Monar­chs and Milk­weed by ANURAG AGRAWAL

Cosmos - - Spectrum - — AN­DREW MASTER­SON

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.

Newspapers in English

Newspapers from Australia

© PressReader. All rights reserved.