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Animal Behaviour 80 (2010) 595–596

Contents lists avai

Animal Behaviour

journal homepage: www.elsevier .com/locate/anbehav

In Focus

Featured Articles in This Month’s Animal Behaviour

Figure 1. Zebra finches engage in discreet duets that help maintain their pair bonds.Photo: Simon Griffith.

Whispering Sweet Nothings

Who doesn’t love a good duet? From John Travolta and OliviaNewton-John declaring ‘You’re the one that I want’ to Marvin Gayeand Tammy Tyrell telling us that ‘It takes two’, duets are a powerfulway for people to convey their sentiments to theworld, as well as toeach other. Duets in the animal kingdom can do a similar job. Theycan serve as a form of collaborative display that enables males andfemales to defend resources or their territory. They can also helpsustainpair bonds bymaintaining contact betweenmale and femalepartners, synchronize and stimulate reproduction and advertisemating status. There is also the possibility that duets are related toconflicts between the sexes, and that they represent a form ofmate guarding ormatemanipulation. Given this variety of functionsand its widespread distribution across bird families (over 40%contain a species that duets), one would expect duetting to becommon, and yet only 3–4% of all bird species do so. But is thisnumber accurate? In this month’s issue (pp. 597–605), ClémentineVignal and colleagues Julie Elie, MylèneMariette, Hédi Soula, SimonGriffith andNicolasMathevon suggest duets couldbemorecommonthan we think. We just need to listen more carefully.

When it comes to duetting, most attention has been given tospecies that engage in complex and conspicuous vocal exchanges,but Vignal and colleagues argue that there could also be more‘private’ duets that perhaps go unnoticed by researchers. To inves-tigate this, theymonitored the quiet and unobtrusive calls male andfemale zebra finches make during reproduction (Fig. 1). Zebrafinches mate for life and are highly monogamous, making thema good test case to investigate more subtle forms of duetting.They engage in two types of calling sequence: meeting sequences,performed when one member of the pair returns to the nest, andsentinel sequences, produced when one mate is inside the nestand the other is outside, keeping watch for predators.

To qualify as a duet, male and female calls must display certaincharacteristics. When the researchers analysed the patterns of calls,they found that both meeting and sentinel sequences showed thehigh degree of precision timing that is distinctive of duets. Malesand females would take turns when calling, and did so muchmore frequently than expected by chance alone. The number ofcalls made by each partner in a given sequence was also positivelycorrelated, and the patterning of the intervals between the maleand female calls confirmed further that the sexes were alternatingtheir calls in a highly ordered sequence. In addition, Vignal andcolleagues found that meeting call sequences were more frequentwhen males and females had been separated for longer, whereassentinel call sequences were more common as separation timedecreased. A playback experiment, in which the calls of either

0003-3472/$38.00 2010 The Association for the Study of Animal Behaviour. Published bdoi:10.1016/j.anbehav.2010.08.003

a bird’s mate or nonmate were played to the bird in the nest whileits matewas away, found that the majority of birds respondedmoreto their mate’s calls. Taken together, these results suggest that duetcalls may contribute to mate recognition, help strengthen the pairbond and synchronize activities between mates.

Unlike other studies on duetting, which have found that maleand female vocal contributions differ, the present study found nosuch sex differences. Instead, different call sequences reflectedwhether a bird was inside or outside the nest. This suggests thatboth partners are involved in the function filled by duetting, againsupporting the idea that it reflects the coordination of their effortsduring breeding. These patterns also help to rule out explanationsbased on the idea of conflict between the sexes, while the quietnessand subtlety of the duets mean that explanations based on territorydefence are equally unlikely.

But why so quiet? Vignal and colleagues suggest that quiet duetsallow communication between a pair while reducing the chances of

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In Focus / Animal Behaviour 80 (2010) 595–596596

eavesdropping by other zebra finches, which would then be able tolocate the nest and parasitize it by laying their own eggs. Sentinelsequences, which also keep the lines of communication openwhen one bird is outside the nest, may help to reduce stress inthe partner that remains in the nest and cannot detect predators,and so increase the efficiency of behaviours such as incubation,brooding or feeding offspring.

