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Social foraging in cliff swallows revisited
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Animal Behaviour, 39, 6
References Brown, C. R. 1988. Social foraging in cliffswallows:local enhancement, risk sensitivity, competition and the avoidance of predators. Anita. Behav., 36, 780 792. Milinski,M. 1988.Games fishplay: making decisionsas a social forager. Trends Ecol. Evol., 3, 325-330. Real, L. & Caraco, T. 1986. Risk and foraging in stochastic environments. A. Rev. Ecol. Syst., 17, 371-390. Stephens, D. W. & Krebs, J. R. 1986. Foraging Theory. Princeton, New Jersey: Princeton University Press. Wittenberger, J. F. & Hunt, G. J., Jr. 1985. The adaptive significance of coloniality in birds. In: Avian Biology. Vol. 8 (Ed. by D. S. Farner, J. R. King & K. C. Parkes), pp. 1--78.New York: Academic Press. (Received20 March 1989; initial acceptance 30 May 1989; final acceptance 21 June 1989; MS. number: ,4s-615)
Social Foraging in Cliff Swallows Revisited Getty (1990) and Templeton & Giraldeau (1990) recently criticized conclusions drawn in my study of the foraging behaviour of cliff swallows, Hirundo pyrrhonota (Brown 1988). The most important criticism that they offer is simply that I did not provide evidence that cliff swallows are risksensitive foragers. Getty describes a 'rate maximization scenario', which he argues is at least an equally plausible interpretation and one that does not require risk sensitivity on the part of these animals. Both of these critiques, however, have overlooked the fact that I (Brown 1988) did not claim that cliff swallows are risk-sensitive. I suggested only that the data were consistent with risk-sensitive models, which is clearly the case, and that cliff swallows might be likely candidates in which to look for risk sensitivity. A general prediction of risk-sensitive models (e.g. Caraco 1981) is that the rate of food encounter should increase, and the variance of food encounter should decrease, with increasing flock size. The cliff swallow data support that prediction per se (Brown 1988). The data are also consistent with Getty's more complicated rate maximizing scenario, and I agree with most of Getty's remarks. However, it is not clear to me how (if at all) Getty's scenario and risk sensitivity could be operationally distinguished in the field with cliff swallows. I appreciate Getty's alternative interpretation, but his criticism of me for claiming that cliff swallows are risk-sensitive is unwarranted. I pointed out (Brown 1988) that we do not as yet have any information on the condition or status of different foragers, which would be essential before any robust claim of risk sensitivity could be made.
My report that cliff swallows exhibit both riskprone and risk-averse foraging strategies was questioned by Getty (1990) and Templeton & Giraldeau (1990). Templeton & Giraldeau cite Caraco (1981) in arguing that mere presence in a flock or as a solitary forager cannot be equated with risk-aversiveness or risk-proneness, respectively. While their argument can hold in certain circumstances, most mathematical models do predict that flocks are associated with reduced foraging risk and solitaries with increased foraging risk (e.g. Caraco 1981; Clark & Mangel 1984). When the observed variance in foraging success declines strongly with group size as predicted by the models (see Fig. 2 in Brown 1988) and when individuals consistently forage in groups of particular sizes (a point ignored by Templeton & Giraldeau), one would need a compelling reason not to view this as a possible example of a risk-sensitive strategy. Templeton & Giraldeau provide no reason why presence in a group of a given size might not equate directly to risk in the case of cliff swallows. Getty's rate maximizing scenario might provide such a reason, but even his scenario ignores the fact that cliffswallows that choose to nest solitarily or in extremely small colonies have no choice in how they forage. These individuals have no groups to join, because there are not enough cliff swallows present to form a group. Without groups, his rate maximizing scenario cannot apply. These solitarily nesting birds always have to forage solitarily where variance and thus risk are great. Birds occupying large colonies have the full range of foraging options available, as illustrated in Fig. 3 of Brown (1988). The fact that cliff swallows actively choose to nest as solitaries and in small colonies where variance in foraging success is high as opposed to in large colonies where variance can be low (because the birds can feed in large groups) is the basis for my conclusion that