talking about life history in ecology, we're thinking about basic demographic features of a population or species – the kind of things that would appear in a life table. That includes when organisms first reproduce, how many offspring they have in each round of reproduction, and how many times reproduction occurs. For humans, life history involves a late start to reproduction, few offspring, and the ability to reproduce multiple times.
We can define the life history of a species as its lifecycle, and in particular, the lifecycle features related to survival and reproductionstart superscript, 1, end superscript. Life history is shaped by natural selection and reflects how members of a species distribute their limited resources among growth, survival, and the production of offspring.

Life history strategies and natural selection

All living things need energy and nutrients to grow, maintain their bodies, and reproduce. In nature, these resources are in limited supply, and there is often competition for access to them (e.g., to sunlight and minerals for plants or food sources for animals). Thus, each organism will have non-infinite resources to divide among activities like growth, body maintenance, and reproduction.
What does it mean for an organism to allocate its limited resources "well" in this context? From an evolutionary standpoint, it means that the resources are distributed among the potential activities (growth, maintenance, reproduction) in a way that maximizes fitness, or the number of offspring the organism leaves in the next generation. Organisms with inherited traits that cause them to distribute their resources in a more effective way will tend to leave more offspring than organisms lacking these traits, causing the traits to increase in the population over generations by natural selectionstart superscript, 2, comma, 3, end superscript.
Over very long periods of time, this process results in species with life history strategies, or collections of life history traits (number of offspring, timing of reproduction, amount of parental care, etc.), that are well-adapted for their role and environment. The optimal life history strategy may be different for each species, depending on its traits, environment, and other constraintssquared.
In this article, we'll examine some tradeoffs in life history strategies and see examples of plants and animals that use strategies of different types.

Parental care and fecundity

One major tradeoff in life history strategies is between number of offspring and a parent's investment in the individual offspring. Basically, this is a "quantity versus quality" question: an organism can have many offspring that each represent a relatively small energy investment, or few offspring that each represent a relatively large energy investment.
To put this more formally, we can say that fecundity tends to be inversely related to the amount of energy invested per offspring. Fecundity is an organism's reproductive capacity (the number of offspring it's capable of producing). The higher the fecundity of an organism, the less energy it's likely to invest in each offspring, both in terms of direct resources – such as fuel reserves placed in an egg or seed – and in terms of parental care.
  • Organisms that produce large numbers of offspring tend to make a relatively small energy investment in each, and don't usually provide much parental care. The offspring are "on their own," and the idea is that enough are produced that some will survive (even if the odds for any one are low).
  • Organisms that make few offspring usually make a large energy investment in each offspring and often provide lots of parental care. These organisms are effectively "putting their eggs in one basket" (literally, in some cases!) and are heavily invested in the survival of each offspring.
As for so many cases in biology, these are general trends and not universal rules. The main point is just that when organisms have many offspring, they can't invest as much energy in any single offspring. When they have fewer, they can (and must) invest more energy to ensure those offspring's survival.

Example: Many offspring, low investment/parental care

A typical sea snail (whelk) produces hundreds of eggs at a pop, and these eggs hatch to yield baby snails that are pretty self-sufficient from the get-go. In fact, the baby snails in the first 10, percent of eggs that hatch will enthusiastically eat their slower-hatching siblings for breakfaststart superscript, 4, end superscript!
Image modified from "Egg cases - Common whelk," by Sarah Smith, CC BY-SA 2.0.
Cannibalism aside, this example is a good illustration of one common type of parental investment strategy. Sea snails and many other marine invertebrates provide little (if any) care to their offspring. Instead, they use most of their energy budget to make lots of offspring, each of which is relatively small. The sea snail isn't even that impressive when it comes to numbers—a female sea urchin might release 100, comma000, comma000 eggs in a single spawningstart superscript, 5, end superscript!
In species with this type of strategy, offspring are often self-sufficient at an early age. Still, since not much energy is invested in each offspring, they tend to be small and come into the world with low energy reserves. This makes the offspring vulnerable to predation, so many or most will not survive—instead, it's their sheer numbers that ensure the survival of the population.

