The life cycle of mammals (including humans), birds and some other organisms is relatively simple. An individual is born or hatched, feeds and grows up, often disperses to a new living space, reproduces, and eventually dies. Young individuals are similar to older ones, albeit with changes in proportions. This is known as direct development.

In sharp contrast are organisms that have complex life cycles — going through drastic developmental changes in their lifetimes. Complex life cycles divide up the tasks of living among two or more morphologically very different stages, so these organisms effectively lead two or more different lives.

Here are a few examples from the many that exist in the animal kingdom.

Think of caterpillars that turn into butterflies, for example. There is no resemblance between a caterpillar and the butterfly it becomes, yet they are the same individual. A caterpillar eats and grows; at some point it shuts itself up in a chrysalis and undergoes a complete reorganization of body structure — a metamorphosis. The butterfly emerges, may (or sometimes may not, depending on the species) feed itself, moves around from place to place, finds a mate and starts the next generation. The life-tasks of the caterpillar are to eat and grow; those of the adult are to disperse and reproduce. This division of labor, so to speak, requires two entirely different sets of adaptations for differing ecologies: different body plans and different behaviors.

Mayflies take this strategy to the extreme. A mayfly larva is aquatic, feeding and growing. It metamorphoses into a flying, aerial adult that does not feed or grow at all; the only role of the adult is dispersal and reproduction. They live only a few days, play the mating game, and die.

A familiar example of this sort of life cycle is seen in (most) frogs and toads, although with a less cataclysmic reorganization than in caterpillars. These animals, along with salamanders, are even called‘amphibian, which means “two lives.” Tadpoles are the juvenile form, typically aquatic, feeding and growing. At some point they lose their tadpole tails, grow four legs, change their digestive systems and diets, and become adults, ready to disperse from pond to pond, find a mate in a suitable pond, and reproduce.

In the eastern part of North America, there is a salamander that divides up the life-tasks into three stages.

The larva is aquatic, eating and growing. It transforms into a terrestrial form (called an “eft”) that eats, grows and disperses overland. The eft again transforms into an adult, returning to the water to find a mate and reproduce.

Barnacles, and many other marine invertebrates, do it differently; it is the larvae that disperse as they swim and drift in the plankton. Barnacles typically have two larval stages, and hence two metamorphoses between the three parts of the life cycle. From an egg comes a nauplius larva, initially having only a tail and a head (with antennae) but eventually adding a midsection with legs. A nauplius feeds and grows as it disperses; this stage changes to a cypris larva, enclosed in a light-weight hinged shell; a cypris does not feed at all but disperses to a settling site, where it settles down, glues its head to a rock or shell, and produces a hard shell — the barnacle that we see on the shorelines. The adult barnacle feeds (with its legs), grows and reproduces, but it is totally stuck in one spot for the rest of its life.

Why do complex life cycles exist? It’s a subject of much scholarly debate. On the one hand, having two (or more) different habitats or two different food types or two different ways to get around means that there are two distinct stages where something might go wrong, so if anything goes wrong with either one of them, the animal is finished. On the other hand, two different sets of adaptations may allow an animal to exploit rich but ephemeral resources (such as temporary pools for tadpoles, seasonally available plants for caterpillars) or fluctuating conditions for dispersal and reproduction. There seems to be no clear answer. Yet, clearly, it is a successful sort of life cycle, because so many animals have it. The advantages of the complexity have outweighed the possible disadvantages as these critters evolved. But somehow, for some reason, this kind of life cycle was apparently not suitable for mammals and birds.

If you find all of this is complicated, just contemplate the life cycles of many parasites, as well as many algae, mosses and ferns. These have the additional complication that generations alternate; individuals of one generation reproduce and are replaced by different individuals in the next. In the case of parasites, this pattern may be repeated for several generations, each with a different host where they feed and may get dispersed by that host, until individuals from the so-called final host disperse to the initial kind of host where the cycle starts over again.

• Mary F. Willson is a retired professor of ecology.