both risk-prone and risk-averse foraging occurs. Getty and Templeton & Giraldeau apparently assume that my conclusion was for birds within the same flock or colony; my conclusion was for birds between colonies only. I agree with them that additional data would be necessary to show the occurrence of different foraging strategies by swallows within the same flock, but I did not address this issue in Brown (1988). Templeton & Giraldeau (1990) make three other criticisms. They question whether cliff swallows really exhibit local enhancement, because convergences of birds at a foraging site could occur when the birds respond to the food resource itself or to other 'stimuli', which they do not specify. The whole issue may be one of semantics. The earliest definition of local enhancement that I can find is by Thorpe (1956, page 121) who defines it as 'an
Short Communications
apparent imitation resulting from directing the animal's attention to a particular object or to a particular part of the environment'. In its original use, it is not clear how the directing of the animal's attention occurs: either other animals or other events produce the apparent imitative behaviour. More recently, Wittenberger & Hunt (1985) defined local enhancement as the finding of food 'by opportunistically joining flocks of foraging or feeding individuals'. Templeton & Giraldeau (1990) apparently use a more restrictive definition of local enhancement, although their working definition is not given. I used local enhancement to refer to the joining of foraging groups by other foragers, a common usage in field studies of groupfeeding birds (see references in Wittenberger & Hunt 1985). Cliff swallows clearly joined foraging groups that had located prey. The mechanism of how they were attracted to these groups (either by other birds or by some other stimulus) is not at issue here. My point was to describe group foraging that occurred independently of information transfer at the colony site (sensu Brown 1986). Distinguishing the basis for group foraging, whether it is the opportunistic joining of already existing groups away from the colony (local enhancement) or the following of other foragers from an information centre to food sources, is critical in evaluating how foraging advantages may have led to the evolution of avian coloniality (Wittenberger & Hunt 1985; Mock et al. 1988). I agree with Templeton & Giraldeau that the assumption that foraging and vigilance are incompatible activities is not ideal. I do not agree that this assumption is any less satisfactory for aerial feeding birds. Why would or could a swallow both forage and watch for predators simultaneously, whereas a ground-feeding sparrow (which may be at much greater risk from ambush predators concealed in shrubbery) would or could not? Templeton & Giraldeau question me for assuming that foraging and vigilance are incompatible for cliff swallows because I did not 'justify' this assumption. But they present no reason why foraging and vigilance should be any less incompatible for swallows than for ground feeders, and at any rate I challenge them to present an operational way of 'justifying' such an assumption in the field. Templeton & Giraldeau invoke 'special circumstances' that might confound my results, namely, the risk of collisions in the centre of flocks. I was not aware of the unpublished paper they cite in support of their criticism, but collision seems highly unlikely for cliff swallows given that these birds almost invariably maintain estimated interindividual distances of at least 2 m while foraging (Brown, personal observation). In hundreds of hours of watching cliff swallows, my
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assistants and I have never observed collisions or even near-collisions in foraging groups. They allude to my supposed 'observation that swallows do not favour central flock positions'. This is incorrect; I have never reported such an observation. I reported only that birds maintained their relative positions in a flock while foraging. Birds may in fact prefer to be in the centre of the flock (which, if so, would suggest that anti-predator advantages are important), but my paper did not address actual preferences of foragers. Edge birds may maintain their positions because they are kept out by more central birds, because they are specialized as peripheral foragers, or for other reasons; the point is simply that we do not know. My conclusion was that social foraging was not primarily antipredator in function. I did not conclude, nor would I agree, that avoidance of predators is not ever important for foraging cliffswallows. Group foraging