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Featured Articles in This Month’s Animal Behaviour
Animal Behaviour 82 (2011) 175–176
Contents lists available at ScienceDirect
Animal Behaviour journal homepage: www.elsevier.com/locate/anbehav
In Focus
Featured Articles in This Month’s Animal Behaviour
The Role of Experience in Nest Building I expect you have encountered a spider’s web, a bird’s nest or an ant mound and admired the sophistication of animal architecture. Animals of diverse species build structures to protect themselves from predators and the elements, to catch prey or to communicate information. Animal architecture amazes us with its shapes and textures, but the most fascinating of all are the decisions involved in building something that could last and be functional. Where to build? On what surface? What materials to use? An interesting issue is whether learning plays a part in making such decisions. Individuals who are capable of making a choice informed by experience are more likely to protect themselves, survive for longer and have more offspring. For example, birds are more likely to return to habitats or particular nest sites if their breeding experience there was successful and less likely to return if their breeding experience was unsuccessful. Despite the evidence that breeding success affects the habitat birds choose for building a nest, the question whether it also affects the building materials birds choose has not been addressed. Felicity Muth and Susan Healy of the University of St Andrews in the U.K. address this interesting question for the first time in the present issue (pp. 185–189). Muth and Healy carried out their experiments on zebra finches, a bird species native to Australia, in which it is the males who take building material to the nest, even though both males and females manipulate building material already brought to the nest. Two tests were carried out. The aim of the first test was to determine whether male zebra finches prefer green or brown nest material in the form of hay and coconut fibres. Birds were presented with equitable access to both fibre colours. Their choices were filmed until each male had taken at least 10 pieces of material to add to the nest. All 35 males included in this experiment, except one, showed a preference for one of the two colours of nest material (Fig. 1). The aim of the second test was to determine whether breeding experience influences the choice of nest material. Half of the breeding pairs were provided with the nest material preferred by the male in the first test and the other half were provided with the male’s nonpreferred nest material, ensuring that each fibre colour preference was equitably represented in these two groups. To manipulate breeding success, Muth and Healy then allowed half of the pairs in the group that built with the preferred colour of nest material and half of the pairs in the group that built with the nonpreferred colour of nest material to incubate their eggs, fledge chicks and care for them. This was the ‘successful breeding experience’ treatment. The other pairs were allowed to incubate
Figure 1. A zebra finch in a nest of predominantly green building material. Photo: Felicity Muth.
eggs for 6–7 days and then the eggs were removed. This was the ‘unsuccessful breeding experience’ treatment. Four weeks after these treatments were completed, the pairs involved were reunited and presented with green and brown fibres as nest-building material. The preference of the males was recorded in the same way as in the first test. The results from the second test showed that males who were allowed to use their preferred colour of nest material and had a successful breeding experience did not change significantly the number of strands they chose from their preferred colour. At the other extreme, males who had to build with their nonpreferred colour of nest material and had an unsuccessful breeding experience also did not change significantly the number of strands they chose from their preferred colour. Both of these outcomes are compatible with the hypothesis that zebra finches can adjust their behaviour in light of experience. However, neither of them leads to any change in behaviour. Therefore, the most powerful results are from the remaining two treatment groups where, according to the hypothesis that zebra finches can associate the type of nest material with their breeding success, there should be a change in behaviour. Thus, males who were allowed to build with their preferred colour of nest material but had an unsuccessful breeding experience would be expected to reduce their preference for that colour. This was not the case. The number of strands these males chose from their preferred colour did not change significantly after
0003-3472/$38.00 Ó 2011 The Association for the Study of Animal Behaviour. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.anbehav.2011.06.001
176
In Focus / Animal Behaviour 82 (2011) 175–176
their poor reproductive experience. In the final group, where males had to build with their nonpreferred colour of nest material but had a successful breeding experience, the learning hypothesis predicts that they will switch their preference to the initially nonpreferred colour. This was indeed the case. So, Muth and Healy have demonstrated clearly that male zebra finches can learn to associate the type of nest material with their breeding experience. But why was there no evidence that birds reduce their preference when they have an unsuccessful breeding experience? Well, the role of learning in the building behaviour of zebra finches might be even more sophisticated than initially hypothesized. Muth and Healy’s results suggest that such learning might be context dependent; in other words, zebra finches might learn differently under different circumstances. For example, unsuccessful breeding attempts are common in nature. Therefore, the selection pressure for abandoning a preference for a particular type of nest-building material after a single unsuccessful breeding attempt might be weak. The study by Muth and Healy demonstrates that learning and memory play a greater role in nest building in birds than has been assumed. Indeed, it begs the question whether this is also the case in other species and opens the way for many new experiments. Ana Sendova-Franks Executive Editor
Performing Mice When a human singer performs, we all have some feel for what aspects of the performance are difficult and therefore worthy of our admiration: hitting and sustaining high notes, for example. When other animals sing, by contrast, it is not always obvious to us what is difficult for them to do, and thus what their fellows might use to judge their performance. If we define song, somewhat loosely, as a long, complex vocal display, then many animals produce song: birds, of course, plus frogs and toads, some whales, and insects such as crickets and cicadas. To this list we may add Neotropical singing mice (Fig. 2), whose performance abilities are described in an article in this issue by Bret Pasch, Andreas S. George, Polly Campbell and Steven M. Phelps of the University of Florida (pp. 177–183). In birds, the biomechanics of song production make it difficult to repeat rapidly syllables that span a large range of frequencies. Therefore, when the frequency bandwidths of songs are plotted against their trill rates, a triangular distribution emerges, with no entries in the upper right quadrant representing the combination of very wide bandwidths and very fast trill rates. In Neotropical singing mice, males produce a trilled vocalization, and Pasch and colleagues demonstrate that the songs of 102 males in their Costa Rican and Panamanian study populations fall into the same kind of triangular distribution as found in birds. When the mouse songs are binned into trill rate categories, maximum bandwidth per bin decreases as trill rate increases. The trade-off between bandwidth and trill rate thus produces a performance limit, which can be estimated by an upper bound regression, a line that passes through the maximum bandwidths in each trill rate category. Songs near the
Figure 2. An adult male Alston’s singing mouse vocalizes at a site in Costa Rica. Photo: Bret Pasch.
