This would have been a very short book if it had stuck to its basic premise: burrowing helps animals survive by providing refuge from predators and climate. Fortunately, it expands its scope a bit, spending a lot of time discussing evolution and the factors that were most likely to have helped creatures survive the great mass extinctions. Not surprisingly, there is evidence, some of it indirect, to support burrowing as a factor in survival in the Permian-Triassic extinction (252 million years ago (Ma)), the Triassic-Jurassic event (201 Ma) and the Cretaceous–Paleogene event (66 Ma).
The Cambrian Explosion, which started about 540 Ma and lasted for around 20 million years, is most notable because some of its fossil sites, such as the Burgess Shale, preserved impressions of soft-bodied creatures. However, it also shows clear evidence that predator-prey relationships had already matured, as teeth and claws were matched against shells and speed. This book succinctly describes the impetus behind this, saying “The biggest difference in the coevolution of predator-prey relations ... is that if a predator fails, it just goes hungry, whereas if the prey animal fails, it dies. This inequity means that prey have greater selection pressure applied to them than predators, as predators can still pass on their genes while malnourished, but dead prey cannot.” (p. 205)
The author, Anthony Martin, is a paleontologist, geologist, and ichnologist, the latter a term I had never heard before, which is the study of the traces animals leave behind. For the very earliest lifeforms, the only evidence of their existence is fossilized burrows, but ichnology is applied even to modern creatures in the study of how they modify their environments. A well known example would be earthworms, which collectively turn over, aerate, and fertilize soil in great abundance. Charles Darwin was so impressed by their abilities that he wrote a book about them.
The author makes the point that humans, too, are burrowers. There are over 200 underground villages and towns carved into the soft rock of Cappadocia, now part of eastern Turkey, the largest having a dozen levels, with sophisticated ventilation systems and capable of protecting thousands of people in times of danger. In the nuclear age modern humans discovered the benefits of hiding underground should a global annihilation event take place, with mountains hollowed out to provide military command and control for whatever was left of the country, and of course to save the precious asses of Congressmen.
There is an entire chapter devoted to an animal I had never heard of, but which is a star of the ichnologist community. The gopher tortoise is a master tunneler, digging huge, elaborate burrows which become their own ecosystems, providing living accommodations for up to 400 different kinds of snakes, lizards, insects, and the occasional small mammal. The amount of soil they excavate is remarkable, and they are true engineers of the animal world, altering the environment both above and below ground.
It is now generally accepted that some dinosaur species used burrows, and the author of this book played a role in uncovering and documenting that fact, although it was initially so far beyond standard paleontological thinking that he had difficulty getting his findings published. Using that idea as a jumping off place, he discusses how burrows might have provided critical shelter during mass extinctions. The generally accepted idea regarding them is that nothing larger than a cat managed to squeeze past the great die-offs and into the next era. All of the great extinction events were catastrophic, but the one at the boundary of the Permian-Triassic 252 Ma was as close as life ever came to perishing, with 95% of species vanishing. To bolster his argument he has an interesting chapter about how life recovered after the devastation of the Mount St. Helens explosion in 1980. Burrowing animals survived in even the most hard-hit areas, and their burrows served as shelters for other animals and plants to re-establish themselves.
So, lots of critters make burrows: crocodiles, turtles, birds, lungfish, amphibians, earthworms, insects, crustaceans, and mammals, and the book spends time on each of them, describing their special adaptations and how they modify their surroundings. For me most of this was moderately interesting, but since I have an interest in evolution, the best parts of the book were the discussions of ancient environment and the strategies animals used to pass along their genes.
The author is a professor at Emory University in Atlanta, Georgia, and I’m sure he is very popular with his students. His writing style is casual, even conversational, but sometimes he strains to make a joke. Two examples from the book:
“cute little alligator babies easily fit on the palm of an average adult human hand when newly hatched. Only later do they grow up to become monsters – much like how human children eventually turn into teenagers.” (p. 8)
“From its shoulder to its front toes, stout bones reinforced what must have been massive musculature and connective tissues. These appendages were not just made for what wildlife biologists call the thee “F’s” – fleeing, fighting, and mating – but also for digging.” (p. 130)
Okay, I don’t want to be too much of a wet blanket, but I could have done without the stand-up comedy bits. Overall I enjoyed the book. It taught me about animal behaviors I was unaware of, and helped reinforce some facts I knew about early evolution.