certainly presents opportunities to detect and avoid predators for these birds in the relatively infrequent circumstances when natural predators are present. But for reasons outlined in Brown (1988), this does not seem to be the main reason why cliff swallows feed in groups. Finally, Templeton & Giraldeau criticize my conclusion that cliff swallows do not experience food competition or resource depression in foraging groups. They suggest that foragers distribute themselves in an ideal free fashion so that competition would not be detected, although competition may have actually accounted for the distribution of birds. This suggestion is possible, and I agree that cliff swallows might at times forage in an ideal free manner. However, my purpose in Brown (1988 ) was to document whether or not these birds suffered a proximate cost in terms of reduced prey intake in larger foraging groups. Competition for insects resulting from resource depression would be the most likely cause of such a cost. No such cost was found. My assumption that competition might lower feeding rates in larger groups is wellfounded. For example, food competition resulting from resource depression has long been thought to be important in regulating the size of seabird colonies (e.g. Ashmole 1963; Furness & Birkhead 1984; Birt et al. 1987), and this competition can be proximately manifested in a number of ways that presumably reflect lowered individual harvest rates (see Wittenberger & Hunt 1985). It is important to emphasize that understanding the proximate costs of social foraging (if any) is a separate issue from the evolutionary forces that accounted for social foraging in the first place. My paper addressed the former issue, while Templeton & Giraldeau focus on the latter. Invoking the notion that competition has distributed the foragers observed in nature in an
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Animal Behaviour, 39, 6
ideal free fashion begs the ~ssue, because it then becomes difficult ever to rule out food competition (the ghost of competition past). Ongoing analyses do suggest that patch duration is independent of the number of foragers exploiting the patch (Brown, unpublished data). The remarks by Getty (1990) provide a useful alternative interpretation of the swallow data, but do not change my original conclusion that the results are equally consistent with predictions of risk sensitivity. The goal in the future should be to design field observations that can distinguish between these two alternatives. Those interested in foraging theory and risk sensitivity in particular should be heartened to know that these ideas may actually apply to animals in the wild, and we need more such empirical tests with both cliff swallows and other species. None of Templeton & Giraldeau's criticisms alter any of my conclusions in Brown (1988). CHARLESR. BROWN Department o f Biology, Yale University, P.O. Box 6666, New Haven, CT06511, U.S.A. References Ashmole, N. P. 1963. The regulation of numbers of tropical oceanic birds. Ibis, 103, 458-473. Birt, V. L., Birt, T. P., Goulet, D., Cairns, D. K. & Montevecchi, W. A. 1987. Ashmole's halo: direct evidence for prey depletion by a seabird. Mar. Ecol. Prog. Ser., 40, 205-208. Brown, C. R. 1986. Cliff swallow colonies as information centers. Science, 234, 83-85. Brown, C. R. 1988. Social foraging in cliff swallows: local enhancement, risk sensitivity, competition and the avoidance of predators. Anim. Behav., 36, 780-792. Caraco, T. 1981. Risk-sensitivity and foraging groups. Ecology, 62, 527-531. Clark, C. W. & Mangel, M. 1984. Foraging and flocking strategies: information in an uncertain environment. Am. Nat., 123, 626-641. Furness, R. W. & Birkhead, T. R. 1984. Seabird colony distributions suggest competition for food supplies during the breeding season. Nature, Lond., 311, 655-656. Getty, T. 1990. Are cliff swallows risk-sensitive, or meanrate maximizing? Anim. Behav., 39, 1214~1216. Mock, D. W., Lamey, T. C. & Thompson, D. B. A. 1988. Falsifiability and the information centre hypothesis. Orn. Scand., 19, 231-248. Templeton, J. J. & Giratdeau, L.-A. 1990. Social foraging in cliff swallows: a critique. Anim. Behav., 39, 12131214. Thorpe, W. H. 1956. Learning and Instinct in Animals. London: Methuen. Wittenberger, J. F. & Hunt, G. L., Jr. 1985. The adaptive significance of coloniality in birds. In: Avian Biology
Vol. 8 (Ed. by D. S. Farner, J. R. King & K. C. Parkes), pp. 1-78. New York: Academic Press. (Received 15 September 1989; initial acceptance 2 October 1989; final acceptance 26 October 1989; MS. number: AS-660)
