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Featured Articles in This Month’s Animal Behaviour
Animal Behaviour 80 (2010) 771–772
Contents lists available at ScienceDirect
Animal Behaviour journal homepage: www.elsevier.com/locate/anbehav
In Focus
Featured Articles in This Month’s Animal Behaviour
Polyandry and Its Discontents In many species of animals, females mate with more than one male during the course of a single reproductive attempt. One category of explanations for such polyandry is that females prefer to mate with multiple males because the costs of multiple mating are outweighed by benefits such as increased fertilization success, greater genetic diversity among the offspring, or additional material benefits provided by the males. According to a second category of hypotheses, females prefer to mate with just one male, but resisting mating attempts by additional males is too difficult or too costly for at least some females. In a paper in this issue (pp. 773–782), Sheri L. Johnson and H. Jane Brockmann of the University of Florida provide an incisive experimental test of these alternatives for horseshoe crabs. Female horseshoe crabs lay their eggs on sandy beaches at high tide. As a female approaches a beach, she is grasped from behind by a male, who maintains his hold as the female moves up the beach and thereby positions himself to fertilize the female’s eggs when she spawns. Additional, ‘satellite’ males are attracted to some mating pairs, and these males also try to position themselves so that they can shed sperm on the eggs as the female releases them. Previous work has shown that satellite males are able to fertilize a substantial proportion of the eggs. Johnson and Brockmann first examined the egg-laying behaviour of females that vary naturally in number of mates (Fig. 1). Multiply mated females (with a mean of 2.1 satellites) did not differ from singly mated females in time spent laying, number of clutches laid or total numbers of eggs produced. Results were very different, however, when the researchers experimentally manipulated numbers of mates. When satellite males were removed from polyandrous females, these females spent more time laying, laid more clutches, and laid more eggs overall than polyandrous female controls. When satellites were induced to join monandrous females, these females for the most part left the beach immediately, and on average spent less time laying and laid fewer clutches and fewer eggs than monandrous controls. Monandrous and polyandrous females differed in reproductive effort when mating status was determined by experimental manipulation, but not when mating status was determined naturally. This pattern suggests that naturally polyandrous females must differ from naturally monandrous ones in ways that obscure the effects of mating status per se, illustrating why experiments were important. The results of the two experiments indicate that female horseshoe crabs as a whole prefer to nest with a single male. This preference supports the hypotheses that explain polyandry as resulting from male–male competition rather than female choice.
Figure 1. A monandrous female horseshoe crab (below) with her one male and a polyandrous female (above) with five males. Photo: Sheri L. Johnson.
The preference suggests there may be a benefit to mating with only one male, but what that benefit might be is not yet clear. Johnson and Brockmann did not find any differences in fertilization success or larval development between the clutches of polyandrous and monandrous females, but it is still possible that the offspring of monandrous females have some advantage later in life. The authors also suggest that polyandrous mating may have some advantage or advantages for the subset of females that accept nesting with multiple males. William A. Searcy Executive Editor
Kea Make the Tool-using Grade The right tools for the job can make any task easier, and this is no different for the rest of the animal kingdom than it is for humans. The ability of other animals beside humans to use tools has long fascinated researchers because it provides a neat way to test how much other animals understand about the physical properties of objects, as well as probe their understanding of more abstract concepts, such as causality. It also allows us to test animals’ problem-solving skills by investigating the degree to which they can combine tools in a task and so gain access to a reward. To date, a lot of work in this area has been conducted on birds that are known to be behaviourally flexible, such as the members of the corvid family, especially those that naturally use tools in
0003-3472/$38.00 Ó 2010 The Association for the Study of Animal Behaviour. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.anbehav.2010.09.002
772
In Focus / Animal Behaviour 80 (2010) 771–772
the wild, such as the New Caledonian crow. This has naturally raised the question of whether the ability to use tools is a byproduct of some form of ‘general intelligence’ or whether tool use itself has acted as a selection pressure for greater intelligence. There has also been the suggestion that tool using may be an extension of the kinds of skills used in nest building, which also involves the manipulation of objects to achieve a particular goal. One way of getting at some of these issues is to investigate the tool-using skills of species that are not natural tool-users in the wild, and that do not build complex nest structures. In this month’s issue (pp. 783–789), Alice Auersperg, Gyula Gajdon and Ludwig Huber from the Department of Cognitive Biology in Vienna, investigate the tool-using abilities of large parrots from New Zealand that neither use tools nor build