What ancient giant sharks can teach us about recovering Australian arid zone mammals

Admin Aridrecovery - Friday, April 07, 2017

The ancestors of whales first entered the seas around 50 million years ago, and quite frankly looked ridiculous (see a great video here). They had the mammalian blueprint, and appeared quite clumsy at first. After 20 million years, they evolved into a handful of species. They altered the mammalian design to become more sea-worthy, with their nose behind the head and no more useless feet. Although they were smaller and slower than modern whales, they were still larger than any other predator in the ocean, so they essentially had the place to themselves.

All those whales swimming through the ocean presented a lucrative prey source for a predator to exploit. Soon enough, one species of shark evolved with an effective, yet crude, way of hunting these whales. They grew big enough to attack them.

This shark is called Megalodon, Carcharodon megalodon, and was at least 15m long.


The terrifying Megalodon, Carcharodon megalodon. At over 15 meters long, it was a specialised whale killer. But this predator ultimately had a positive effect on whales, and forced them to become stronger and more diverse. Illustration by Elia Pritle


Considering this shark was the size of a bus, hunting the small, slow whales of that era did not need to be complicated. Megalodons simply ripped off flippers, crushed lungs, or took huge bites from the whales’ sides and waited for them to die. Many of these ancient whales would have become endangered by the predation pressure of this giant shark. Just as in the present day Australian landscape where cats and foxes have been introduced, an entire faunal assemblage found itself suddenly unequipped to deal with a new and vicious predator.

Despite the terror, the Megalodon shark ultimately had a monumental influence on whale evolution. Whales diversified like crazy, and went from what was likely around 6 species to over 20. They became faster, larger and started living and working packs. And they possibly started spending more time in areas Megalodon seemed to avoid, like the polar seas. Ultimately, these ancestral whales became like the whales we know today.

The sharks couldn’t keep up with the speed of evolutionary change in the whales. Their main advantage was their size, but the constraints of the fish blueprint meant they couldn’t evolve to be much larger or faster. Fish hearts are not as effective as mammalian hearts. To get any bigger than the Megalodon, a shark heart simply could not develop to the calibre required to pump blood around the whole enormous body and still move at speed. 

Eventually, the Megalodon ate all the slow moving whales to extinction, and were unable to hunt the better adapted whales of the new era (see here). They lost their primary food source, and went extinct around 2.6 million years ago (see here).


The Megalodon lost.


As far as the whales are concerned, it all worked out for the best. Though countless whales would have died terrible deaths to these brutal predators and species went extinct, as a collective they evolved and won. They adapted, overcame the threat, and ruled the oceans again until humans with harpoons came along a few million years later.

A similar event is occurring in Australia’s arid zone. Until recently, we had a diverse range of native mammals including bilbies, bettongs and stick-nest rats. New predators, the feral cat and red fox, have swept across the country, and most natives did not have effective strategies for dealing with them.

Feral cats have come to Australia and, like the Megalodon sharks, decimated the unadapted mammals like the above Plains Mouse. Can our natives become stronger like the whales did? Photo by Zac Richardson

Unlike the Megalodon versus Whale arms race that occurred over millions of years, the cats and native marsupials have co-occurred together over a mere 200 years at most. This is not enough time for the native animals to slowly evolve avoidance strategies or to develop new physical adaptations. Not only that, the arid zone has been besieged by many other changes such as rabbit plagues, intense cattle and sheep grazing,  and humans hunting with guns. Many of these species have gone extinct as a result.

But the other difference between the ‘Shark vs Whale’ and the ‘Cat vs Marsupial’ stories is that humans are now managing the landscape. We have the ability to jump on Wikipedia and Google Scholar to learn from the ancient whale story! All known knowledge of evolutionary history is available to us, and we can jump in and play referees for a fair and balanced arms race.

At Arid Recovery, our hope is that one day introduced predators and natives can coexist. Like the ancient whales, we know some species may be doomed to persist only on predator-free islands or in reserves. But for others, there is still hope for them to adapt and diversify. We need to find an equilibrium where the threat of feral cats and foxes is low enough to allow some natives to survive and adapt.



Native mammals like the burrowing bettong will hopefully learn to survive in a feral cat infested landscape. Photo by Nathan Beerkens


If we don't take action, will evolution take its own course? Can natives adapt as the whales did? Without conservation management, probably not. Cat and fox densities in Australia are being artificially supplemented. This is both from animals living in towns moving out to the wild (cats), and from the abundance of introduced prey species. For example, plague rabbits can increase cat and fox densities almost 10-fold. I doubt the ancient whales would have survived if they were dealing with 10 times as many Megalodons prowling the oceans.

Using the fences at Arid Recovery, we have been taking the next step forward in preparing native animals for co-existence with introduced predators. In two of our compartments, we add predators to a predetermined threshold, and hope to nudge evolution along in the right direction. By exposing native animals to feral cats, we can encourage natural selection to run its course in a controlled environment, and facilitate the evolution of traits that enable natives to co-exist with cats (read more).

While whales grew bigger and faster, what can we expect native mammals to do? Will they learn to build better burrows for protection? Become better at smelling whether cats are nearby? We can’t be sure, we hope that by setting cat predation at a controlled level, native animals will develop anti-predator traits (see here). The smarter animals, the faster animals, are likely to be rewarded so that the species is pushed to new heights.

In this vein, we’re pleased to announce the next stage of this research. Arid Recovery is partnered with the University of NSW and Bush Heritage through an ARC Linkage project that kicks off again this year. Hopefully, just as the ancient whales did, some of our native arid zone mammals can adapt to this new predator and forge a new future.

Written by Dr Hugh McGregor and Dr Katherine Tuft


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Arid recovery is a conservation initiative supported by:
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