Slave-Species Ant Colonies Recognize Slavemakers as Enemies Some ant species specialize in preying upon or otherwise exploiting other ant species (Wilson 1971). Other frequently exploited ant species have evolved the capacity to recognize and take specific defensive measures against especially dangerous ant enemies. Examples of such 'enemy recognition' include responses of colonies of Pheidole, Novomessor and Camponotus to Neivamyrmex army ants LaMon & Topoff 1981; Droual 1984; McDonald & Topoff 1986) and responses of Pheidole dentata to ants of the genus Solenopsis (Wilson 1975, 1976). Slave-making ants are social parasites that raid nests of certain so-called 'slave' species. Slavemakers kill or drive away the adults in raided nests and carry offtheir broods of eggs, larvae and pupae to the parasites' nest. There some of the immature individuals are reared to become 'slaves', i.e. slavespecies workers that perform for the slavemakers the functions that they would ordinarily perform for their parental colony (Wilson 1971; Altoway 1979, 1980). Harpagoxenus americanus (Emery) is an obligatory slavemaker that enslaves workers of Leptothorax ambiguus Emery, L. curvispinosus Mayr and L. longispinosus Roger. In southern Ontario, L. ambiguus and L. longispinosus colonies complete with one another for nest sites (Alloway 1980), but L. longispinosus workers occur more commonly as slaves in H. americanus nests (Buschinger & Alloway 1977). In this situation, L. longispinosus colonies should attempt to repel incursions by both H. americanus and L. ambiguus workers. However, the response to incursions by workers of the two species should be different if L. longispinosus has evolved special defences against the greater danger posed by slave raids. To determine whether L. longispinosus colonies respond differently to workers of the two species, 30 L. longispinosus nests were observed after the introduction of a L. ambiguus intruder and 30 others after the introduction of a H. americanus intruder. Subject colonies lived in plastic petri dishes, measuring 15 x 150 mm, which served as arenas for culture and observation. Each petri dish contained
Animal Behaviour, 39, 6
References Brown, C. R. 1988. Social foraging in cliffswallows:local enhancement, risk sensitivity, competition and the avoidance of predators. Anita. Behav., 36, 780 792. Milinski,M. 1988.Games fishplay: making decisionsas a social forager. Trends Ecol. Evol., 3, 325-330. Real, L. & Caraco, T. 1986. Risk and foraging in stochastic environments. A. Rev. Ecol. Syst., 17, 371-390. Stephens, D. W. & Krebs, J. R. 1986. Foraging Theory. Princeton, New Jersey: Princeton University Press. Wittenberger, J. F. & Hunt, G. J., Jr. 1985. The adaptive significance of coloniality in birds. In: Avian Biology. Vol. 8 (Ed. by D. S. Farner, J. R. King & K. C. Parkes), pp. 1--78.New York: Academic Press. (Received20 March 1989; initial acceptance 30 May 1989; final acceptance 21 June 1989; MS. number: ,4s-615)
Social Foraging in Cliff Swallows Revisited Getty (1990) and Templeton & Giraldeau (1990) recently criticized conclusions drawn in my study of the foraging behaviour of cliff swallows, Hirundo pyrrhonota (Brown 1988). The most important criticism that they offer is simply that I did not provide evidence that cliff swallows are risksensitive foragers. Getty describes a 'rate maximization scenario', which he argues is at least an equally plausible interpretation and one that does not require risk sensitivity on the part of these animals. Both of these critiques, however, have overlooked the fact that I (Brown 1988) did not claim that cliff swallows are risk-sensitive. I suggested only that the data were consistent with risk-sensitive models, which is clearly the case, and that cliff swallows might be likely candidates in which to look for risk sensitivity. A general prediction of risk-sensitive models (e.g. Caraco 1981) is that the rate of food encounter should increase, and the variance of food encounter should decrease, with increasing flock size. The cliff swallow data support that prediction per se (Brown 1988). The data are also consistent with Getty's more complicated rate maximizing scenario, and I agree with most of Getty's remarks. However, it is not clear to me how (if at all) Getty's scenario and risk sensitivity could be operationally distinguished in the field with cliff swallows. I appreciate Getty's alternative interpretation, but his criticism of me for claiming that cliff swallows are risk-sensitive is unwarranted. I pointed out (Brown 1988) that we do not as yet have any information on the condition or status of different foragers, which would be essential before any robust claim of risk sensitivity could be made.