nests (they live in burrows instead). Specifically, Auersperg and her colleagues investigated the ability of kea to solve a ‘second-order’ tool use problem. They presented the birds with two Plexiglas tubes, one of which contained a food reward. The reward was secured in the middle of the tube by skewering it through the middle with a piece of spaghetti. To solve the task, the birds had to insert an object of the correct size and shape into one or other of the open ends of the baited tube, and then lift the tube up from the same end so that the object rolled down the tube, hit the reward and broke the spaghetti. The birds then had to lift the tube up so that the reward would roll out of the tube. The birds were given a choice of a small ball, a large ball and a small cube as objects, with the small ball most suitable for solving the task. The researchers presented the task to nine captive kea, seven of which inserted the correct object into the tube and obtained the reward on the very first trial. After 16 trials, all the birds were consistently responding correctly. In addition, despite showing an equal tendency to touch the large ball and the small ball during initial preference trials, the birds consistently chose the small ball to insert into the tube and break the spaghetti in their test trials. Auersperg and her colleagues then tried a variant of the spaghetti-breaking task. This time, they inserted a stopper into one end of the tube, approximately 1.5 cm from the end, so that, although it was still possible to insert an object into the tube at that end, doing so wouldn’t enable the object to break the spaghetti and so retrieve the reward. The task was therefore more complex to
Figure 2. Kea, a species of parrot from New Zealand, are able to solve complex ‘second-order’ tool-using tasks. Photo: Alice Auersperg.
solve because the reward could only be retrieved if the object was inserted into the nonstoppered end of the tube. Despite this extra complication, all birds quickly mastered the multistep procedure needed to retrieve the reward. These results are impressive because kea are the first noncorvid species that has been shown to master these complex second-order tool-using tasks (Fig. 2). As well as demonstrating that tool-using skills are not limited to the corvid family, the fact that kea live in burrows also reveals that nest-building ability is not crucial to success in tool-using tasks. Although they are not tool-users or nest-builders, kea do have relatively large brains for their body size (the parrots, like the corvids, are well known for being brainy), and their flexibility with respect to tool using suggests that, in their case at least, the ability reflects a more general capacity to behave in a flexible and ‘intelligent’ fashion, rather than a more domain-specific specialization. Louise Barrett 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
Polyandry and Its Discontents In many species of animals, females mate with more than one male during the course of a single reproductive attempt. One category of explanations for such polyandry is that females prefer to mate with multiple males because the costs of multiple mating are outweighed by benefits such as increased fertilization success, greater genetic diversity among the offspring, or additional material benefits provided by the males. According to a second category of hypotheses, females prefer to mate with just one male, but resisting mating attempts by additional males is too difficult or too costly for at least some females. In a paper in this issue (pp. 773–782), Sheri L. Johnson and H. Jane Brockmann of the University of Florida provide an incisive experimental test of these alternatives for horseshoe crabs. Female horseshoe crabs lay their eggs on sandy beaches at high tide. As a female approaches a beach, she is grasped from behind by a male, who maintains his hold as the female moves up the beach and thereby positions himself to fertilize the female’s eggs when she spawns. Additional, ‘satellite’ males are attracted to some mating pairs, and these males also try to position themselves so that they can shed sperm on the eggs as the female releases them. Previous work has shown that satellite males are able to fertilize a substantial proportion of the eggs. Johnson and Brockmann first examined the egg-laying behaviour of females that vary naturally in number of mates (Fig. 1). Multiply mated females (with a mean of 2.1 satellites) did not differ from singly mated females in time spent laying, number of clutches laid or total numbers of eggs produced. Results were very different, however, when the researchers experimentally manipulated numbers of mates. When satellite males were removed from polyandrous females, these females spent more time laying, laid more clutches, and laid more eggs overall than polyandrous female controls. When satellites were induced to join monandrous females, these females for the most part left the beach immediately, and on average spent less time laying and laid fewer clutches and fewer eggs than monandrous controls. Monandrous and polyandrous females differed in reproductive effort when mating status was determined by experimental manipulation, but not when mating status was determined naturally. This pattern suggests that naturally polyandrous females must differ from naturally monandrous ones in ways that obscure the effects of mating status per se, illustrating why experiments were important. The results of the two experiments indicate that female horseshoe crabs as a whole prefer to nest with a single male. This preference supports the hypotheses that explain polyandry as resulting from male–male competition rather than female choice.
