Speed of Life
Christmas Island Robber Crab
Size Matters?
Energy Chart
Right across the natural world, the size you are has a profound effect on your metabolic rate – or your "speed of life". For Australia's small
marsupial mouse, even at rest, his heart is racing away. But the fox-sized Tasmanian Devil, he ticks along at a much slower rate. And then there's
me, living life at a languid 60 beats a minute.
Looking beyond heart rate, your size influences the amount of energy you need to consume, and the rate at which you need to consume it.
Bigger bodies have more cells to feed. So, you might expect that the total amount of energy needed goes up at the same rate as any increase in
size. But that's not what happens.
If you plot the amount of energy and animal uses against its mass, for a huge range of sizes, from animals as small as flies, and even smaller,
all the way up to whales, then you do get a straight line, but the slope is less than one. So, that implies that gram for gram, large animals
use less energy than small animals.
This relationship between the metabolism and size significantly affects the amount of food larger animals have to consume to stay alive.
Kookaburra
Now, if my metabolic rate is scaled one-to-one with that of a mouse, then I would need to eat about 4 kg of food a day. In my language, that's
around 67,000 kJ of energy, which more colloquially is 16,000 cal. That is 8 times the amount that I take in on average on a daily basis.
Each of the cells in my body requires less energy than the equivalent cells in a smaller-sized mammal. The reason why this should be so is
not fully understood. It's also not clear whether this rule of nature gives an advantage to big things, or is actually a constraint placed
on larger animals.
Take the relationship between an animal's surface area and its volume. Big animals have a much smaller surface area to volume ratio than small
animals, and that means that their rate of heat loss is much smaller. And that means that there is an opportunity there for large animals. They
don't have to eat as much food to stay warm, and therefore they can afford a lower metabolic rate.
Now if this helps explain the lives of large, warm-blooded endotherms, like birds and mammals, but doesn't hold so well for large ectotherms,
life's cold-blooded giants.
Now, there is another theory that says that it wasn't really an evolutionary opportunity that large animals took to lower their metabolic rate.
It was forced on them. It was a constraint, if you like. The capillaries, the supply network to cells, branches in such a way that it gets more
and more difficult to get oxygen and nutrients to cells in a big animal than in a small animal. Therefore, those cells must run at a lower
rate. They must have a lower metabolic rate.
Femur Bones
Or it could just be that as you get bigger, then more of your mass is taken up by the stuff that supports you, and the support structures, like
bones, are relatively inert. They don't use much energy.
But whatever the reason, it's certainly true to say that the only way that large animals can exist on planet Earth is to operate at a reduced
metabolic rate.
If this wasn't the case, the maximum size of a warm-blooded endotherm like me or you would be around that of a goat. And cold-blooded animals, or
ectotherms like dinosaurs, could only get as big as a pony. Any bigger, and giants would simply overheat.
Now, there's one last consequence of all these scaling laws that I suspect you will care about more than anything else, and it's this – there
is a strong correlation between the effective cellular metabolic rate of an animal and its lifespan. In other words, as things get bigger, they
tend to live longer.
To explore this connection between size and longevity, I've left the mainland behind. For my final destination, I've come to one of Australia's
remotest outposts. Named Christmas Island when it was spotted on Christmas Day in 1643, this isolated lump of rock in the Indian Ocean is a land
of crabs. And in their midst lurks a giant wonder of the natural world.
Christmas Island Robber Crab, Birgus latro
This is a Christmas Island robber crab, the largest land crab anywhere on the planet. These things can grow to around 50 cm in length, they
can weigh over 4 kg, and they are supremely adapted as an adult to life on land. They can even climb trees.
Over the years, the crabs have become well adapted to human co-habitation. These things are called robber crabs because they have a reputation for
curiosity and for stealing things, anything that isn't bolted down. They'll steal food and cameras if they can get half a chance.
These giants live on a diet of seeds and fruit, and occasionally other small crabs. Their large, powerful claws mean they can also rip open
fallen coconuts.
They are really quite a menacing animal, actually, for a crab!
What's wonderful about these crabs is that they live through a range of scales. At different times of their lives, they have a completely
different relationship with the world around them, simply down to their size.
This Chap - Mature Robber Crab
Throughout their lives, robber crabs take on many different forms. They begin their lives as small larvae, swept around by the ocean
currents, and as they grow, some of them get swept up onto the beaches of Christmas Island, where they find a shell, because they are, in fact,
hermit crabs. They live inside their shell for a while, they continue to grow, and eventually, as adults, they roam the forests like this chap
here. So these crabs, over that lifespan, inhabit many different worlds.
Hermit Crab
On land, the adults continue to grow and now have to support their weight against the gravity . compared to the smaller crabs whizzing around,
these giants move about much more slowly, but they also live far longer.
Of all the species of land crab here on Christmas Island, robber crabs are not only the biggest, they're also the longest-living. So this
chap here is probably about as old as me, and he might live to 60, 70, even 80 years old.
Because of the robber crab's overall body size, its individual cells use less energy and they run at a slower rate than the cells of their
much smaller, shorter-lived cousins.
The pace of life is slower for robber crabs, and it's this that's thought to allow them to live to a ripe old age.
Your size influences every aspect of your life… from the way you were built… to the way you move… and even how long you live. Your size
dictates how you interact with the universal laws of nature.
So there's a minimum size, which is set ultimately by the size of atoms and molecules, the fundamental building blocks of the universe. And
there is a maximum size which, certainly on land, is set by the size and the mass of our planet, because it's gravity that restricts the
emergence of giants. But within those constraints, evolution has conspired to produce a huge range in size of animals and plants, each
beautifully adapted to exploit the niche is available to them.
Your size influences your form and construction. It determines how you experience the world, and ultimately, how long you have to enjoy it.
