Why aren’t there are super pygmy vertebrates?
For most vertebrate animals, their early embryonic development unfolds in extremely congruent fashions.
Embryo growth is stem cell-dependent and in large part, despite different gestational periods, the molecular and cellular machinery that controls vertebrate development is almost perfectly conserved.
Yet at maturity vertebrates end up with a range of different body sizes and masses. Even amongst individual types of animals such as chameleons, there can be a wide range of body sizes as discussed in a recent NY Times article (see image at right of Brookesia micra chameleon).
Of course everyone knows about just how big vertebrates can get as exemplified by the extinct dinosaurs, but also the now living blue whale, the largest animal to have ever lived.
But how small can vertebrates get?
It turns out the answer is “not very”.
In fact, vertebrates do not end up at maturity much smaller than the tiny chameleon shown in the image above. This chameleon, reported in a paper in PLoS One, reaches a maximum length for males of 16mm at maturity. Yeah, this chameleon is pretty small, but gargantuan in contrast to small invertebrates, which can be uber tiny such as rotifer, which can be 50 μm.
Why are there not more tiny vertebrates cruising around on Earth…what we might call “super pygmies? Could there be more and they are just so tiny we just don’t know about them yet precisely because they are so tiny?
The kid in me hopes that is true and that there are all kinds of super pygmy vertebrates hiding all over Earth, but I doubt it.
The smallest known vertebrates are tiny fish and frogs. According to Wikipedia, the currently recognized smallest vertebrate is a type of frog called Paedophryne amauensis . It is on average on 7.7mm.
Why doesn’t nature make vertebrates smaller than just under 1cm (10mm)?
One can imagine why there would be upper limits on the size of vertebrates. As their size increases, the body mass to surface area ratio increases dramatically. Basically animals, especially on land, can only be so big before they would collapse under their own mass due to gravity and relatively small surface area. Of course specialized skeletons or living in the seas can modulate the upper limits, but the concept of an upper limit of vertebrate size on Earth makes good sense based on facts and the laws of nature as we understand them.
Flipping things around, why might there be a lower limit on vertebrate size? What would be deleterious about being super small for a vertebrate?
To the contrary, it seem as our super pygmy vertebrate gets smaller and smaller, in theory it would gain some benefits such as being too small for predators to see or even care about.
Its body surface area to mass ratio would be potentially more ideal rather than less, making for a sturdier, tougher animal.
But what might be the downsides to ultra-smallness for vertebrates that would have made evolution not produce any that are alive today?
I guess the smallness would open the door to new predators as well.
One other negative of über-smallness might be that a vertebrate below say 7mm in length with the accompanying tiny mass might be so small that it is in some sense at the “whim” of environmental forces to such a degree that it could not control its own fate.
Wind, water, dust, etc. could interfere with the ultra small creature’s daily life in oversized ways.
While this reason is at least in theory plausible, another explanation has more to do with molecular and developmental biology. In this way of thinking, the evolutionary “ban” on ultra-small vertebrates is due to conservation of key elements of developmental programs. In other words, the vertebrate developmental program so exquisitely honed by evolution simply does not work to produce mature vertebrate animals <7mm in size.
Is this possible?
Some clues might come from answering the critical question: are tiny animals smaller because they have fewer cells or smaller cells?
It turns out that both can be the case.
Perhaps there is a lower limit to just how small a vertebrate cell can be and still function.
In addition it is reasonable to theorize that vertebrates need a minimum number of total cells to be healthy and functional. Not every job in a vertebrate tissue or body as a whole can be downsized to be performed by fewer cells.
Now if you throw in the previously mentioned possibility that super pygmy vertebrates could be injured by the environment by dust and debris that larger vertebrates don’t even notice or literally blown around by wind, perhaps we have a theory for why super pygmy vertebrates don’t exist.
I still wish they did, but I don’t know why. Maybe it is the cuteness factor. Take a look at Harry the baby Pygmy Hippo…can you keep yourself from smiling? Now imagine Harry, if once full grown, was the size of a ladybug…that just doesn’t work. In reality, once fully grown, Harry will weigh around 500 pounds.