More Than Cute: Inside the Complex World of Animal Infancy

“There was a child went forth every day
And the first object he look’d upon, that object he became,
And that object became part of him for the day or a certain
part of the day,
Or for many years or stretching cycles of years.”
–Walt Whitman, Leaves of Grass
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Baby animals are undeniably cute: puppies tumbling over each other, joeys peeking out of kangaroo pouches, ducklings paddling in a wobbly line. Baby animals are also incredibly bizarre: Moth larvae mimic both snakes and feces, featherless finch chicks beg with beaks like Mondrian paintings—and let’s not forget baby humans, with our squishy skulls and taste buds on our tonsils.

Baby animals are very sensitive to the environment: Bird embryos die when their eggshells are thinned by DDT or smothered by spilled oil. Shellfish larvae can’t grow into edible adults until they find the perfect home in a sea full of increasingly imperfect habitat.

But baby animals are also powerful enough to change the environment, for both good and ill from the human perspective. Agricultural pests like rootworms and borers are actually infant insects, and they devastate crops on every continent. At the same time, numerous beetle larvae can actually digest plastic, offering hope for pollution cleanup.

Even human babies embody this combination of vulnerability and voracity. When we’re born, we don’t know a single language, leaving us at the mercy of our adult caretakers. Yet we are so ravenous to learn that we can pick up as many languages as we are exposed to, an ability the adults around us have lost. Similarly, as babies, our incomplete immune systems put us at risk from infections that rarely affect adults.

But these same baby immune systems are primed to build relationships with beneficial bacteria that will serve us the rest of our lives. If we’re exposed to too many dangers or deprived of necessary resources, our sensitivity becomes a weakness, but in the right environment, it blooms into a superpower.

Baby animals are also powerful enough to change the environment, for both good and ill from the human perspective.

Scientists who study early life stages, by dripping chemicals on tadpoles or feeding chicks experimental diets or injecting genes into fly eggs, are known as developmental biologists. Developmental biology is the study of how animals build their bodies—an examination of every process between fertilization and maturity. As a field of research, it has progressed through its own fascinating and sometimes turbulent developmental stages.

In the nineteenth century, it was called embryology, and its practitioners peered through microscopes to watch an egg cell cleave into two, four, eight, and many more cells, then eventually grow a gut and a brain. The work of embryologists expanded to produce and bud off the entire field of genetics in the twentieth century, and developmental biology is now expanding again to link the microscopic stages of animal growth with global environment and ecology.

Unexpected connections are the forte of developmental biology. “Adult-onset” diseases like cancer and diabetes are increasingly understood to result from influences in early life, even as early as the womb. Not only humans but many other animals face challenges to health, longevity, and survival that trace their roots to chemical exposure or resource limitation in babyhood. And although these challenges highlight the vulnerability of youth, they also illuminate its adaptability.

When a developing baby doesn’t get enough nutrition, it can preferentially devote its limited energy to growing critical organs like the heart and brain, leaving more redundant organs like kidneys to suffer the brunt of starvation. This postpones problematic symptoms until later in life, giving the animal a chance to reproduce first. Thus, surviving long enough to grow up and manifest kidney disease is a triumph of the flexibility of early development.

At no other stage in our lives are animals more capable of perceiving and responding to changes in the environment. In intimate conversation with our inanimate surroundings as well as with our fellow cohabitors of Earth, we mold our bodies to match our world. Within this capacity for change lies the future of life as we know it.

The diversity of developmental forms
When we think of the world’s diverse animal life, we usually think of adult animals: frogs and butterflies, jellyfish and echidnas. Not tadpoles and caterpillars, ephyra and puggles. (Believe it or not, those baby names match the parents that precede them.) Even when we turn our minds to baby animals, we tend to forget that they don’t always look like their parents. This leads to the occasional amusing contradiction in children’s books, like a “daddy caterpillar” or a “baby bee.” In reality, daddy caterpillars are moths or butterflies, and baby bees are wingless white larvae.

What is a larva? (Besides the singular form of larvae.) The word describes a baby that goes through a distinct metamorphosis to become an adult. Most animals have one or more larval stages. Because larvae can look so different from adults, this creates a constant puzzle for biologists, who must piece disparate forms into a single life cycle. Some species’ larvae have never been seen. Other larvae are not yet associated with an adult.

Larvae don’t have to be tiny, nor are all tiny babies necessarily larval forms. Bluefin tuna and kangaroos, both of which can grow to well over 3 feet (1 m) as adults, produce babies about an inch (2.5 cm) long. The tuna hatchling is a larva, the kangaroo joey is not. On the other end of the scale are the chick of a kiwi (not a larva) and the maggot of a tsetse fly (a larva), both born nearly the same size as their parents.

This size variation arises because animals can’t invest infinite energy in producing offspring. There’s a trade-off between size and number. Kiwis lay one gigantic egg at a time, while tuna spawn millions of mini eggs. In both cases, the babies that hatch from the eggs are independent, striking out on their own. Kiwi parents allocate their reproductive effort to building mass, producing a baby large enough to have a good chance of solo survival. Tuna parents allocate theirs to quantity, producing enough babies that it doesn’t matter if only a few survive.

Kangaroos follow a third route, pouring their resources into parental care. Although each baby is minuscule, it receives the warm protection of a pouch and a constant infusion of milk for up to a year after birth. Thus, although a newborn kangaroo is as tiny as a newborn tuna, an independent kangaroo is far closer in size to an adult.

