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Behavioral Ecology Vol. 14 No. 1: 127-134
© 2003 International Society for Behavioral Ecology
Evolution of fighting behavior under asymmetric competition: an experimental test with juvenile salmonids
Forest Sciences Department, Centre for Applied Conservation Biology, University of British Columbia, 3041-2424 Main Mall, Vancouver, BC V6T 1Z4, Canada
Address correspondence to K.A. Young, who is now at the Department of Biological Sciences, Simon Fraser University, 8888 University Drive, Burnaby, BC V5A 1S6, Canada. E-mail: kayoung{at}sfu.ca.
Received 9 October 2001; revised 11 June 2002; accepted 14 June 2002.
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
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Large-dominant and small-subordinate species engaging in asymmetric interference competition may optimize behavior under different trade-offs between the chance of winning and the cost of fighting. If fighting behavior is heritable and under selection, theory suggests that large-dominant and small-subordinate species should evolve aggressive and passive fighting behaviors, respectively. To test this prediction, I manipulated the size and competitive asymmetry of juveniles from sympatric populations of large-dominant coho salmon (Oncorhynchus kisutch) and small-subordinate steelhead trout (O. mykiss) and asked whether differences in fighting behavior persisted independently of competitive ability. I observed fighting behavior during dyadic contests in two habitats, mutually preferred pools and energetically demanding riffles, under each of three size treatments: natural size asymmetry, asymmetry removed, and reversed size asymmetry. The results supported the prediction. Competitive ability depended primarily on size; large individuals of both species dominated smaller heterospecifics, and neither species dominated when size matched. Fighting behavior depended primarily on species identity; coho salmon used a higher proportion of aggressive chases, whereas steelhead trout used a higher proportion of passive displays. Large individuals were more likely to chase, and small individuals were more likely to display. As evidence that asymmetric competition is associated with behavioral divergence, these results complement previous work on morphological divergence under asymmetric competition and provide a richer context for other features of the coho–steelhead system.
Key words: asymmetric competition, fighting behavior, interspecific competition, salmonids, Oncorhynchus kisutch, Oncorhynchus mykiss.
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
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Interspecific competition is often asymmetric, with individuals of one species reducing the fitness of individuals of another species more than the reciprocal effect (
ij > ji) (Connell, 1983
; Lawton and Hassell, 1981; Schoener, 1983). Competitive asymmetries can result from exploitation competition if one species uses a limiting resource more efficiently than the other species (Giller and Doube, 1989; Morin and Johnson, 1988; Tilman, 1982) or from interference competition if individuals of one species dominate heterospecific contests for mutually preferred habitat (Case and Gilpin, 1974; Gill, 1974; Morse, 1974). In animals, interference asymmetry generally results from a size asymmetry, with individuals of the larger species dominating fighting contests against smaller heterospecifics (Morse, 1974; Schoener, 1983).
Theories designed to understand fighting behavior during intraspecific contests offer predictions for how fighting behavior should evolve under interspecific asymmetric competition (Enquist, 1985; Maynard Smith and Parker, 1976). When confronted with a heterospecific competitor, individuals should optimize fighting behavior based on the perceived trade-off between the chance and value of winning the contest and some combination of the cost of fighting and chance of losing the contest (Maynard Smith and Parker, 1976; Parker, 1974). When interspecific contests are asymmetric, this trade-off differs for the two species. If size is a reliable indicator of competitive ability, then individuals of the large-dominant species should be willing to escalate using aggressive fighting behaviors, whereas individuals of the small-subordinate species should be less willing to escalate (Persson, 1985). If fighting behavior is heritable and influences lifetime fitness, species engaging in asymmetric interference competition should evolve different fighting behaviors.
Testing the prediction that asymmetric competition promotes evolutionary divergence in fighting behavior is difficult because environmental conditions, interspecific competition per se, and the strength of intraspecific competition may allinfluence fighting behavior. Two approaches can provide informative tests. The first involves comparing two pairs of sympatric populations, one where competition is asymmetric and one where competition is either symmetric or asymmetric in the opposite direction, and showing that the fighting behaviors of individuals from the four populations differ in the predicted direction. This is equivalent to a doubly sym patric test of character displacement (Brown and Wilson, 1956), where fighting behavior is the character, and asymmetric competition, not interspecific competition, is the factor of interest. The approach is well established, but in the present context limited by the requirement that other factors that can influence behavior (e.g., productivity, intra- and interspecific density, community composition, predators) must be held constant across the four populations (Grant, 1972).
The second approach is to study sympatric populations that experience asymmetric competition and manipulate the phenotypic trait (i.e., size) that determines competitive ability. The prediction would be supported if interspecific differences in fighting behavior persist when the size/competitive asymmetry is removed and reversed. Numerous studies show that individuals of large-dominant and small-subordinate species use aggressive and passive fighting behaviors, respectively (e.g., Alatalo and Moreno, 1987; Bleich and Price, 1995; Marvin, 1998; Nishikawa, 1985), but such studies say little about evolved differences in fighting behavior, only that large and small individuals use different fighting behaviors. One cannot rule out the possibility that large and small individuals are merely adopting conditional strategies associated with the underlying size asymmetry. The questions one must address are (1) do individuals of a large-dominant species use aggressive fighting behaviors when the size/competitive asymmetry is removed and reversed? and (2) is the same true for use of passive fighting behaviors by the small-subordinate species? In this study, I used this phenotypic manipulation approach with juvenile coho salmon (Oncorhynchus kisutch) and steelhead trout (O. mykiss) to test the prediction that asymmetric competition is associated with evolved differences in fighting behavior.