upper bound are high performance, whereas those far from the limit are low performance. Pasch et al. go on to investigate the effects of androgens on this measure of song performance. Androgens such as testosterone and DHT (dihydrotestosterone) are known to stimulate the development of a variety of sexual display characters, but their effects on song performance have not previously been tested. Pasch and colleagues castrated a sample of male mice, and then treated them with either an empty implant or an implant containing testosterone or DHT. Songs of males in the empty implant group declined in performance following castration, while those in the androgen groups did not. Consequently, androgen-treated males tended to have higher performance levels after castration than the controls. Finally, Pasch and colleagues tested whether female mice prefer songs of higher performance. Performance of a sample of songs was manipulated by shortening the silent gaps between syllables, producing songs of increased trill rate and thus increased performance. Reproductively experienced females were given a choice of approaching either an unmanipulated control song or the corresponding experimental song of heightened performance. Females overall showed a strong preference for the high-performance songs, spending more than triple as much time near those songs as near the control songs. The higher the performance of the experimental song, the stronger was the female preference for it. The study thus shows that a particular vocal performance limit, first found in birds, also operates in Neotropical singing mice, and that this aspect of vocal performance is important to female preferences in the mice, again as in birds. The study is the first to demonstrate that androgens affect vocal performance, and it will be interesting to see whether this conclusion also applies to birds. More globally, the study adds further support to the emerging generalization that females often judge males on aspects of motor performance. William A. Searcy Executive Editor
Contents lists available at ScienceDirect
Animal Behaviour journal homepage: www.elsevier.com/locate/anbehav
In Focus
Featured Articles in This Month’s Animal Behaviour
The Role of Experience in Nest Building I expect you have encountered a spider’s web, a bird’s nest or an ant mound and admired the sophistication of animal architecture. Animals of diverse species build structures to protect themselves from predators and the elements, to catch prey or to communicate information. Animal architecture amazes us with its shapes and textures, but the most fascinating of all are the decisions involved in building something that could last and be functional. Where to build? On what surface? What materials to use? An interesting issue is whether learning plays a part in making such decisions. Individuals who are capable of making a choice informed by experience are more likely to protect themselves, survive for longer and have more offspring. For example, birds are more likely to return to habitats or particular nest sites if their breeding experience there was successful and less likely to return if their breeding experience was unsuccessful. Despite the evidence that breeding success affects the habitat birds choose for building a nest, the question whether it also affects the building materials birds choose has not been addressed. Felicity Muth and Susan Healy of the University of St Andrews in the U.K. address this interesting question for the first time in the present issue (pp. 185–189). Muth and Healy carried out their experiments on zebra finches, a bird species native to Australia, in which it is the males who take building material to the nest, even though both males and females manipulate building material already brought to the nest. Two tests were carried out. The aim of the first test was to determine whether male zebra finches prefer green or brown nest material in the form of hay and coconut fibres. Birds were presented with equitable access to both fibre colours. Their choices were filmed until each male had taken at least 10 pieces of material to add to the nest. All 35 males included in this experiment, except one, showed a preference for one of the two colours of nest material (Fig. 1). The aim of the second test was to determine whether breeding experience influences the choice of nest material. Half of the breeding pairs were provided with the nest material preferred by the male in the first test and the other half were provided with the male’s nonpreferred nest material, ensuring that each fibre colour preference was equitably represented in these two groups. To manipulate breeding success, Muth and Healy then allowed half of the pairs in the group that built with the preferred colour of nest material and half of the pairs in the group that built with the nonpreferred colour of nest material to incubate their eggs, fledge chicks and care for them. This was the ‘successful breeding experience’ treatment. The other pairs were allowed to incubate
Figure 1. A zebra finch in a nest of predominantly green building material. Photo: Felicity Muth.