My report that cliff swallows exhibit both riskprone and risk-averse foraging strategies was questioned by Getty (1990) and Templeton & Giraldeau (1990). Templeton & Giraldeau cite Caraco (1981) in arguing that mere presence in a flock or as a solitary forager cannot be equated with risk-aversiveness or risk-proneness, respectively. While their argument can hold in certain circumstances, most mathematical models do predict that flocks are associated with reduced foraging risk and solitaries with increased foraging risk (e.g. Caraco 1981; Clark & Mangel 1984). When the observed variance in foraging success declines strongly with group size as predicted by the models (see Fig. 2 in Brown 1988) and when individuals consistently forage in groups of particular sizes (a point ignored by Templeton & Giraldeau), one would need a compelling reason not to view this as a possible example of a risk-sensitive strategy. Templeton & Giraldeau provide no reason why presence in a group of a given size might not equate directly to risk in the case of cliff swallows. Getty's rate maximizing scenario might provide such a reason, but even his scenario ignores the fact that cliffswallows that choose to nest solitarily or in extremely small colonies have no choice in how they forage. These individuals have no groups to join, because there are not enough cliff swallows present to form a group. Without groups, his rate maximizing scenario cannot apply. These solitarily nesting birds always have to forage solitarily where variance and thus risk are great. Birds occupying large colonies have the full range of foraging options available, as illustrated in Fig. 3 of Brown (1988). The fact that cliff swallows actively choose to nest as solitaries and in small colonies where variance in foraging success is high as opposed to in large colonies where variance can be low (because the birds can feed in large groups) is the basis for my conclusion that both risk-prone and risk-averse foraging occurs. Getty and Templeton & Giraldeau apparently assume that my conclusion was for birds within the same flock or colony; my conclusion was for birds between colonies only. I agree with them that additional data would be necessary to show the occurrence of different foraging strategies by swallows within the same flock, but I did not address this issue in Brown (1988). Templeton & Giraldeau (1990) make three other criticisms. They question whether cliff swallows really exhibit local enhancement, because convergences of birds at a foraging site could occur when the birds respond to the food resource itself or to other 'stimuli', which they do not specify. The whole issue may be one of semantics. The earliest definition of local enhancement that I can find is by Thorpe (1956, page 121) who defines it as 'an
Short Communications
apparent imitation resulting from directing the animal's attention to a particular object or to a particular part of the environment'. In its original use, it is not clear how the directing of the animal's attention occurs: either other animals or other events produce the apparent imitative behaviour. More recently, Wittenberger & Hunt (1985) defined local enhancement as the finding of food 'by opportunistically joining flocks of foraging or feeding individuals'. Templeton & Giraldeau (1990) apparently use a more restrictive definition of local enhancement, although their working definition is not given. I used local enhancement to refer to the joining of foraging groups by other foragers, a common usage in field studies of groupfeeding birds (see references in Wittenberger & Hunt 1985). Cliff swallows clearly joined foraging groups that had located prey. The mechanism of how they were attracted to these groups (either by other birds or by some other stimulus) is not at issue here. My point was to describe group foraging that occurred independently of information transfer at the colony site (sensu Brown 1986). Distinguishing the basis for group foraging, whether it is the opportunistic joining of already existing groups away from the colony (local enhancement) or the following of other foragers from an information centre to food sources, is critical in evaluating how foraging advantages may have led to the evolution of avian coloniality (Wittenberger & Hunt 1985; Mock et al. 1988). I agree with Templeton & Giraldeau that the assumption that foraging and vigilance are incompatible activities