Whether a baby turns to its parents or to the wild world to supply its needs, it is exquisitely well-equipped to do so. Kangaroo joeys have tough arms for climbing from the birth canal to the pouch. Larval tuna have (relatively) enormous jaws for swallowing prey nearly as big as they are. Larval parasites may be some of the most specialized babies of all, built to create links between disparate forms of life.

As the tiny seeds of mighty trees are adapted to find a new home by hooking onto an animal carrier or hitching a ride on the wind, so baby tapeworms help themselves travel to a new host. A tiny “worm seed” can infect a human through the consumption of undercooked pork, as cleverly described by the embryologist-poet Walter Garstang:

He’s very small, a mere pin’s head, beset with six small hooklets,
Is whirled about by wind and rain through puddles, fields and
brooklets;
But if a pig should swallow him, as many porkers do,
He’s made a start with no mistake: he’s on the road to you!

A tapeworm baby needs to be eaten in order to complete its life cycle, a typical parasite feature. As it passes from host to host, a single parasite can infect a wide range of animals in a wide range of habitats, connecting snails to birds and wetlands to forests. Similar connections are made even by animal babies that die when they’re eaten. Young life-forms are easier prey than their parents, accessible to a greater range of predators. Many hungry animals depend for their meals on the profusion of progeny produced by their fellows.

The hidden abundance of youth
For many species, babies comprise the majority of their life cycle. Most animals on Earth are, in fact, babies.

It might be easiest to understand this as an ephemeral springtime truth, since we’re used to seeing one duck parent trailed by a dozen fuzzy offspring. The children’s classic Make Way for Ducklings contains four times as many babies as adults. In March, a square meter of water (about 11 sq ft) in a North Carolina pond can hold fifteen thousand tadpoles, the product of breeding by only a few hundred adults.

In both cases, these babies grow to adulthood in a matter of weeks, and for the rest of the year, no ducklings or tadpoles can be found. So we might conclude that only during certain limited times are adults outnumbered by babies. However, as climate change brings spring weather earlier and earlier, the breeding season of many species is extended, and so is the period of time during which babies rule the roost.

What’s more, other animals often linger much longer in childhood and youth. Salmon are born in fresh water and live in streams for up to two years as fry before they mature and move out to the ocean. Many types of salmon spend no more time as seafaring adults than they did as river-dwelling babies. Given the natural population attrition over time, as salmon are eaten by predators or succumb to parasites, the total number of fry is typically greater than that of adults at any time of year.

Our developmental biology illuminates our kinship with the rest of the kingdom.

Although salmon babies stick to rivers, the ocean serves as a giant nursery for uncounted other species. Surface waters froth with billions of baby fish, squid, crabs, and more, and it seems like each new expedition to the deep sea uncovers another astonishing cradle of life.

In 2021, in Antarctica’s Weddell Sea, cameras towed at hundreds of meters’ depth revealed an icefish breeding colony of 93 square miles (240 sq km) filled with an estimated 60 million nests. The average number of eggs per nest was 1,735, making the total number of babies in this nursery well over a hundred billion. This previously unknown jackpot of baby fish revised our whole understanding of the Antarctic ecosystem.

As for humans? Currently, about 22 percent of the world’s Homo sapiens population is under the age of eighteen—likely the lowest this percentage has ever been. Less than a century ago, it was 31 percent. The relative proportion of children and teens varies geographically, with only 17 percent of the Japanese population under the age of twenty, while 60 percent of the Nigerien population falls into this category. On average, adult humans outnumber children, but in many places—from the country of Niger to any schoolyard—the reverse is true.

Including humans with other animals can be a touchy subject. Merriam-Webster offers multiple definitions for “animal”: first, any of a kingdom (Animalia), and second, one of the lower animals as distinguished from human beings. Both definitions have their uses. The species Homo sapiens belongs indisputably to the order Primates, phylum Chordata, kingdom Animalia, a fact encompassed in the first definition.

At the same time, Homo sapiens is the only species that engineers global-environment-altering technology, and engages in moral debates about said technology (among many other topics). The second definition allows us to refer to all the animals that don’t do this with a single word.

However, our developmental biology illuminates our kinship with the rest of the kingdom. As a human embryo, I looked pretty fishy for a while. I also at times resembled a reptile and a chick. The similarities are eye-catching enough that some early biologists encoded them in law, contending that each animal displays the evolutionary history of its species over the course of its development.

We now know that this superficially compelling “law” fails to capture the true intersection of development and evolution, but shared embryonic features still inform our understanding of relationships between animals—including humans. While this book is not about human development (many other excellent texts are available on that subject), our species will come up from time to time.

After all, I am a human, and you most likely are one, too. I, like you, began life as a baby. Also probably like you, I don’t remember it. I know that I depended on my parents and other caregivers, and I remain grateful to them for keeping me alive. I know that I was complicit in the process, crying for the attention I needed, producing attractive facial expressions and postures to garner care.

I absorbed environmental input, both actively when I put dirt in my mouth and passively as I experienced the hot summers, poor air quality, and minimal rainfall of Los Angeles in the late twentieth century. I was fortunate to be given consistent affection and nutrition and to be brought periodically to a beach where I could enrich my sampling of dirt and dry grass with salt water and sand.

How much of who I am today is shaped by the genes in my mother’s egg cell and my father’s sperm cell, and how much by my experiences from the womb onward? This age-old question of nature versus nurture sits at the very heart of developmental biology.

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