eggs for 6–7 days and then the eggs were removed. This was the ‘unsuccessful breeding experience’ treatment. Four weeks after these treatments were completed, the pairs involved were reunited and presented with green and brown fibres as nest-building material. The preference of the males was recorded in the same way as in the first test. The results from the second test showed that males who were allowed to use their preferred colour of nest material and had a successful breeding experience did not change significantly the number of strands they chose from their preferred colour. At the other extreme, males who had to build with their nonpreferred colour of nest material and had an unsuccessful breeding experience also did not change significantly the number of strands they chose from their preferred colour. Both of these outcomes are compatible with the hypothesis that zebra finches can adjust their behaviour in light of experience. However, neither of them leads to any change in behaviour. Therefore, the most powerful results are from the remaining two treatment groups where, according to the hypothesis that zebra finches can associate the type of nest material with their breeding success, there should be a change in behaviour. Thus, males who were allowed to build with their preferred colour of nest material but had an unsuccessful breeding experience would be expected to reduce their preference for that colour. This was not the case. The number of strands these males chose from their preferred colour did not change significantly after
0003-3472/$38.00 Ó 2011 The Association for the Study of Animal Behaviour. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.anbehav.2011.06.001
176
In Focus / Animal Behaviour 82 (2011) 175–176
their poor reproductive experience. In the final group, where males had to build with their nonpreferred colour of nest material but had a successful breeding experience, the learning hypothesis predicts that they will switch their preference to the initially nonpreferred colour. This was indeed the case. So, Muth and Healy have demonstrated clearly that male zebra finches can learn to associate the type of nest material with their breeding experience. But why was there no evidence that birds reduce their preference when they have an unsuccessful breeding experience? Well, the role of learning in the building behaviour of zebra finches might be even more sophisticated than initially hypothesized. Muth and Healy’s results suggest that such learning might be context dependent; in other words, zebra finches might learn differently under different circumstances. For example, unsuccessful breeding attempts are common in nature. Therefore, the selection pressure for abandoning a preference for a particular type of nest-building material after a single unsuccessful breeding attempt might be weak. The study by Muth and Healy demonstrates that learning and memory play a greater role in nest building in birds than has been assumed. Indeed, it begs the question whether this is also the case in other species and opens the way for many new experiments. Ana Sendova-Franks Executive Editor
Performing Mice When a human singer performs, we all have some feel for what aspects of the performance are difficult and therefore worthy of our admiration: hitting and sustaining high notes, for example. When other animals sing, by contrast, it is not always obvious to us what is difficult for them to do, and thus what their fellows might use to judge their performance. If we define song, somewhat loosely, as a long, complex vocal display, then many animals produce song: birds, of course, plus frogs and toads, some whales, and insects such as crickets and cicadas. To this list we may add Neotropical singing mice (Fig. 2), whose performance abilities are described in an article in this issue by Bret Pasch, Andreas S. George, Polly Campbell and Steven M. Phelps of the University of Florida (pp. 177–183). In birds, the biomechanics of song production make it difficult to repeat rapidly syllables that span a large range of frequencies. Therefore, when the frequency bandwidths of songs are plotted against their trill rates, a triangular distribution emerges, with no entries in the upper right quadrant representing the combination of very wide bandwidths and very fast trill rates. In Neotropical singing mice, males produce a trilled vocalization, and Pasch and colleagues demonstrate that the songs of 102 males in their Costa Rican and Panamanian study populations fall into the same kind of triangular distribution as found in birds. When the mouse songs are binned into trill rate categories, maximum bandwidth per bin decreases as trill rate increases. The trade-off between bandwidth and trill rate thus produces a performance limit, which can be estimated by an upper bound regression, a line that passes through the maximum bandwidths in each trill rate category. Songs near the
Figure 2. An adult male Alston’s singing mouse vocalizes at a site in Costa Rica. Photo: Bret Pasch.
upper bound are high performance, whereas those far from the limit are low performance. Pasch et al. go on to investigate the effects of androgens on this measure of song performance. Androgens such as testosterone and DHT (dihydrotestosterone) are known to stimulate the development of a variety of sexual display characters, but their effects on song performance have not previously been tested. Pasch and colleagues castrated a sample of male mice, and then treated them with either an empty implant or an implant containing testosterone or DHT. Songs of males in the empty implant group declined in performance following castration, while those in the androgen groups did not. Consequently, androgen-treated males tended to have higher performance levels after castration than the controls. Finally, Pasch and colleagues tested whether female mice prefer songs of higher performance. Performance of a sample of songs was manipulated by shortening the silent gaps between syllables, producing songs of increased trill rate and thus increased performance. Reproductively experienced females were given a choice of approaching either an unmanipulated control song or the corresponding experimental song of heightened performance. Females overall showed a strong preference for the high-performance songs, spending more than triple as much time near those songs as near the control songs. The higher the performance of the experimental song, the stronger was the female preference for it. The study thus shows that a particular vocal performance limit, first found in birds, also operates in Neotropical singing mice, and that this aspect of vocal performance is important to female preferences in the mice, again as in birds. The study is the first to demonstrate that androgens affect vocal performance, and it will be interesting to see whether this conclusion also applies to birds. More globally, the study adds further support to the emerging generalization that females often judge males on aspects of motor performance. William A. Searcy Executive Editor