The Opposing Thumb

An opinionated digit leafs through the biological literature
Views of Life

On Being the Right Size


Warning: will splash on impact (Image: Madalena Parreira).

In his classic 1926 essay On Being the Right Size, J.B.S. Haldane remarked that zoologists rarely pay attention to one of the most salient differences between animals, their size.  In his words, “(…) higher animals are not larger than the lower because they are more complicated. They are more complicated because they are larger”. A living being’s size dictates how it breathes, how it absorbs and circulates nutrients, how much heat it must generate or dissipate, and much else. Size also, of course, determines the creature’s fate when you toss it down a mine shaft (like you do): “You can drop a mouse down a thousand-yard mine shaft; and, on arriving at the bottom, it gets a slight shock and walks away, provided that the ground is fairly soft. A rat is killed, a man is broken, a horse splashes.”

Within a species, size is largely determined by nutrition, within limits set by genetics. In humans, height is a great indicator of a population’s relative wealth. Dramatic changes in appearance can take place in the absence of significant genetic change: “The Netherlands, as any European can tell you, has become a land of giants. In a century’s time, the Dutch have gone from being among the smallest people in Europe to the largest in the world.” Add to this plasticity problems such as symmetric growth- the tall Dutch we know and their diminutive forefathers had pairs of matching long and short arms– and you have a fascinating biological puzzle.

While periods of growth spurts are a familiar part of human development, holometabolous* insects must decide when they have enough larval stuff to go into a cocoon and make and adult. How much caterpillar does it take to build a butterfly? Christen Mirth and colleagues examine how the interplay of three signals, insulin, juvenile hormone (JH) and ecdysone sets the size threshold for fruitfly metamorphosis. According to Mirth ‘“JH acts to fine-tune growth rates” by influencing the effects of the other two hormones’. The insulin pathway is a conserved regulator of growth in animals, and perhaps replacing JH and ecdysone of insects with analogous steroid hormones in vertebrate development will give us the physiological mechanisms that have made Holland a land of giants.

OT’s disclaimer: these studies were conducted by professionals under controlled conditions. Under no circumstances should readers throw Dutch citizens down a mine shaft.

* Insects that undergo full metamorphosis.


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