A female bear with her cubs: bears have direct-development life cycles, looking like bears from the time they are born. (Photo by Jos Bakker)

A female bear with her cubs: bears have direct-development life cycles, looking like bears from the time they are born. (Photo by Jos Bakker)

On the Trails: Animal life cycles

There are two basic life-cycle patterns among animals. Many animals have complex life cycles with two or more very distinct stages; metamorphosis is the usual term for the often-radical changes that occur between stages. Think, for instance, of caterpillars turning into butterflies or tadpoles becoming frogs. Contrast that with what’s called direct development: a puppy is clearly a dog for all its life, and a chick is obviously a bird, growing and changing gradually, without distinct stages. Like dogs and chickens, bears and humans have a direct-development life cycle.

For starters, consider some of the overwhelming variety of critters that have complex life cycles — enough variations to fill a huge catalog (and all with special names sufficient for a big dictionary—but I’ll leave those out). Here are some examples:

• Amphibian larvae (tadpoles) typically lose their tails and grow legs as they change to adult form.

• Fish larvae usually look like “typical” fish with some modifications, typically living off food stored in their yolk sac. They then change to juvenile forms, in some cases looking much like miniature adults, although their physiology may differ (e.g., salmon). But in some cases the juveniles look quite different. For example, in certain wrasses, the small juvenile has numerous frills on the fins that disguise its underlying fishy shape, and then all those frills are lost when becoming adult. And flatfish larvae change from a symmetrical fishy shape to an asymmetrical arrangement with both eyes on one side.

• Caterpillars (lepidopteran larvae) transition to an immobile pupa, in which the body is totally reorganized (digestive tract, brain, all of it) to form a winged adult.

• Dragonflies do a less total transformation, developing adult wings and legs inside the larval skin.

• Jellyfish have a little oval, short-lived planktonic stage that swims using minute hairs (cilia) on its surface. It then settles on a surface and develops into a polyp, which buds off the adult-form medusa.

• Most molluscs have one or two planktonic stages that feed and swim, using cilia or ciliated flaps. The final larval stage typically disperses and settles; then it develops into an adult, often with a shell (such as a clam). Some, however, have an additional, parasitic stage (attaching to fish), before they settle and mature.

• Crustaceans often pass through several free-living juvenile stages, molting between stages and having varied morphology, until the last one morphs into a small adult form (such as a crab or a barnacle) that just grows larger. For example, barnacles have a planktonic form that feeds and swims, using long bristle-like hairs. It then transforms, after several molts, into a different, non-feeding, multi-legged, swimming form that eventually settles onto a hard surface, gluing its head down and leaving the legs free for capturing food, as it creates a hard shell around the now-sedentary, adult body.

• Sea urchins have a planktonic stage with bilateral symmetry; this form settles onto a surface and develops five-sided radial symmetry in the familiar prickly juvenile-to-adult phase.

Barnacle larvae temporarily conceal their swimming legs in the shell; this non-feeding planktonic stage drifts and swims, opening its shell to fully expose the legs, and settles head-down on a hard surface, becoming adult. (Photo courtesy of NOAA)

Barnacle larvae temporarily conceal their swimming legs in the shell; this non-feeding planktonic stage drifts and swims, opening its shell to fully expose the legs, and settles head-down on a hard surface, becoming adult. (Photo courtesy of NOAA)

Complex life cycles dominate among insects (estimated to be more than 80% of the species) and other invertebrates. Clearly, that plan for living is highly successful. Some researchers suggest that the ‘invention’ of complex life cycles may even have led to the evolution of the huge diversity of insects. All this begs the question: what are the advantages of such life cycles?

Although physiologists have unraveled some of the intricate physiological orchestrations that govern the process of metamorphosis, the possible ecological advantages of having distinct life-history phases are less amenable to experimentation. They can be explored with ecological, mathematical models, but remain more a matter of conjecture. We certainly observe morphological, behavioral, and ecological differences between life-cycle stages, but are those differences the ones that led to the evolution of the distinct stages (and if so, how)?

