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.

Viruses R Us

kw: medicine, viruses, embryology, symbiosis

I'll just get this out of the way before I even finish the book. I find the idea behind this image rather unsettling. This shows a portion of the syncytium (pronounced sin-sigh-tee-um), the multinuclear membrane, effectively composed of a single cell, that is the boundary between fetal blood and maternal blood in the placenta of all mammals. It is the reddish layer surrounding the purplish blobs, which are folds of placental tissue.

What you don't see in this light microscope image are the viruses that induce the syncytium to form. Animal tissue doesn't "know" how to form a syncytium, or any multinuclear cell. Its formation is mediated by viruses called HERVs, for Human Endogenous RetroViruses. The retrovirus most of us have heard about is HIV, the cause of AIDS. It is related to HERVs. Our DNA is host to many, many related retrovirus genomes, and certain ones are expressed and work together with "our" DNA at many stages of our life, including setting up the placenta that makes most mammalian pregnancies work.

I'm reading a book on evolutionary virology, which I'll review more fully in a few days. Meantime, I could not get this image out of my mind once I saw it at the author's website. Many of our tissues and organs develop with the help of symbiotic viruses. I never knew viruses could be symbiotic! Not only that, they may be the dominant partner!!

This is one more demotion of our vaunted humanity. First, we were at the center of the Universe. Copernicus and Galileo moved Earth to "third rock from the Sun". Then, we were the peak of creation. Darwin, Wallace and others showed we're smart apes, but apes all the same. In recent decades it has become clear that 9/10 of the living cells in our bodies are bacteria, although each of "our" cells weighs hundreds of times what a bacterium does. Now I read that, while "vertebrate DNA" makes up only 1.5% of our total genome, various total and partial virus sequences make up 45%, or 30 times as much. I am starting to think that if you took away everything that is not "human" from us, we would be nearly weightless shells, ready to collapse under our own negligible mass.

I'm tempted to write more about the book now, but I suspect the author has more surprises waiting in the last few chapters, so I must simply say, "Stay tuned."

The mouth of plants

kw: musings, embryology

This morning I did the mini-delivery of the newspapers. A few neighbors have an agreement: whoever gets out first will put the newspapers on the porch for the others. Sometimes the papers get delivered a little later, and my neighbor across the street has gone to work already. Then I do it. Today was such a day. It was nice to be out on a spring-like morning, seeing the just-past-full moon about to set.

I happened to be thinking about plants. A friend of my wife's gave her flowers yesterday. We had them in a vase, and seeing them standing in the water reminded me of something I read a few years ago.

All metazoans, multi-celled creatures, all plants and animals, develop in a similar way. Once the ovum is fertilized it begins to divide. People with great diligence have watched this early development process, carefully noting where the embryo's features are formed, in relation to the original cell. At a certain point, the embryo is a hollow sphere, and then one side buckles in to form the "inside". In animals, this "inside" becomes the alimentary canal. In plants, it is the vascular system. One point becomes the mouth of animals, and that same point becomes the root of a plant. In either case, that particular point is the location where the male gamete entered the ovum.

What I was thinking about was this. The "mouth" of a plant is its root system. Not only does it make sense—that is the locus of intake—but embryology shows it. As I walked down my driveway, I looked at the dogwood tree on the lawn, imagining it as an animal perched on its mouth, sucking from the ground. Then I saw all the little grass plants as tiny versions of the same thing.

I dunno. Is this too nerdy? I just thought it is cool.