"Long before it's in the papers"
September 14, 2015


Birds “in love” found to be more successful breeders

Sept. 14, 2015
Courtesy of PLoS
and World Science staff

Ev­o­lu­tion bas­ic­ally di­rects us to mate so that we’ll have chil­dren. So, as Ti­na Turn­er asked, what’s love got to do with it? What con­crete ben­e­fits does love pro­vide that out­weigh the many and con­sid­er­able dif­fi­cul­ties that the search for love en­tails?

In short, why did evo­lu­tion create love?

Sci­en­tists have asked the ques­tion, but the nec­es­sary ex­pe­ri­ments to an­swer it, if done rig­or­ously on peo­ple, might lead to a night­mare in terms of sci­en­tif­ic eth­ics.

The zebra finch forms long-term pair bonds. (© Max Planck Inst. for Or­nith­o­logy)

So a team of re­search­ers opted in­stead to test out the mat­ter on zeb­ra finches, small Aus­tral­ian birds whose mat­ing prac­tices are sur­pris­ingly hu­man-like in some ways. The sci­en­tists di­vid­ed the finches in­to to groups cor­re­spond­ing—more or less—to mar­riages of free choice ver­sus ar­ranged mar­riages.

The re­ported re­sult: “lov­ing” pairings led to more sex, bet­ter pa­ren­tal care and ul­ti­mately more suc­cess­ful breed­ing. The find­ings are con­sist­ent with some stud­ies on the dif­fer­ences be­tween love-based and ar­ranged mar­riages in hu­man so­ci­e­ty, the in­ves­ti­ga­tors said.

Zeb­ra finches mate mo­nog­a­mously for life and share par­ent­ing du­ties. Fe­males choose mates in an indi­vidual-specific way, with little agree­ment among them as to who the cut­est male is. The re­search­ers—Malika Ihle and col­leagues at the Max Planck In­sti­tute for Or­nith­ol­o­gy, Seewiesen, German­y—used a popula­t­ion of 160 birds. They set up a speed-dating ses­sion, leav­ing groups of 20 females to choose freely be­tween 20 ma­les. 

Once the birds had paired off, half of the cou­ples were al­lowed to go off in­to a life of wed­ded bliss. For the oth­er half, how­ev­er, the au­thors in­ter­vened like over­bear­ing Vic­to­ri­an par­ents, split­ting up the hap­py pair, and forcibly pair­ing them with oth­er bro­ken-heart­ed in­di­vid­u­als.

Bird cou­ples, wheth­er hap­py or some­what dis­grun­tled, were then left to breed in avi­ar­ies, and the au­thors as­sessed cou­ples’ be­hav­ior and the num­ber and pa­tern­ity of dead em­bryos, dead chicks and sur­viv­ing off­spring.

The fi­nal num­ber of sur­viv­ing chicks was 37 per­cent high­er for in­di­vid­u­als in cho­sen pairs than those in non-cho­sen pairs, the re­search­ers said, in find­ings they re­ported Sept. 14 in the re­search jour­nal PLoS Biol­o­gy.

The nests of non-cho­sen pairs had al­most three times as many un­fer­ti­lized eggs as the cho­sen ones, they found. More­o­ver, a great­er num­ber of eggs were ei­ther bur­ied or lost, and markedly more chicks died af­ter hatch­ing. Most deaths oc­curred with­in the chicks’ first 48 hours, the in­ves­ti­ga­tors found, amid markedly less dil­i­gent fa­therly care.

The court­ships themselves showed no­tice­a­ble dif­fer­ences, the sci­en­tists said. 

Al­though non-cho­sen males paid the same amount of at­ten­tion to their mates as the cho­sen ones did, non-cho­sen females were far less re­cep­tive to their ad­vanc­es, and tended to cop­u­late less of­ten. An anal­y­sis of har­mo­ni­ous be­hav­ior con­clud­ed that non-cho­sen cou­ples were gen­er­ally sig­nif­i­cantly less lov­ey-dov­ey than the cho­sen ones. There was al­so more in­fi­del­ity in birds from non-cho­sen pairs—in­ter­est­ing­ly, more of it from the ma­les, and in­creas­ing over time.

Sound fa­mil­iar?

* * *

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Evolution basically directs us to mate so that we’ll have children. So, as Tina Turner asked, what’s love got to do with it? What concrete benefits does love provide that outweigh the many difficulties, to say the least, that the search for love entails? Scientists have asked the same question, but the necessary experiments, if done rigorously on people, might lead to a nightmare in terms of scientific ethics. So a team of researchers opted instead to test out the matter on zebra finches, small Australian birds whose mating practices are surprisingly human-like in some ways. The scientists divided the finches into to groups corresponding—more or less—to marriages of free choice versus arranged marriages. The reported result: “loving” marriages, if you can call it that, led to more copulation, better parental care and ultimately more successful breeding. The findings are consistent with some studies on the differences between love-based and arranged marriages in human society, the investigators said. Zebra finches mate monogamously for life and share parenting duties. Females choose mates in an individual-specific way, with no wide agreement among them as to who the cutest male is. The researchers—Malika Ihle and colleagues at the Max Planck Institute for Ornithology, Seewiesen, Germany—used a population of 160 birds. They set up a speed-dating session, leaving groups of 20 females to choose freely between 20 males. Once the birds had paired off, half of the couples were allowed to go off into a life of wedded bliss. For the other half, however, the authors intervened like overbearing Victorian parents, splitting up the happy pair, and forcibly pairing them with other broken-hearted individuals. Bird couples, whether happy or somewhat disgruntled, were then left to breed in aviaries, and the authors assessed couples’ behavior and the number and paternity of dead embryos, dead chicks and surviving offspring. The final number of surviving chicks was 37% higher for individuals in chosen pairs than those in non-chosen pairs, the researchers said, in findings they reported Sept. 14 in the research journal PLoS Biology. The nests of non-chosen pairs had almost three times as many unfertilized eggs as the chosen ones, they found. Moreover, a greater number of eggs were either buried or lost, and markedly more chicks died after hatching. Most deaths occurred within the chicks’ first 48 hours, the investigators found, amid markedly less diligent fatherly care. Watching the couples’ courtship showed some noticeable differences, the scientists said. Although non-chosen males paid the same amount of attention to their mates as the chosen ones did, non-chosen females were far less receptive to their advances, and tended to copulate less often. An analysis of harmonious behavior concluded that non-chosen couples were generally significantly less lovey-dovey than the chosen ones. There was also more infidelity in birds from non-chosen pairs—interestingly, more of it from the males, and increasing over time. Sound familiar?