Recent discoveries of a Lilliputian lizard and elfin amphibian, fascinating in their own right, highlight one of the most enduring questions in biology: what controls the evolution of body size? Why do some taxa grow smaller and smaller, while others grow larger and larger, as if they’d tumbled down the rabbit hole with Alice and devoured all the curious potions and cakes she found there?
The question endures in large part because body size affects nearly every aspect of an organism’s existence, from physiology (temperature regulation and metabolism) to ecology (life history and foraging strategies) and evolution (reproductive success over time).
For more than a century, biologists thought evolutionary taxa, or lineages, grew larger and larger over time, a phenomenon known as Cope’s Rule, illustrated most often by horse evolution. Modern equids, scientists believe, evolved from the diminutive Hyracotherium (commonly known as eohippus, or “dawn horse”), which appeared in the fossil record some 55 million years ago. Many textbooks mistakenly liken Hydracotherium to a fox terrier (think Asta of The Thin Man movies), but the ancestral horse was more Lassie than Asta, as Stephen Jay Gould famously explained in his essay "The Case of the Creeping Fox Terrier Clone."
In 1997, though, David Jablonski showed that (as usual) biology rarely follows hard and fast rules. In a 10-year review of fossils covering 16 million years and 1,000 species from 191 lineages of bivalves (clams and scallops) and gastropods (snails and slugs), Jablonski found that just as many taxa decreased in body size over time as increased. And even the horse example has come under fire. A 2004 study analyzed horse fossils in light of recently resolved relationships among evolutionary groups and showed that while the lineage that gave rise to the modern horse grew larger, others shrank.
Still, examples of lineages evolving toward larger body size abound, with evidence linking greater size to higher fitness (better survival and mating success for individuals). If you’re big (say, a lion or other large carnivore), it’s easier to catch prey, avoid predation (though elephants, like mammoths before them, may perish at the hands of human hunters), survive tough conditions, attract mates (silverback gorillas claim exclusive breeding rights to females), and claim more resources than your competitors.
Given the advantages of size, one might think the tiny frog and chameleon are simply freaks, outliers among a field of giants. But the fossil record offers plenty of examples of large animals shrinking over millennia (known as “phyletic dwarfism”), often after winding up on islands or other restricted ranges.
Until about 10,000 years ago, dwarf elephants inhabited Crete and other Mediterranean islands, which favored smaller, nimbler forms that could survive on less food and manage the rocky terrain. Even dwarf mammoths (the oxymoron notwithstanding), dinosaurs, and hominids (Homo floresiensis) once inhabited isolated islands.
If you’re small, you might reproduce quickly, offer too little reward for a predator’s effort, and maybe even prove too hard to see.
That seems to be the case for a pint-sized amphibian found, through no small effort, in the forests of Papua New Guinea, which its discoverers claimed as the “world’s smallest vertebrate.” Because the largest vertebrate, the blue whale, and (previously) smallest, a fish, are aquatic species, some biologists thought a water-based lifestyle may facilitate the evolution of extreme size. But, as the scientists argue in the paper describing the frog, this doesn’t explain how extreme miniaturization evolved at least 11 times in terrestrial frogs.
The 7-8 millimeter frog, named Paedophryne amauensis, is active mostly at dawn and dusk, sounding more like a cricket than a frog when it calls out to potential mates from the leafy detritus of the forest floor. (The authors dubbed the species “amauensis” after the region near Amau Village where it was found.) Leaf litter in tropical forests stays moist throughout the year, keeping the minute amphibian safe from desiccation and likely explaining the evolution of its life history: offspring bypass the tadpole stage, emerging fully formed, though even tinier, avoiding fish, insects, and other aquatic predators. Of course, teeny adults would be at higher risk from predators if they lived in the water, too, which might explain why the species carved out a niche in upland areas with a lower diversity of such threats.
And just last month, another group of researchers reported their discovery of four new species of dwarf chameleons, one so small it can balance on the tip of a match head. The mini chameleon, Brookesia micra, measures a smidgen over an inch from snout to tail, and seems restricted to Nosy Hara, a small (naturally) island off the coast of Madagascar. An extensive survey of Nosy Hara and adjacent islands in 2007 failed to spot the little lizard, which scampers around limestone rocks and dry forest leaf litter during the day and roosts on low-lying branches a few inches above the ground at night.
Unlike their amphibian counterparts, the minuscule reptiles inhabit relatively dry tropical areas. Because small body size carries a higher risk of desiccation from the proportionally higher body surface area, it’s surprising the lizards live in a dry environment, the scientists explain in their report. It’s possible they’ve adapted to certain features of the landscape that retain moisture, like leaf-filled fissures in limestone.
The tiny frog and chameleons may or may not win the title for smallest of their kind, but the distinction is beside the point. The discovery of these new species offers a rare ray of hope amid ongoing reports of devastating declines in amphibian and reptile populations around the world, mostly due to habitat destruction. These dwarf species have likely benefited from minimal space and resource requirements, and being too tiny to spot. And for me, it’s no small comfort to know that we can still find wonders, both beautiful and strange, on this side of the looking glass.