Babies outnumber all

 kw: book reviews, nonfiction, biology, zoology, population, embryology

I couldn't think of a better illustration of the book's theme than the cover art. It shows the larval or infant form of several dozen animals, from tadpoles to veligers to baby monkeys and birds. 

"Veligers?", you ask? A veliger (soft "g": "vell-uh-jer") is the larval form of most kinds of mollusk, like this tiny snail shown at 50x.

The book is Nursery Earth: The Wondrous Lives of Baby Animals and the Extraordinary Ways They Shape Our World, by Danna Staaf. The author's enthusiasm for these small-to-tiny-to-invisible animals will soon become your own as you read.

We seldom pay much attention to baby animals of any kinds besides kittens and puppies, because they are small and mostly unseen. However, in numbers they dominate the biosphere! Think about it: we usually relate everything to our human milieu and to the most familiar animals, which are mostly domestic. These familiar animals live a long time as adults (if not slaughtered for food), compared to their lives as infants and juveniles. 

When we think "animal", what comes to mind is mainly mammals and possibly birds…and maybe lizards and fish. Mammals and birds, in particular, care for their offspring, and we were all told in a beginning science class that "other animals" such as fish and turtles and "everything else" simply leave newborns to fend for themselves. Maybe we've seen documentaries of newly-hatched, nickel-sized sea turtles struggling down the beach to reach the water. Now, step back a moment: How many of those little sea turtles will survive to adulthood and produce more baby turtles? A few out of hundreds, or of thousands? It is easy to conclude that, by numbers, the vast majority of sea turtles alive at any one time are the babies, even as they are being gobbled up by predatory fish or dying of diseases. This is true for nearly every living animals species. Most animals alive now are babies, but most are hidden.

Even for backyard birds, the nestlings may number four or five or six, like these little robins (there are four, but one had just closed its beak) in a nest outside our kitchen window. But on average, only two grow up and have their own families, from a lifetime of nesting, not just from one nest. A pair of robins may produce five or six clutches of eggs in their lifetime; only two nestlings will survive to reproduce. Birds care for their young with great diligence, but they still need to lay many eggs to ensure a stable population. It's a similar case with most mammals. Infant and juvenile mortality is very high, so they must have many cubs or kits or joeys or puggles so that the next generation will not be less numerous than the present one. 

Now, what of fishes? There are a few notable species of fish that care for their young, but only a few. Salmon may represent the opposite end of the spectrum: they struggle upstream to their birthplace and lay millions of eggs, and then die. The fry (newborns) have been bequeathed a yolk sac, which nourishes them until they learn to catch their food. They look like fish, but not much like they will appear when grown. This is because of a theme of the book, that the environment of a newborn animal is quite different from the adults' environment, so they need a different kind of body to thrive in it. This is more evident among animals that develop through stages, with partial or full metamorphosis. The conversion of a caterpillar into a moth or butterfly, or of a grub or mealworm into a beetle, are familiar examples. Even baby grasshoppers, that have "partial metamorphosis", and thus look a lot like adults, don't grow wings until they reach full size.

Most people have seen caterpillars, or inchworms, or lawn grubs. Particularly for insects, the larval stage (or stages) of life can last much longer than the adult period. A mayfly nymph grows underwater for several months, then surfaces and metamorphoses into the adult, flying form, which lives just a few days, mates, and dies. Periodical cicada larvae live underground for 13 or 17 years. When they emerge, the adults "serenade" us (really, each other) for a month or so, and die before winter arrives. Therefore, at any one time, there are trillions of cicada babies hidden away underground, and then for a short time, this year's crop emerges to amuse and irritate us while they hurry to reproduce. Crops of other years remain hidden until their time comes.

Many details about many of these baby animals fill this very enjoyable book. The author, who has children of her own, circles back to the human condition. We don't think of mammals, or humans in particular, as experiencing metamorphosis. While a human baby doesn't pupate and melt away, to be radically reorganized to a new form, we do change a lot between birth as a seemingly helpless wiggle-wormy, squirmy baby, and the competent (we hope!!) grownup we become after 15-25 years. Baby humans are actually very well adapted to the environment into which they are born. And at birth they have already undergone the greatest period of growth of their lives: from a single cell to around 3 kg, complete with all major organs, the motivation to find a nipple and suckle at it, and a brain about 1/3 adult size; everything is primed to go through the decades-long metamorphosis we call "growing up." As adults, we may not remember that much of going through puberty. It is a huge metamorphic change in both body and mind. (For neurotics, many of the outdated defense mechanisms that plague us were formed during adolescence.)

Here's the takeaway: The vast majority, in number, of animals alive at any time are babies.