Rodents of Unusual Size? I don’t believe they exist*… At least, not in the way that some media outlets would have you think. Last week journalists reported A LOT on the possibility of giant sheep-sized rats evolving in the future. See here, here and depressingly** also here.

I was asked for comment by the BBC a couple of of times — the first time I refused as I was busy (I had 30 mins notice and at that point I had no idea what the story was about). Plus, it sounded like a tabloidy link-bait story where I would find it hard to get a complicated point across if the angle had already been established as ZOMG EVIL KILLER GIANT RATS OF THE FUTURE.

However, afterwards the guilt got to me. Surely this is *exactly* when I should be joining in the discussion & trying to improve the quality of coverage?

So when the next request came through I decided to make the time for the journalist’s questions. They were hard to answer in a way which satisfied me, as they felt like they were pushing me to provide quotes that would still miss what I felt was the fundamental point of the R.O.U.S. story: the impact of the Anthropocene, especially on biodiversity.

As Henry Nicholls blogged (thank you Henry!): the mass extinction crisis we are facing in the Anthropocene is no laughing matter.

As I have no idea if any of what I wrote will even be used, The BBC used some of my replies in this article. I thought I’d pop up my full reply here (typos included) so this effort actually goes *somewhere*. It’s one of the many examples of the ‘hidden service’ that researchers do for free all the time.

An imagined giant rodent of the future, from the Gallery of Evolution at the Royal Belgium Institute of Natural Sciences. Copyright RBINS. Thanks to Mark Carnall of the Grant Museum for bringing this to my attention.

* This, by the way, is a Princess Bride quote. Rodents of unusual size do indeed exist, and even larger ones have existed. Evolution is amazing like that.

** to clarify why I find this depressing, it is because of the focus on the giant rat aspect, rather than the broader topic of which species will be ‘ future ancestors’ (which is also an important story about which species are going extinct right now) and how they will give rise to many different sizes, and differently adapted descendents. This gets a brief mention, leaving the overwhelming impression that sheep-sized rats is all the future holds…

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Dear [BBC journalist],

The museum press office passed on your questions to me. To give you some background to my expetise, my research investigates the evolution of dwarf elephants (now extinct) that used live on islands like Sicily, Malta, Cyprus and Crete (e.g. see here). Interestingly, on islands while large mammals such as elephants and hippos evolve to become smaller (1m-tall adult elephants!), small mammals like rodents evolve to become bigger. But no island rat has approached anywhere near the size of a sheep.

The sort of evolution scenarios that Dr Zalasiewicz is speculating about would occur over huge swathes of time – tens of millions of years. Those sorts of timescales would allow for a myriad of wondrous forms to evolve: just look at the enormous diversity of the mammals that have evolved in the last 65 million years. I only have the University of Leicester press release to go on, but it seems to me that Dr Zalasiewicz has set up an interesting thought experiment, but one that is quite specific in its scenario and doesn’t lend itself to the broader Qs you have asked. Hence my answers wont fit perfectly with your questions I’m afraid.

Q8: Is there anything else he/she would like to add on the subject? 

It’s really fun to speculate about what the distant future might hold for the evolution of life on earth – the possibilities are endless. But we know from the fossil record that it takes, on average, ten MILLION years for life on Earth to bounce back from a mass extinction event. So while it is fun to think about the wondrous new forms that might arise, it is far more chilling to think of the species we are losing right now, and how long it will take to replace them. Certainly, given the average duration of a species, no human would get to see this imagined future ecosystem.

Q1: If certain animals went extinct because of climate change, would the evolutionary principle of niche-filling really extend to just one taxon?

Q2: Or would it be more wide spread with lots of other animals rushing to fill the space? (like after the KT extinction)



Only if all species bar one went extinct. Any species surviving an extinction event may eventually evolve to fill empty ecological niches. So the question is really “Which species are currently the most vulnerable to extinction” (because we’re set to lose those) and “which species are not at risk at all” (because that what will be left for natural selection to work on, the ancestral species of the future).

Q3: Would rats really be the ideal candidates for the taxa that would fill any ecological gaps?

Certain rat species, like the brown rat Rattus norvegicus, have a large, globally distributed population, and so are very far from extinction right now. The brown rat is also a generalist, flexible in its diet and ability to live in different environments – factors key to its success as an invasive species across the world.

Q4: or is their current niche too much of a success for them to need to adapt?



The crux of this question is time: in the very short-term, as we are seeing with invasive rat species today, rats are able to spread into unoccupied niches because they are generalists. Once in a new niche, natural selection continues to act – hence on islands, rats tend to evolve to be come a little bit larger. This might occur over hundreds to thousands of years. But when you zoom out to 100 million years in the future, we aren’t just talking about evolutionary changes in the rat – but the whole environment, throwing up new challenges the whole time. Continental drift; new mountain ranges; significant changes in climate; unpredictable catastrophic events like a meteor hit. And set against all this a continuous, complex evolutionary interplay between individuals competing for space to live, eat and breed. This is what Dr Zalasiewicz is talking about – but when you get to this stage, it makes no sense to call that theoretical sheep-sized animal of the future a ‘rat’ at all. It may have had a rat as an ancestor, but it will be something totally new, in a different ecological niche.

Q5: As global warming continues would an increase in global temperatures (and CO2) due to climate change result in animals getting bigger?

Q6: or would animals actually be likely to get smaller?


It’s impossible to generalise in such a way: What animals? Where? Over what timescales?

The size of an animal affects everything about it – where it can live, how it moves, what it can eat, how many offspring it can have, and how fast it can have them. This means that the size of an animal is a trade-off between all these things, and will evolve to best fit the ecological niche available to it.

Q7: Apart from the famous KT extinction, are there other examples of certain species/ groups filling an ecological niche after an extinction event?




Many. For example, the biggest mass-extinction event – the Permo-Triassic extinction, or the “Great Dying”, approx 250 million years ago – killed up to 97% of all life on Earth. It took up to 20 million years for life to recover in terms of species number and diversity – on land, some of the groups that were very successful were the Archosaurs, a group which includes the dinosaurs (including birds!) and crocodiles. The dinosaurs would go on to to diversify into a wide range of ecolgical niches.