Monarchs and Milkweed by ANURAG AGRAWAL
Princeton University Press (2017) RRP $29.95
FLITTING ABOUT with its bright orange wings, the Monarch butterfly has often been depicted as a poster-insect for peace, love and happiness. This, as it turns out, is a spectacularly inappropriate characterisation. For millennia the Monarch ( Danaus plexippus) has been locked in an arms race with a toxic plant called milkweed ( Asclepias syriaca).
The martial terminology is appropriate. As Cornell University entomologist Anurag Agrawal reveals in this entertaining book, we’re not talking symbiosis here. The milkweed gains nothing from the association – no pollination, nothing – while the butterfly gains nutrition, a nursery and a very effective defence mechanism.
In this highly detailed case study of coevolution, Agrawal traces the history of research into relationship between plant and insect, revealing the sometimes fanatical devotion of the scientists who have made it their speciality.
Key to the coevolutionary tussle are cardenolides – a type of poisonous steroid produced by the milkweed. Very few insects feed on the plant because of its toxicity, and the Monarch has very few natural predators either – its brightly coloured wings are a textbook example of a wildlife warning signal.
For much of the 20th century a central question for entomologists was whether the plant and the butterfly employed the same toxin, and if so, how.
In the 1960s, an Oxford graduate student succeeded in isolating cardenolides in Monarch pupae and adults. He then demonstrated, writes Agrawal with evident if understated delight, “that they had toxic effects on frog hearts, guinea pig intestines, the blood pressure of cats, the enzymatic activity of human blood cells, and they also caused starlings to vomit”.
He follows this up by detailing further research that established that the butterflies acquired their steroids directly from the milkweed – illustrated by possibly the world’s only photograph of a blue jay throwing up.
Agrawal is a scientist first and a writer second, but his style is for the most part direct and approachable. And these loving digressions into the habits and passions of the generations of researchers save the book from becoming too dry for nonspecialist readers, and there are passages of fascinating illustrative detail. These range from the rivalry between competing scientists in the thrall of the Monarch, to highly specific descriptions of the “ritual assessment” made by female Monarchs of the potential egg-sustaining properties of fresh milkweed plants.
Monarchs and Milkweed maintains its focus on this single coevolutionary interaction with unwavering concentration. In doing so, however, it serves a greater purpose: identifying the general principles that pertain in any and all evolutionary relationships.
The author is clear and eloquent as he describes the idea of trade-offs, the concept that each optimised trait – be that for defence or fecundity or nutrient conservation – comes at a cost and involves a proportionate downgrading of some other potentially beneficial ability.
It is in this constantly shifting set of checks and balances that the war between Monarchs and milkweed has been fought for millions of years. And it is here, too, that Agrawal finds a professional delight that he is generous in sharing.