Figure 1. A monandrous female horseshoe crab (below) with her one male and a polyandrous female (above) with five males. Photo: Sheri L. Johnson.
The preference suggests there may be a benefit to mating with only one male, but what that benefit might be is not yet clear. Johnson and Brockmann did not find any differences in fertilization success or larval development between the clutches of polyandrous and monandrous females, but it is still possible that the offspring of monandrous females have some advantage later in life. The authors also suggest that polyandrous mating may have some advantage or advantages for the subset of females that accept nesting with multiple males. William A. Searcy Executive Editor
Kea Make the Tool-using Grade The right tools for the job can make any task easier, and this is no different for the rest of the animal kingdom than it is for humans. The ability of other animals beside humans to use tools has long fascinated researchers because it provides a neat way to test how much other animals understand about the physical properties of objects, as well as probe their understanding of more abstract concepts, such as causality. It also allows us to test animals’ problem-solving skills by investigating the degree to which they can combine tools in a task and so gain access to a reward. To date, a lot of work in this area has been conducted on birds that are known to be behaviourally flexible, such as the members of the corvid family, especially those that naturally use tools in
0003-3472/$38.00 Ó 2010 The Association for the Study of Animal Behaviour. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.anbehav.2010.09.002
772
In Focus / Animal Behaviour 80 (2010) 771–772
the wild, such as the New Caledonian crow. This has naturally raised the question of whether the ability to use tools is a byproduct of some form of ‘general intelligence’ or whether tool use itself has acted as a selection pressure for greater intelligence. There has also been the suggestion that tool using may be an extension of the kinds of skills used in nest building, which also involves the manipulation of objects to achieve a particular goal. One way of getting at some of these issues is to investigate the tool-using skills of species that are not natural tool-users in the wild, and that do not build complex nest structures. In this month’s issue (pp. 783–789), Alice Auersperg, Gyula Gajdon and Ludwig Huber from the Department of Cognitive Biology in Vienna, investigate the tool-using abilities of large parrots from New Zealand that neither use tools nor build nests (they live in burrows instead). Specifically, Auersperg and her colleagues investigated the ability of kea to solve a ‘second-order’ tool use problem. They presented the birds with two Plexiglas tubes, one of which contained a food reward. The reward was secured in the middle of the tube by skewering it through the middle with a piece of spaghetti. To solve the task, the birds had to insert an object of the correct size and shape into one or other of the open ends of the baited tube, and then lift the tube up from the same end so that the object rolled down the tube, hit the reward and broke the spaghetti. The birds then had to lift the tube up so that the reward would roll out of the tube. The birds were given a choice of a small ball, a large ball and a small cube as objects, with the small ball most suitable for solving the task. The researchers presented the task to nine captive kea, seven of which inserted the correct object into the tube and obtained the reward on the very first trial. After 16 trials, all the birds were consistently responding correctly. In addition, despite showing an equal tendency to touch the large ball and the small ball during initial preference trials, the birds consistently chose the small ball to insert into the tube and break the spaghetti in their test trials. Auersperg and her colleagues then tried a variant of the spaghetti-breaking task. This time, they inserted a stopper into one end of the tube, approximately 1.5 cm from the end, so that, although it was still possible to insert an object into the tube at that end, doing so wouldn’t enable the object to break the spaghetti and so retrieve the reward. The task was therefore more complex to
Figure 2. Kea, a species of parrot from New Zealand, are able to solve complex ‘second-order’ tool-using tasks. Photo: Alice Auersperg.
solve because the reward could only be retrieved if the object was inserted into the nonstoppered end of the tube. Despite this extra complication, all birds quickly mastered the multistep procedure needed to retrieve the reward. These results are impressive because kea are the first noncorvid species that has been shown to master these complex second-order tool-using tasks (Fig. 2). As well as demonstrating that tool-using skills are not limited to the corvid family, the fact that kea live in burrows also reveals that nest-building ability is not crucial to success in tool-using tasks. Although they are not tool-users or nest-builders, kea do have relatively large brains for their body size (the parrots, like the corvids, are well known for being brainy), and their flexibility with respect to tool using suggests that, in their case at least, the ability reflects a more general capacity to behave in a flexible and ‘intelligent’ fashion, rather than a more domain-specific specialization. Louise Barrett Executive Editor