An Inordinate Fondness for Beetles

There is a story, possibly apocryphal, of the distinguished British biologist, J.B.S. Haldane, who found himself in the company of a group of theologians. On being asked what one could conclude as to the nature of the Creator from a study of his creation, Haldane is said to have answered, “An inordinate fondness for beetles.”

1959 May-June, The American Naturalist, “Homage to Santa Rosalia or Why Are There So Many Kinds of Animals?” by G. E. Hutchinson, Page 146, Volume XCIII, Number 870. (JSTOR)

Scientists have identified and named about 1.2 million living animal species on Earth. And about 400,000 of those living species are beetles. The quote attributed to Haldane is, if anything, a serious understatement.¹ There are some 8,000 species of dung beetles alone. How did it happen that a third of the world’s animals are from one single order of life, Coleoptera?²

Coleopterists – the scientists who study beetles – don’t have any single answer, but they can point to some possible reasons for the incredible speciation of beetles.

Beetles have been around a long time. A really long time. Perhaps 350 million years. That’s enough time to evolve plenty of different species. On the other hand, Coelacanth fish have been swimming in the ocean for approximately 360 million years, but reached a maximum of around 90 species before declining to the present two species known to be living today. So simply having been around a long time isn’t necessarily a ticket to speciation.

Another possibility is that beetles, which for the greater part are herbivores, began their speciation about the time an explosion of new flowering plant species spread across the Earth’s surface, colonizing many different habitats. Today, plants make up about 80 percent of the mass of Earth’s life. The chemical warfare between plants and the beetles that eat them might have further encouraged the speciation. But there were other insect species around when flowering plants evolved, but the insect species didn’t undergo a similarly extravagant speciation. So that doesn’t seem to be the reason, either.

Beetles survived two gigantic mass extinctions. While you and WC might point to the incredible loss of diversity in a mass extinction – about 8/9ths of insect life vanished in the Permian Extinction 252 million years ago – evolutionary biologists focus on the explosion of diversity following a mass extinction event. Beetles have been through two of those. But the paleontology of the Permian event is a muddle; a lot of insect life seems to have vanished in the Late Permian, before the mess extinction event, whatever caused it. So that’s a bit dubious, too.

The final theory is that beetles early on learned the advantage of symbiosis. Herbivores have a challenge: to succeed on plant-based diet, you have to find a way to break down the cellulose walls of plant cells, which enclose all the nourishing food. Beetles, early on, and possibly more than one time, incorporated bacteria species’ ability to break down plant cell walls, moving the ability into the beetles’ genetics. Other beetle species aded cellulose-munching bacteria to their intestinal flora. Still others abandoned leaf-eating entirely and moved to farming fungi, in effect letting the fungi do the hard work of breaking down the cellulose, and then eating the fungi.

Or perhaps it has been a combination of two or more of these factors that has allowed the Coleoptera to be so wildly successful.

Unless, of course, you’re a Biblical literalist and you think the Creator is simply inordinately fond of beetles.³

Note: WC apologizes to Derek Sikes and WC’s other entomologist friends for the gross simplifications in this blog post.

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¹ Quote Investigator is fairly certain that J.B.S. Haldane never actually said exactly that. But the late Stephen J. Gould discussed a letter in the August 1992 issue of The Linnean from a friend of Haldane named Kenneth Kermack who insists that Haldane did say “God has an inordinate fondness for beetles” and more often than not he said, “God has an inordinate fondness for stars and beetles.”
² That’s the total named and classified species. Some scientists estimate there could be 8.7 million living animal species and between 0.9 and 2.1 million total species of beetles.
³ Of course, then you have to explain how Noah crammed at least two of each of the 400,000 beetles on his ark. In addition to two each of the other 800,000 species of animal.