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2018-05-05 16:11 三立在线 admin

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摘要:回想 3 月 10 日这场 SAT 考试,试题难度一度让很多考生担心,真正等考生走出考场,大家才发现这个担心是多余 , 下面我们一起来看看考情回忆吧。 文章原文: In the spring of 1879, Hermann Lau shot two white-winged choughs,Corcorax melanorhamphos, off their nest in Queensland, Australia. He watched asadditional choughs continued to attend the nest, proving that a cooperativegroup shared parent




In the spring of 1879, Hermann Lau shot two white-winged choughs,Corcorax melanorhamphos, off their nest in Queensland, Australia. He watched asadditional choughs continued to attend the nest, proving that a cooperativegroup shared parental care ( 1). Since then, cooperatively breeding birds havehad a starring role in efforts to explain the evolution of complex animalsocieties. We now know that helpersat-the-nest who forgo reproduction areoften relatives of the breeding pair. Genetic payoff is, thus, one of severaladvantages that helpers can gain from their super cially altruistic behavior (2). On page 1506 of this issue, Feeney et al. ( 3) show that collective defenseagainst brood parasites (see the gure) can enhance such bene ts ofcooperation. Why do some bird species cooperate and others do not? Globalanalyses have shown that cooperative breeding (now known from 9% of species) isassociated with a slow pace of life (characterized by high survival rates andlow turnover of breeding territories) ( 4), monogamy (which facilitates kinselection within families) ( 5), and unpredictable environments (such as aridzones) that might favor cooperation as a bet-hedging strategy ( 6). But thesefactors often fail to predict the incidence of cooperation among relatedspecies or within geographical regions ( 7). Feeney et al.s study is built onthe premise that brood parasitismreproductive cheating by species such ascuckoos and cowbirds, which exploit other birds to raise their youngis asevere selection pressure on their hosts breeding strategies. Parasitizedparents typically not only lose their current offspring but also waste a wholebreeding season raising a demanding impostor. The best way to avoid parasitismis to repel adult parasites from the nest. Feeney et al. show that socialitycan be pivotal to this process. The authors begin by unfolding a new map. Usingdata compiled by BirdLife International, they show that the global distributionof cooperatively breeding birds overlaps strikingly with that of broodparasites. This overlap need not re ect a causal relationship:

The same unpredictable environments thatfavor cooperation could also favor alternative breeding strategies such asparasitism. However, the authors go on to show that even within geographicalregions rich in both parasites and cooperatorsAustralia and southern Africacooperativebreeders are much more likely than noncooperative species to be targeted bybrood parasites. To determine the reasons for this correlation, Feeney et al.studied cooperative breeding in superb fairy-wrens (Malurus cyaneus) inAustralia. Horsfields bronze-cuckoos (Chalcites basalis) should bene t fromtargeting larger groups of fairy-wrens because more helpers mean faster chickgrowth. Yet, data from a 6-year field study show that in practice, cuckoosrarely experience this advantage, because larger groups of fairywrens much moreeffectively detect and repel egg-laying intrusions by cuckoo females,mobilizing group defenses with a cuckoospeci c alarm call. Thus, cooperationand parasitism could reciprocally in uence one another: Cooperators might bemore attractive targets because they make better foster parents, but once

exploited by parasites, they are alsobetter able to ght back, helping cooperation to persist ( 8). Feeney et al. nd that superior anticuckoo defenses in larger groups account for 0.2 moreyoung edged per season on average than smaller groupsa substantial boostgiven the fairy-wrens low annual fecundity. These results show convincinglythat defense against brood parasites augments the bene ts of helping, promotingthe persistence of cooperation. But as the authors note, they cannot revealwhat caused cooperation to evolve initially. Brood parasitism alone cannotresolve the question of why some birds breed cooperatively. For example,cooperative king shers and bee-eaters are heavily parasitized in Africa butnot in Australasia, showing that other advantages of helping behavior are sufcient for cooperation to persist. But we should take parasitism seriously as animportant force in a cooperative life. Indeed, it may provide a mechanismcontributing to the previously discovered global correlates of cooperation (46). Some insight into the likely order of evolution might come from furthercomparative predictions. For instance, if cooperation arose fi rst as a defenseagainst parasitism, cooperators may be most prevalent among hosts that relyheavily on repelling adult parasites, rather than on antiparasite strategies atlater reproductive stages, such as egg or chick discrimination ( 9). Incontrast, if parasites target existing cooperators because they providesuperior care, this should be especially true of parasites whose chicks havethe most pressing needsfor instance, those in parasitic families with largebody size relative to their hosts or those whose chicks do not kill host youngand therefore must share their foster parents care. Could there be a similarassociation between cooperation and parasitism among other highly socialanimals? Cooperation in mammals clearly persists irrespective of parasitism, giventhat there are no known brood-parasitic mammals (perhaps because it would bediffi cult for a mammal to insert live young into anothers care). Butrepelling parasitic egg-laying intrusions is crucial to many hosts of sociallyparasitic insects and has shaped sophisticated adaptations and counterdefensesfor and against brute force and secrecy ( 10). It will be fascinating toexplore how selection for antiparasitic defense has interacted with monogamyand defensible resources as forces favoring kin-selected cooperation ininvertebrates, touching on an active debate in evolutionary biology. Answers tosuch comparative questions will ultimately be limited by our knowledge ofnatural history. The work by Feeney et al. is testament to the evolutionaryinsights enabled by careful long-term fi eld studies, together with thecumulative legacy of those naturalists who made the unglamorous effort torecord and publish observations of real animals in real places.



























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