These intriguing results raise the obvious question of whetherprivate duets are also found in other species. Perhaps duetting is,in fact, more common than we have been led to believe. As wehumans know from our own experience, a duet doesn’t need tobe a show-stopper to do its job, and a soft and subtle ballad canalso work wonders. It clearly works for zebra finches, and perhapsthe same is true for other birds as well.

Louise BarrettExecutive Editor

Figure 2. A male burying beetle confronted with a female forewing. Photo: J. K. Müller.

Recognizing Partners in Burying Beetles

Recognizing individuals or classes of individuals is a challengefaced by many animals. Social animals benefit from recognizingmembers of their own as opposed to other colonies, territorialanimals benefit from recognizing neighbouring territory ownersas opposed to strangers, parents benefit from recognizing theirown eggs as opposed to those of brood parasites, and so forth.Recognition can be accomplished by establishing a template thatdefines the characteristics of the desired category and comparingtarget individuals to that template, accepting those that exceedsome threshold of similarity. Theory suggests that the decisionrules for acceptance in the favoured category should be madeflexible, allowing adjustment to factors such as the frequency ofacceptable versus unacceptable targets in the environment. A paperin this issue by Sandra Steiger and Josef K. Müller (pp. 607–613)provides a particularly compelling test of these ideas, based onwork with burying beetles.

Burying beetles are named for their habit of burying thecarcasses of small vertebrates, which are then consumed by thedeveloping beetle larvae. A pair of burying beetles whose youngare on a carcass defend that carcass against other burying beetles,which may try to take over the carcass, killing the existing youngin the process. Only beetles lacking their own larvae representa threat to a carcass, and previous work has shown that parentsaccept the presence of any breeding beetle of the opposite sex,including unfamiliar individuals as well as their own mates.Discrimination is accomplished using chemical cues that differbetween breeding and nonbreeding adults.

Steiger and Müller test whether exposure to a strange beetleshifts the recognition rules followed by male beetles. In their firstexperiment, the researchers removed the female of a pair andplaced on the carcass the forewing of either the removed mate,a novel breeding female or a nonbreeding female (Fig. 2). Pairedmales were much more aggressive towards forewings ofnonbreeding females than towards forewings of their own matesor of novel breeding females, but did not discriminate betweenthe latter two categories. This result confirms that baseline discrim-ination is between the class of nonbreeding females and the class ofbreeding females.

In a second experiment, Steiger and Müller primed males byexposing them to forewings of either nonbreeding or breedingfemales. Males primed with a threatening stimulus (froma nonbreeding female) were subsequently more aggressivetowards forewings of novel breeding females than were malesprimed with the nonthreatening stimulus. In a third experiment,males primed with a threatening stimulus were more aggressivetowards forewings of novel breeding females than towards fore-wings of their own mates. Exposure to a threat thus shiftedmale behaviour from a baseline state in which they accepted allbreeding females (experiment 1) to a state in which they showedmore exacting discrimination, rejecting novel breeding females(experiment 2) and recognizing and accepting only their ownmates (experiment 3).

Theory suggests that recognition can be altered either by modi-fying the template used to judge targets or by shifting the thresholdused to judge whether a target is sufficiently similar to thetemplate. Steiger andMüller are able to show that even in the base-line condition in which all breeding females are accepted, maleburying beetles inspect novel breeding females longer than theyinspect their own mates. These results strongly suggest that themales are always able to distinguish their own mates from others,and that it is the threshold for acceptance that changes under threatrather than the nature of the template. The Steiger andMüller studythus gives insight not only into the adaptive flexibility of recogni-tion rules in burying beetles but also into the proximatemechanisms by which that flexibility is accomplished.

William A. SearcyExecutive Editor