Amazing adaptations: does one size fit best?

Giant Gippsland Earthworm cartoon

In Australia, we have a lot of big things.

The Big Banana, the Big Prawn, the Big Pineapple, the list goes on. I checked, there’s over 150 ‘big things’ all over the country!

But what about our big animals? (And no, I’m not talking about the Big Koala!)

The Giant Gippsland Earthworm, star of last week’s Look at Me podcast, is just one such animal. What possible need could an earthworm have to grow up to three metres long? Or, on the flip side, why don’t all worms get this big?

At first glance, it may seem like a no-brainer to be a big animal. The bigger you are, the harder it is to eat you. And the easier it is to eat other things.

But life isn’t always that simple – as with everything in evolution there is a trade-off. Or, in most cases, several trade-offs.

Firstly, the bigger you are, the more resources you need to grow and maintain that size. It takes longer to grow into an adult and you have fewer resources to allocate to other things, like reproduction.

Because of this, larger animals also require more food overall – although, surprisingly, less in relation to their body weight. A mouse needs to eat all day to keep up energy levels, while an elephant will only need to eat approximately 5% of its body weight to keep going. This is because larger endotherms (warm-blooded animals) have a slower metabolic rate, so they burn less energy. They can also maintain their internal temperature with less effort, as the bulk acts as excellent insulation.

The trade-off will often tip in favour of larger animals under certain conditions. One of the most famous examples of this is islands. On islands, smaller carnivores often can grow quite big to fill niches in the ecosystem usually filled by absent mammalian predators – like a lizard evolving into a Komodo Dragon. Herbivores can also reach gigantic sizes (like the Moa of New Zealand) because they do not have to worry about hiding from predators. Unfortunately, the Moa is now extinct because a novel mammalian predator – humans – entered the ecosystem.

The ocean, too, is a haven for larger animals. At a certain size, it just becomes an effort to exist on land – but the buoyancy of the ocean can alleviate this pressure (you can imagine how well a blue whale would do on land, for instance). Amongst invertebrates, there is a tendency to be larger in deeper waters, likely as an escape from the high pressure and lower temperature of the deep-sea environment.

In general, however, invertebrates struggle to grow too large (you may be pleased to know there is a reason you would not come across a giant spider like Shelob from Lord of the Rings).

In reality, the exoskeleton of a giant arachnid could not support the weight of something so massive, and as they use diffusion instead of blood vessels to move oxygen around the body, a giant spider would also have trouble breathing when it got too big.

Occasionally, though, invertebrates can get larger than we would expect – the Giant Gippsland Earthworm being a case in point.

Although not much is known about this amazing creature that lives underground in a small part of Gippsland in Victoria’s south-east, the Giant Gippsland Earthworm is definitely subject to some large animal trade-offs.

On such a large scale, things tend to move at a slower pace. It takes time for the worms to grow this big, with studies suggesting they can take five years to reach sexual maturity.

Once they reach maturity, each worm only lays one egg per year, which then takes 12 months to hatch. This egg is extremely large for a worm (4–6 cm long and 2 cm wide) and this egg produces a baby worm that is 20 cm long by the time it hatches.

It seems the Giant Gippsland Earthworms have chosen quality over quantity when it comes to having babies. But the earthworm’s slow maturation and reproductive rates could be contributing to the decline of this incredible species, which is found within an area of only 40 000 ha in Gippsland and occurs nowhere else in the world.

If you missed last week’s episode of Look at Me, make sure you listen via the Guardian Australia website or subscribe through your podcast provider to learn about this amazing and often-overlooked animal.

Tune in to the final episode of Look at Me (out Thursday April 5) to hear Benjamin Law and Remember the Wild’s own Chris McCormack find out about Australia’s lesser known animals.

Banner image courtesy of Chris McCormack

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