Clearly, the different stages can play different roles; they may specialize in feeding, or dispersal, or settling, or (in what we call adults) reproduction. A feeding stage like a caterpillar may be efficient at feeding but have limited mobility, making it hard to find mates. So a mobile dispersal phase — such as a butterfly — could be more effective in reproduction.

In addition, perhaps one stage might have found a way to exploit some different, rich food source that allows faster growth, maybe achieving reproductive status more quickly. Also, if immature and mature stages have different diets or habitats, there is less competition between them, possibly enhancing survival. Different stages could be exposed to different predators or diseases. If the threats are stage-dependent, such species might escape the possibility of species-wide devastation, by differing in behavior or habitat. However, we are left with lots of conjecture and little concrete documentation.

Complex life cycles are very common, but what about the animals that have direct development? That, too, is a successful way of life for some species — we see it in the abundance and diversity of reptiles, birds, and mammals. Furthermore, within some taxonomic groups, certain lineages have lost one metamorphic stage although their close relatives retain them. For instance, although most amphibians have complex life cycles, some salamanders have given up the adult form and reproduce in their larval form. Females (but not males) of a few species of flies and beetles have no typical adult stage, but reproduce in larval form. Among the molluscs, pulmonate snails generally have direct development: an egg produces not a larva, but a mini-snail. In these critters, one body form accomplishes all the activities performed by multiple stages in complex life cycles. All of these animals are apparently successful too.

But what were the ecological and historical situations that led to the evolution of such different life histories? There is a lot to wonder about!

• Mary F. Willson is a retired professor of ecology. “On The Trails” appears every Wednesday in the Juneau Empire.

More in Neighbors

A rainbow spans the University of Alaska Southeast campus in September of 2024. (University of Alaska Southeast photo)
Sustainable Alaska: Reading relations

For the program’s 14th iteration, UAS’s One Campus, One Book committee selected… Continue reading

(Juneau Empire File)
Community calendar of upcoming events

This is a calendar updated daily of upcoming local events during the… Continue reading

(Photo provided by Gina Del Rosario)
Living and Growing: Holy Week

Filipinos are known all over the world for their strong faith in… Continue reading

The downtown Juneau cruise ship dock on a clear March day. (Mark Sabbatini / Juneau Empire file photo)
Living and Growing: Seeking joy during times of great uncertainty

“This is the greatest act of power I have come to know:… Continue reading

Mary’s extreme bars, ready to slice. (Photo by Patty Schied)
Cooking For Pleasure: Mary’s extreme bars

For at least 20 years, my sister Mary Watson has been making… Continue reading

Sabrina Donnellan and her family attend a community luncheon for federal employees at Shepherd of the Valley Lutheran Church on Saturday, March 8, 2025. (Jasz Garrett / Juneau Empire file photo)
Living and Growing: Choose empathy during these difficult times

“It is your concern when your neighbor’s wall is on fire.” —… Continue reading

On a nice day it’s always safe to talk about the weather. (City and Borough of Juneau photo)
Gimme A Smile: What to say when you’ve got nothing to say

It could happen, right? Despite your very best efforts, you could find… Continue reading

Braised carrots with garlic and thyme, freshly cooked. (Photo by Patty Schied)
Cooking For Pleasure: Braised carrots with garlic and thyme

When I was growing up, my parents never, ever served cooked carrots… Continue reading

A black bear sow and her cub walk along the Trail of Time at the Mendenhall Glacier Visitor Center. (Michael Penn / Juneau Empire file photo)
Living and Growing: The bear

The folks of Southeast Alaska are fortunate in that we sometimes experience… Continue reading

Laura Rorem is a member of The Evangelical Lutheran Church in America. (Courtesy photo)
Living and Growing: Practicing true patience

“Have patience, have patience, Don’t be in such a hurry, When you… Continue reading

Just-baked cinnamon rolls ready to serve. (Photo by Patty Schied)
Cooking For Pleasure: Easy cinnamon rolls

My father really loved cinnamon rolls. In his later years I would… Continue reading

The Rev. Tim Harrison is the senior pastor at Chapel by the Lake. (Courtesy photo)
Living and Growing: The numbers tell the story

I love numbers and math. One of my first career aspirations was… Continue reading