is not ideal. I do not agree that this assumption is any less satisfactory for aerial feeding birds. Why would or could a swallow both forage and watch for predators simultaneously, whereas a ground-feeding sparrow (which may be at much greater risk from ambush predators concealed in shrubbery) would or could not? Templeton & Giraldeau question me for assuming that foraging and vigilance are incompatible for cliff swallows because I did not 'justify' this assumption. But they present no reason why foraging and vigilance should be any less incompatible for swallows than for ground feeders, and at any rate I challenge them to present an operational way of 'justifying' such an assumption in the field. Templeton & Giraldeau invoke 'special circumstances' that might confound my results, namely, the risk of collisions in the centre of flocks. I was not aware of the unpublished paper they cite in support of their criticism, but collision seems highly unlikely for cliff swallows given that these birds almost invariably maintain estimated interindividual distances of at least 2 m while foraging (Brown, personal observation). In hundreds of hours of watching cliff swallows, my
1217
assistants and I have never observed collisions or even near-collisions in foraging groups. They allude to my supposed 'observation that swallows do not favour central flock positions'. This is incorrect; I have never reported such an observation. I reported only that birds maintained their relative positions in a flock while foraging. Birds may in fact prefer to be in the centre of the flock (which, if so, would suggest that anti-predator advantages are important), but my paper did not address actual preferences of foragers. Edge birds may maintain their positions because they are kept out by more central birds, because they are specialized as peripheral foragers, or for other reasons; the point is simply that we do not know. My conclusion was that social foraging was not primarily antipredator in function. I did not conclude, nor would I agree, that avoidance of predators is not ever important for foraging cliffswallows. Group foraging certainly presents opportunities to detect and avoid predators for these birds in the relatively infrequent circumstances when natural predators are present. But for reasons outlined in Brown (1988), this does not seem to be the main reason why cliff swallows feed in groups. Finally, Templeton & Giraldeau criticize my conclusion that cliff swallows do not experience food competition or resource depression in foraging groups. They suggest that foragers distribute themselves in an ideal free fashion so that competition would not be detected, although competition may have actually accounted for the distribution of birds. This suggestion is possible, and I agree that cliff swallows might at times forage in an ideal free manner. However, my purpose in Brown (1988 ) was to document whether or not these birds suffered a proximate cost in terms of reduced prey intake in larger foraging groups. Competition for insects resulting from resource depression would be the most likely cause of such a cost. No such cost was found. My assumption that competition might lower feeding rates in larger groups is wellfounded. For example, food competition resulting from resource depression has long been thought to be important in regulating the size of seabird colonies (e.g. Ashmole 1963; Furness & Birkhead 1984; Birt et al. 1987), and this competition can be proximately manifested in a number of ways that presumably reflect lowered individual harvest rates (see Wittenberger & Hunt 1985). It is important to emphasize that understanding the proximate costs of social foraging (if any) is a separate issue from the evolutionary forces that accounted for social foraging in the first place. My paper addressed the former issue, while Templeton & Giraldeau focus on the latter. Invoking the notion that competition has distributed the foragers observed in nature in an
1218
Animal Behaviour, 39, 6