Example: Few offspring, high investment/parental care

To see a strategy at the opposite end of the spectrum, let's consider the giant panda. Panda females typically have just one cub each time they reproduce, and the young cub is far from self-sufficientstart superscript, 6, end superscript. That pink thing in the picture below isn't a mouse or a kitten...it's actually a newborn panda!
Image credit: "Newborn panda cub - Chengdu Panda Base," by Buster&Bubby, CC BY-NC 2.0
Animal species like the panda, which have few offspring during each reproductive event, often give extensive parental care. They may also produce larger, more energetically "expensive" offspring. The newborn panda above may look tiny, but compared to a hatching sea snail, it's massive! Species with this type of high-investment strategy use much of their energy budget to care for their offspring, sometimes at the expense of their own health.
This type of strategy is common in mammals, including humans and kangaroos as well as pandas. The babies of these species are relatively helpless at birth and need to develop quite a bit before they become self-sufficient.
Image modified from "Ailuropoda melanoleuca," by Shealah Craighead (public domain).

Fecundity and investment tradeoffs in plants

The same broad patterns seen in animals also apply to plants. Of course, plants aren't going to provide parental care in the same way that animals do. However, they can still produce either large numbers of energetically "cheap" seeds or small numbers of energetically "expensive" seeds.
For example, plants with low fecundity, such as coconuts and chestnuts, produce small numbers of energy-rich seeds, each of which has a good chance of germinating into a new organism. Plants with high fecundity, such as orchids, take the opposite approach: they usually make many small, energy-poor seeds, each of which has a relatively low chance of surviving.

Timing of first reproduction (early vs. late)

When a species starts reproducing is another important part of its life history—and another place where we see trade-offs and lots of variation among species. Some types of plants and animals start reproducing early, while others delay much longer. What are the pros and cons of these strategies?
Organisms that reproduce early have less risk of leaving no offspring at all, but this may be at the expense of their growth or health. For example, small fish like guppies use their energy to reproduce early in life, but since they throw all their energy to reproduction, they don't reach the size that would give them defense against predators. (An intimidating guppy is kind of hard to picture!)
Organisms that reproduce at a later age often have greater fecundity or are better able to provide parental care. On the flip side, they run a greater risk of not surviving to reproductive age. For example, larger fish, like the bluegill or shark, use their energy to grow to a size that gives them more protection. As a consequence, they delay reproduction, so there's more chance that they will die before reproducing (or before they've reproduced to their maximum).
In general, age at first reproduction is linked to the lifespan of a speciesstart superscript, 7, end superscript. Short-lived species often start reproducing early, while long-lived species are more likely to delay reproduction. This is a good reminder that a life history strategy is an integrated "solution" to the problem of leaving as many offspring as possible, and that any one part (e.g., age of first reproduction) only makes sense in light of others (e.g., lifespan).

Single vs. multiple reproductive events

Another important characteristic of life history relates to how many times an organism reproduces over its lifetime. For some species, reproduction is a one-time, all-out event, and the organism doesn't survive much beyond that one event. In other species, opportunities for reproduction come around multiple times, or even many times, throughout the organism's lifetime.
To apply a little ecology vocab, we can split species into two groups:
  • Those that can reproduce only once (semelparity)
  • Those that can reproduce multiple times over their lifetime (iteroparity)

Semelparity

In semelparity, a member of a species reproduces only once during its lifetime and then dies. Species with this pattern use up most of their resource budget in a single reproductive event, sacrificing their health to the point that they do not survive.
Examples of species that display semelparity are bamboo, which flowers once and then dies, and the Chinook salmon, which uses most of its energy reserves to migrate from the ocean to its freshwater nesting area, where it reproduces and then dies.
Image credit: "Life histories and natural selection: Figure 1," by OpenStax College, Biology, CC BY 4.0. Modification of work by Roger Tabor, USFWS.

Iteroparity

In iteroparity, individuals of a species reproduce repeatedly during their lives. Iteroparity can take different forms, depending on the reproductive cycles of the organisms involved. Species that display iteroparity don't put all of their resources into a single reproductive event, as there's a fitness benefit (an opportunity to have more offspring) in surviving to reproduce more times.
Some animals are able to mate only once per year, but can survive through multiple mating seasons. The pronghorn antelope is an example of an animal that has a seasonal estrus cycle (“heat”). Estrus is a hormonally induced physiological condition that prepares the body for successful mating. Females of species with estrus cycles mate only during the estrus phase of the cycle.
Image credit: "Life histories and natural selection: Figure 1," by OpenStax College, Biology, CC BY 4.0. Modification of work by Mark Gocke, USDA.
A different pattern is observed in primates, including humans and chimpanzees, which may attempt reproduction at any time during their reproductive years. However, the menstrual cycles of the females make pregnancy likely only a few days per month during ovulation.
Image credit: "Life histories and natural selection: Figure 1," by OpenStax College, Biology, CC BY 4.0. Modification of work by “Shiny Things”/Flickr.

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