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In Focus
ANIMAL BEHAVIOUR, 2007, 74, 359e361 doi:10.1016/j.anbehav.2007.06.003
IN FOCUS Featured Articles in This Month’s Animal Behaviour
Noise Pollution and the Breakdown of Family Values in Finches Asleep, I missed the spring dawn Birds singing everywhere woke me* Birds are indeed singing outside my window as I write this, but they compete with three lawnmowers and a leaf blower. How do animals that rely on sound for communicating with other members of their species cope with everincreasing levels of noise pollution? This critical question in conservation and behavioural biology is addressed in this issue (pp. 363e368). Swaddle & Page investigate how pair bonds in zebra finches (Fig. 1), which are maintained in part by vocal interchanges between the bonded birds, fare in an environment dominated by white noise. Zebra finches are normally monogamous, although field and laboratory studies have found that sometimes birds copulate outside their pair bond (about 3% of offspring in the field are sired by nonpair males). Swaddle & Page allowed finches to form pair bonds in a laboratory context, and then gave female finches choices between their mate and another male. During the choice trials, the birds were exposed to low-level ambient noise (45 dB, the noise level typical of a library or quiet office), moderate ambient noise (75 dB, like noise from a motorway), or very loud noise (90 dB, like a loud truck or power tool). Finch vocalizations are often in the 90-dB range. The authors counted courtship displays by the female and the males; these data yielded a measure of relative female preference for her pair-bonded mate or the other male. In the low-noise condition, females preferred their pairbonded male. However, female preferences for their mate deteriorated in the moderate noise condition and nearly disappeared under high ambient noise. A number of factors could explain this result, including an inability of females to recognize their mate because of the interference of the white noise with the male’s vocalizations, or that under difficult auditory conditions females shift to more reliance on visual signals in choosing their mate. With either explanation, the result is the potential for a higher
*(From Spring Morning by Meng Haoran, AD 689e740, translated by M. Breed.) 0003e 3472/07/$30.00/0
rate of copulations outside the pair bond, with potential impacts on the population genetics and social structure of this species. Human-generated ambient noise has the potential to affect mating behaviour and other social interactions in animals ranging from marine mammals to songbirds. This study is of particular interest because it points to specific behavioural disruptions caused by such noise. Michael Breed Executive Editor
She’s Leaving Home Leaving home is a big decision for young animals, whether they are baboons, bears or human ‘boomerang kids’ who can’t quite decide whether they’ve left home or not. In this month’s issue (pp. 369e376), Andreas Zedrosser and colleagues present the results of a 20-year study into patterns of natal dispersal among juvenile male and female brown bears in Scandinavia (Fig. 2). Female bears are particularly interesting in this regard because, unlike their North American counterparts, which rarely disperse from their natal ranges, around 32e46% of Scandinavian bears leave the area into which they were born. Zedrosser et al. compared bears in two parts of Scandinavia, over 600 km apart. In the northern study area, the bears had been exposed to more illegal hunting in spring, with the result that the sex ratio was more female biased than in the southern study area. This quasiexperimental set-up permitted a number of competing hypotheses about dispersal to be tested. Specifically, the authors investigated whether dispersal patterns were best explained by inbreeding avoidance, competition with other bears for space, mates and resources or, in the case of females, the ‘resident fitness’ hypothesis, which argues that, provided competition remains below the point at which daughters would impose a cost on the mother’s reproductive success, adult females may benefit from their maturing daughters remaining near their natal range, as this will tend to reduce competition with unrelated females. As expected, males were more likely to disperse than females (94% versus 41%), but the results also revealed that there was no difference in age of dispersal between
359 Ó 2007 The Association for the Study of Animal Behaviour. Published by Elsevier Ltd. All rights reserved.
360
ANIMAL BEHAVIOUR, 74, 3
Figure 1. Maleefemale pair of zebra finches and a nearby extrapair male. Photo: John Swaddle.
study areas, nor was there any difference in dispersal probability. These findings tend to support inbreeding avoidance as the main cause of dispersal among males; if males were attempting to avoid competition with other males, then one would expect dispersal rates to be reduced in the low-density area, and also that dispersal would occur at an older age. For females, the patterns of dispersal suggested something different. Here, it was found that smaller individuals
had a higher probability of dispersing, whereas the probability of dispersal was lowered for females with older mothers. There was also an interaction between these variables: the likelihood that smaller offspring would disperse was reduced in those with older mothers. This pattern of dispersal is consistent with the resident fitness hypothesis, as it suggests that female offspring compete among themselves to remain close to their mothers, with the result that the smaller, less competitive offspring
Figure 2. A young female brown bear from Dalarna, Sweden. Photo: Staffan Widstrand (www.staffanwidstrand.se).
