Why do some animals evolve to become miniature?

Flea toads, despite being just seven millimeters in length, are among the smallest vertebrates on the planet. Regardless of their tiny size, their organs and bodily functions are almost identical to those of animals that are hundreds or even thousands of times larger. 

While extreme miniaturization is a phenomenon seen in nature, the exact mechanisms behind how animals become so small have long remained a mystery.

Now, thanks to a grant of around $1.7 million from the European Research Council (ERC), a University of Copenhagen researcher is setting out to solve this puzzle.

Miniature animals with optimal function 

What do flea toads and smartphones have in common? Both have undergone extreme miniaturization while still maintaining all their essential components.

In the tech world, we’ve grown accustomed to devices becoming smaller while retaining or even increasing functionality. 

Similarly, in nature, creatures like flea toads, dwarf pygmy goby fish, and bumblebee bats have evolved to shrink while keeping their vital organs and functions intact, much like their larger relatives. 

But what exactly happens within these creatures, especially at the genetic level, when they undergo such evolutionary changes to reduce in size? And are there any biological limits to how small these animals can get?

These are some of the questions Mark D. Scherz, a researcher at the Natural History Museum of Denmark, will explore over the next five years. His research will focus on uncovering the genetic processes that govern miniaturization in vertebrates.

Understanding how animals become miniature 

“Large animals are often the ones to grab our attention. But I think it to be just as fascinating how nature has managed to miniaturize the exact same vital organs and cram them into a less than one-centimeter-long frog. Today, we know surprisingly little about how it all happens, and I want to change that,” Scherz explained.

Much like the tech industry has continually reduced the size of transistors to make devices smaller and more efficient, nature has found ways to make animals miniature over millions of years.

Scientists believe that extreme miniaturization is linked to evolutionary innovation, although the exact mechanisms remain poorly understood.

Genetic changes during miniaturization 

Scherz’s research project, GEMINI (Genomics of Miniaturisation in Vertebrates), aims to uncover the specific genetic changes that occur as animals become smaller.

The goal is to create a sort of genetic “template” that will reveal what happens in the DNA of vertebrates when they shrink in size.

“In several independent studies that looked at the genomes of miniaturized animals, a kind of clean-up and innovation takes place, where the genome becomes smaller. A lot of this happens in repetitive bits of the genome that we used to call ‘junk’ DNA. But some of it happens in other genes as well, which is what we need to learn more about,” Scherz said.

Small size, big innovations 

In the past, scientists believed that animals generally evolved to become larger over time, a concept known as “Cope’s rule.” However, it is now clear that this isn’t always the case – and for a good reason.

“Animals can’t just keep growing bigger and bigger. At some point, their physiology reaches a limit, and so does gravity. That’s why there are phases where body size reduces, allowing for a trend toward increased size later on,” Scherz said. 

“We also think that small size might be where the really big innovations happen, which we then see expanded when the descendants get bigger again. That makes these miniaturized animals particularly exciting to study when we’re trying to figure out how innovations evolve in nature.”

An overlooked phenomenon 

For decades, miniature animals have been somewhat overlooked in favor of studying large species like blue whales and elephants.

Some of the biggest discoveries in the field of miniaturization date back to the 1990s, with fewer recent advancements.

“Everyone’s attention is on blue whales and elephants. Any child you ask can tell you about the largest land mammal and the largest marine mammal and the largest dinosaur that has ever lived,” said Scherz.

“But scaling up and getting bigger is not a big problem. It is a far more impressive feat to have (practically) everything in a twenty-three-ton blue whale compressed into a seven-millimeter package.”

However, interest in miniaturization is experiencing a resurgence, partly due to exciting new discoveries.

Earlier this year, researchers in Brazil found the world’s smallest vertebrate, the flea frog, measuring just seven millimeters in length. This discovery has reignited curiosity about the biological processes behind miniaturization.

“We live at a time when probably the smallest vertebrates ever are around us, but they are too easy to overlook, so we just forget about them. This project will allow us to gain a genetic understanding of what controls this ‘miniaturization process’ and investigate whether there is something that keeps animals from becoming even smaller,” Scherz concluded.

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