The inside story on adder sex

Originally published 24 February 1992

It is time to present the Sec­ond Annu­al Gullible Gull Award for the most bizarre sci­en­tif­ic research of the past year involv­ing exper­i­men­ta­tion with animals.

Com­pe­ti­tion was intense. The British were strong con­tenders, skulk­ing about in hedgerows doing odd exper­i­ments with bad­gers and cuck­oos. As usu­al, Scan­di­na­vian zool­o­gists spent the brief north­ern sum­mer inves­ti­gat­ing the sex lives of rein­deer and mos­qui­toes. Amer­i­cans devised inge­nious new ways to inflict psy­cho­log­i­cal con­fu­sion on the low­er primates.

May I have the enve­lope, please?

And the win­ners are — Thomas Mad­sen, Richard Shine, Jon Loman, and Thomas Håkans­son for their paper “Why do female adders cop­u­late so fre­quent­ly?,” pub­lished in the Jan­u­ary 30 [1992] issue of Nature.

But before I tell you about the win­ning research, let me remind you how the Gullible Gull Award got its name.

Intrepid research

Some years ago, Neal Grif­fith Smith of the Smith­son­ian Insti­tu­tion pub­lished the results of exper­i­ments with four species of gulls that co-inhab­it the rocky cliffs of Baf­fin Island, in the Cana­di­an Arctic.

The four kinds of gulls do not inter­breed, yet the only appar­ent dif­fer­ence between them is the col­or of the fleshy rings around the eyes. Is this how the birds rec­og­nize their own kind?

Smith cap­tured hun­dreds of gulls by lac­ing bait with drugs. Once the birds had been ren­dered immo­bile, he paint­ed their eye-rings the col­or of anoth­er species. Sure enough, the pho­ny col­ors fooled birds into choos­ing mates whose paint­ed eye-rings matched their own. Appar­ent­ly, it is eye-ring col­or alone that keeps the species distinct.

But how did the gulls know the col­ors of their own eye rings? Smith’s con­clu­sion: Since mir­rors are gen­er­al­ly absent on the cliffs of Baf­fin Island, gulls must “imprint” on their par­ents soon after birth, and there­after seek mates with eyes like mom and dad.

I named the Gullible Gull Award to hon­or this imag­i­na­tive zool­o­gist, who trav­eled the Arc­tic by dog sled and kayak, with paint box and sleep­ing potions, bring­ing the light of sci­ence to bear upon one more mys­tery of nature.

And now for our winners.

Mad­sen, Shine, Loman, and Håkans­son stud­ied the sex habits of adders liv­ing in grassy mead­ows of south­ern Swe­den. Adders are small ven­omous snakes wide­ly dis­trib­uted through­out Europe. The four sci­en­tists sought answers to a ques­tion that has long baf­fled evo­lu­tion­ists: Why are females of some ani­mal species — such as adders — promiscuous?

Cer­tain­ly, evo­lu­tion favors promis­cu­ous males. Accord­ing to good Dar­win­ian prin­ci­ples, males have every­thing to gain by cop­u­lat­ing with as many part­ners as pos­si­ble. They typ­i­cal­ly invest lit­tle ener­gy in rear­ing off­spring, and sperm is quick­ly replen­ished. By spread­ing their seed wide­ly males help ensure that their genes will reproduced.

Females, on the oth­er hand, spend weeks or months ges­tat­ing young, and must also bear most of the costs of rear­ing off­spring. A sin­gle well-timed cop­u­la­tion is gen­er­al­ly suf­fi­cient to obtain the sperm nec­es­sary for fer­til­iza­tion. It would seem females have noth­ing to gain from promiscuity.

A female adder typ­i­cal­ly has sev­er­al mates, although her first cop­u­la­tion pro­vides more than enough sperm to insem­i­nate all her ova. Is she sim­ply unable to reject the atten­tions of randy suit­ors? Or does she active­ly seek mul­ti­ple part­ners? And, if so, why?

Persistent study

For 10 years, our win­ning researchers have been pry­ing into the sex lives of adders with the earnest­ness of polit­i­cal reporters in an elec­tion year. All snakes in the study area were indi­vid­u­al­ly marked for ready iden­ti­fi­ca­tion, and repro­duc­tive females were force-fed with tiny radio trans­mit­ters so that their sex­u­al activ­i­ties could be mon­i­tored through­out the three-week mat­ing peri­od each spring.

The results? Lit­ter size and weight showed no cor­re­la­tion with the num­ber of mates cho­sen by female adders. Nor did the num­ber of times a female mat­ed affect the per­cent­age of her ova that were fer­til­ized. Promis­cu­ous females dif­fered from their less sex­u­al­ly active cousins in only one respect — they had more live births among their litters.

Our researchers con­clude: Female adders are dri­ven by evo­lu­tion to offer their bod­ies as are­nas for com­pet­ing sperm from sev­er­al males. The strongest sperm fer­til­ize the eggs, and the female is reward­ed with more live births and increased chances that her own genes will be reproduced.

The form of the sup­posed interuter­ine com­pe­ti­tion between sperm is not made clear. It is intrigu­ing to imag­ine them lash­ing about at each oth­er like angry kung-fu tad­poles, des­per­ate­ly assert­ing their self­ish claim on posterity.

Of course, alter­nate inter­pre­ta­tions of these obser­va­tions are pos­si­ble. But it is not for the the­o­ret­i­cal impli­ca­tions of this work that we present the Gullible Gull Award to Mad­sen, Shine, Loman, and Håkans­son. Rather, it is for the image of these intre­pid researchers stuff­ing radio trans­mit­ters down the throats of ven­omous vipers, then sit­ting down by their receivers to lis­ten in on knot­ty rites of ophid­i­an love.

The award-win­ning research rais­es a fur­ther inter­est­ing ques­tion for evo­lu­tion­ists: What repro­duc­tive ben­e­fits accrue to a species who pries elec­tron­i­cal­ly upon the slith­ery cop­u­la­tions of adders?

A round of applause, please, for our winners.