IN FOCUS
are forced to disperse. The decline in female dispersal probability with maternal age (especially among smaller individuals) also points to resident fitness as the most likely explanation: daughters of older mothers may face less antagonism if they stay close by because older mothers have had time to form ‘matrilineal assemblages’ of females that are tolerant of each other. This in turn
suggests that females have the ability to discriminate between kin and nonkin. How they do this is, at present, a matter of speculation, although Zedrosser et al. suggest odour cues as a possible explanation. Louise Barrett Executive Editor
361
IN FOCUS Featured Articles in This Month’s Animal Behaviour
Noise Pollution and the Breakdown of Family Values in Finches Asleep, I missed the spring dawn Birds singing everywhere woke me* Birds are indeed singing outside my window as I write this, but they compete with three lawnmowers and a leaf blower. How do animals that rely on sound for communicating with other members of their species cope with everincreasing levels of noise pollution? This critical question in conservation and behavioural biology is addressed in this issue (pp. 363e368). Swaddle & Page investigate how pair bonds in zebra finches (Fig. 1), which are maintained in part by vocal interchanges between the bonded birds, fare in an environment dominated by white noise. Zebra finches are normally monogamous, although field and laboratory studies have found that sometimes birds copulate outside their pair bond (about 3% of offspring in the field are sired by nonpair males). Swaddle & Page allowed finches to form pair bonds in a laboratory context, and then gave female finches choices between their mate and another male. During the choice trials, the birds were exposed to low-level ambient noise (45 dB, the noise level typical of a library or quiet office), moderate ambient noise (75 dB, like noise from a motorway), or very loud noise (90 dB, like a loud truck or power tool). Finch vocalizations are often in the 90-dB range. The authors counted courtship displays by the female and the males; these data yielded a measure of relative female preference for her pair-bonded mate or the other male. In the low-noise condition, females preferred their pairbonded male. However, female preferences for their mate deteriorated in the moderate noise condition and nearly disappeared under high ambient noise. A number of factors could explain this result, including an inability of females to recognize their mate because of the interference of the white noise with the male’s vocalizations, or that under difficult auditory conditions females shift to more reliance on visual signals in choosing their mate. With either explanation, the result is the potential for a higher
*(From Spring Morning by Meng Haoran, AD 689e740, translated by M. Breed.) 0003e 3472/07/$30.00/0
rate of copulations outside the pair bond, with potential impacts on the population genetics and social structure of this species. Human-generated ambient noise has the potential to affect mating behaviour and other social interactions in animals ranging from marine mammals to songbirds. This study is of particular interest because it points to specific behavioural disruptions caused by such noise. Michael Breed Executive Editor
She’s Leaving Home Leaving home is a big decision for young animals, whether they are baboons, bears or human ‘boomerang kids’ who can’t quite decide whether they’ve left home or not. In this month’s issue (pp. 369e376), Andreas Zedrosser and colleagues present the results of a 20-year study into patterns of natal dispersal among juvenile male and female brown bears in Scandinavia (Fig. 2). Female bears are particularly interesting in this regard because, unlike their North American counterparts, which rarely disperse from their natal ranges, around 32e46% of Scandinavian bears leave the area into which they were born. Zedrosser et al. compared bears in two parts of Scandinavia, over 600 km apart. In the northern study area, the bears had been exposed to more illegal hunting in spring, with the result that the sex ratio was more female biased than in the southern study area. This quasiexperimental set-up permitted a number of competing hypotheses about dispersal to be tested. Specifically, the authors investigated whether dispersal patterns were best explained by inbreeding avoidance, competition with other bears for space, mates and resources or, in the case of females, the ‘resident fitness’ hypothesis, which argues that, provided competition remains below the point at which daughters would impose a cost on the mother’s reproductive success, adult females may benefit from their maturing daughters remaining near their natal range, as this will tend to reduce competition with unrelated females. As expected, males were more likely to disperse than females (94% versus 41%), but the results also revealed that there was no difference in age of dispersal between
359 Ó 2007 The Association for the Study of Animal Behaviour. Published by Elsevier Ltd. All rights reserved.
360
ANIMAL BEHAVIOUR, 74, 3
Figure 1. Maleefemale pair of zebra finches and a nearby extrapair male. Photo: John Swaddle.
study areas, nor was there any difference in dispersal probability. These findings tend to support inbreeding avoidance as the main cause of dispersal among males; if males were attempting to avoid competition with other males, then one would expect dispersal rates to be reduced in the low-density area, and also that dispersal would occur at an older age. For females, the patterns of dispersal suggested something different. Here, it was found that smaller individuals
had a higher probability of dispersing, whereas the probability of dispersal was lowered for females with older mothers. There was also an interaction between these variables: the likelihood that smaller offspring would disperse was reduced in those with older mothers. This pattern of dispersal is consistent with the resident fitness hypothesis, as it suggests that female offspring compete among themselves to remain close to their mothers, with the result that the smaller, less competitive offspring
Figure 2. A young female brown bear from Dalarna, Sweden. Photo: Staffan Widstrand (www.staffanwidstrand.se).
IN FOCUS
are forced to disperse. The decline in female dispersal probability with maternal age (especially among smaller individuals) also points to resident fitness as the most likely explanation: daughters of older mothers may face less antagonism if they stay close by because older mothers have had time to form ‘matrilineal assemblages’ of females that are tolerant of each other. This in turn
suggests that females have the ability to discriminate between kin and nonkin. How they do this is, at present, a matter of speculation, although Zedrosser et al. suggest odour cues as a possible explanation. Louise Barrett Executive Editor
361