ideal free fashion begs the ~ssue, because it then becomes difficult ever to rule out food competition (the ghost of competition past). Ongoing analyses do suggest that patch duration is independent of the number of foragers exploiting the patch (Brown, unpublished data). The remarks by Getty (1990) provide a useful alternative interpretation of the swallow data, but do not change my original conclusion that the results are equally consistent with predictions of risk sensitivity. The goal in the future should be to design field observations that can distinguish between these two alternatives. Those interested in foraging theory and risk sensitivity in particular should be heartened to know that these ideas may actually apply to animals in the wild, and we need more such empirical tests with both cliff swallows and other species. None of Templeton & Giraldeau's criticisms alter any of my conclusions in Brown (1988). CHARLESR. BROWN Department o f Biology, Yale University, P.O. Box 6666, New Haven, CT06511, U.S.A. References Ashmole, N. P. 1963. The regulation of numbers of tropical oceanic birds. Ibis, 103, 458-473. Birt, V. L., Birt, T. P., Goulet, D., Cairns, D. K. & Montevecchi, W. A. 1987. Ashmole's halo: direct evidence for prey depletion by a seabird. Mar. Ecol. Prog. Ser., 40, 205-208. Brown, C. R. 1986. Cliff swallow colonies as information centers. Science, 234, 83-85. Brown, C. R. 1988. Social foraging in cliff swallows: local enhancement, risk sensitivity, competition and the avoidance of predators. Anim. Behav., 36, 780-792. Caraco, T. 1981. Risk-sensitivity and foraging groups. Ecology, 62, 527-531. Clark, C. W. & Mangel, M. 1984. Foraging and flocking strategies: information in an uncertain environment. Am. Nat., 123, 626-641. Furness, R. W. & Birkhead, T. R. 1984. Seabird colony distributions suggest competition for food supplies during the breeding season. Nature, Lond., 311, 655-656. Getty, T. 1990. Are cliff swallows risk-sensitive, or meanrate maximizing? Anim. Behav., 39, 1214~1216. Mock, D. W., Lamey, T. C. & Thompson, D. B. A. 1988. Falsifiability and the information centre hypothesis. Orn. Scand., 19, 231-248. Templeton, J. J. & Giratdeau, L.-A. 1990. Social foraging in cliff swallows: a critique. Anim. Behav., 39, 12131214. Thorpe, W. H. 1956. Learning and Instinct in Animals. London: Methuen. Wittenberger, J. F. & Hunt, G. L., Jr. 1985. The adaptive significance of coloniality in birds. In: Avian Biology
Vol. 8 (Ed. by D. S. Farner, J. R. King & K. C. Parkes), pp. 1-78. New York: Academic Press. (Received 15 September 1989; initial acceptance 2 October 1989; final acceptance 26 October 1989; MS. number: AS-660)
Slave-Species Ant Colonies Recognize Slavemakers as Enemies Some ant species specialize in preying upon or otherwise exploiting other ant species (Wilson 1971). Other frequently exploited ant species have evolved the capacity to recognize and take specific defensive measures against especially dangerous ant enemies. Examples of such 'enemy recognition' include responses of colonies of Pheidole, Novomessor and Camponotus to Neivamyrmex army ants LaMon & Topoff 1981; Droual 1984; McDonald & Topoff 1986) and responses of Pheidole dentata to ants of the genus Solenopsis (Wilson 1975, 1976). Slave-making ants are social parasites that raid nests of certain so-called 'slave' species. Slavemakers kill or drive away the adults in raided nests and carry offtheir broods of eggs, larvae and pupae to the parasites' nest. There some of the immature individuals are reared to become 'slaves', i.e. slavespecies workers that perform for the slavemakers the functions that they would ordinarily perform for their parental colony (Wilson 1971; Altoway 1979, 1980). Harpagoxenus americanus (Emery) is an obligatory slavemaker that enslaves workers of Leptothorax ambiguus Emery, L. curvispinosus Mayr and L. longispinosus Roger. In southern Ontario, L. ambiguus and L. longispinosus colonies complete with one another for nest sites (Alloway 1980), but L. longispinosus workers occur more commonly as slaves in H. americanus nests (Buschinger & Alloway 1977). In this situation, L. longispinosus colonies should attempt to repel incursions by both H. americanus and L. ambiguus workers. However, the response to incursions by workers of the two species should be different if L. longispinosus has evolved special defences against the greater danger posed by slave raids. To determine whether L. longispinosus colonies respond differently to workers of the two species, 30 L. longispinosus nests were observed after the introduction of a L. ambiguus intruder and 30 others after the introduction of a H. americanus intruder. Subject colonies lived in plastic petri dishes, measuring 15 x 150 mm, which served as arenas for culture and observation. Each petri dish contained