Juvenile coho and steelhead occur sympatrically in coastal rivers and streams of western North America and engage in asymmetric interference competition. Anadromous, semelparous coho salmon return to freshwater to spawn July–January, with spawning occurring later at lower latitudes (Sandercock, 1991; Weitkamp et al., 1995). In contrast, ana dromous, iteroparous steelhead trout spawn January–April throughout their range (Busby et al., 1996; Withler, 1966). Interspecific differences in the timing of adult spawning andfry emergence result in early-emerging salmon having a latitude-dependent size advantage over late-emerging trout during the first summer in freshwater. The size advantage ofcoho salmon fry is large in northern populations and negligible near the southern end of their sympatric range (Bugert et al., 1991; Burns, 1972; Hartman, 1965).
Juvenile salmonids use a variety of fighting behaviors dur ingintra- and interspecific interference competition to gain energetically favorable foraging positions in the water column (Fausch, 1984; Hartman, 1965; Nielsen, 1992; Sabo and Pauley, 1997; Taylor and Larkin, 1986). Numerous studies have demonstrated that salmonid fighting behavior is heritable and can evolve rapidly (four to seven generations) in response to artificial selection (Berijikian et al., 1996; Fleming and Einum, 1997; Swain and Holtby, 1990). In natural streams coho salmon and steelhead trout fry tend to occupy pools and riffles, respectively (Bisson et al., 1988; Bugert et al., 1991; Hartman, 1965). Both salmon and trout grow faster in deep, low-velocity pools than in shallow, high-velocity riffles (Harvey and Nakamoto, 1997; Quinn and Peterson, 1996; Young, 2001). Given their natural size advantage, salmon are competitively dominant and increase the proportion of trout occupying riffle habitat, whereas trout have little effect on habitat selection by salmon (Bugert and Bjornn, 1991).
Because large-dominant coho salmon and small-subordinate steelhead trout optimize behavior under different trade-offs during interspecific contests, salmon should evolve more aggressive fighting behaviors than trout. To test for differences in fighting behavior between juveniles from sympatric populations of salmon and trout, I observed fighting behavior during dyadic contests in pools and riffles under each of three size treatments: salmon larger than trout (natural asymmetry), salmon equal to trout (asymmetry removed), and salmon smaller than trout (reversed asymmetry).
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MATERIALS AND METHODS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
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Study area and populations
I studied populations of coho salmon and steelhead trout from the Chilliwack River in southwestern British Columbia, Canada (49.1° N, 121.6° W), near the center of their sympatric range (Hartman, 1965
). The Chilliwack River drains a slightly glaciated mountainous basin of 1250 km2 and enters the Fraser River approximately 100 km from the Pacific Ocean. Peak flows occur in the fall and winter during the rainy season and in the spring during snow melt. Water temperatures range from 2° to 15°C. Adult coho salmon spawn in the fall andfry begin emerging March and April; adult steelhead trout spawn in the winter and fry emerge May–July. A fed eralhatchery located at river kilometer 32 propagates wild steelhead trout for conservation and recreation purposes.
Fish collection and rearing
To minimize the chance of environmental differences affecting fighting behavior, I reared individuals of each species in similar conditions before and during the experiment. On 23 April 1999, I seined approximately 200 newly emerged coho salmon fry from small tributaries and side channels between river kilometers 50 and 60, where Hartman (1965) observed the highest densities of coho salmon and steelhead trout fry and where wild adults of both species spawn. Because of conservation concerns, I was not permitted to collect steelhead trout fry from the same area. On 25 May, Icollected approximately 200 newly emerged steelhead trout fry from the Chilliwack hatchery. The fry were a random sample from 12 families of wild parents (one female x one male). In the laboratory, salmon were placed in identical holding troughs (3.7 length x 0.35 width x 0.20 m depth) in groups of 100 and held at 9.5°C and fed maintenance rations of standard hatchery feed (Moore-Clark #1 Crumble) until trout were collected. From 25 May until 10 June, salmon and trout were held at similar densities (
100 fish/trough) at 9.5° and 11°C, respectively. Salmon and one group of trout were fed to satiation once daily; the other group of trout was fed to satiation twice daily. On 10 June, 1 month before the beginning of the experiment, I separated fry of each species into small, medium, and large groups based on visual estimates (Table 1). Fifty of each size class of each species were placed in separate troughs. I grouped fish by size to minimize the chance of prior size asymmetries and dominance hierarchies influencing fighting behavior. From 10 June until the completion of the experiment, all fish were held at 11°C and fed maintenance rations once daily.
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Experimental apparatus
Dyadic contests were conducted in a flow-through aquarium with a 25 cm wide x 25 cm deep x 50 cm long clear plexiglass observation chamber screened at both ends. Water depth wascontrolled with an adjustable dam 50 cm behind the backscreen. Small gravel (mean diameter = 10.8 mm, n = 25, SD = 1.9) was spread across the bottom of the observation chamber. Standard hatchery feed (same as above) was intro duced at the water surface at the front of the observation chamber through a tube draining a bucket sitting atopa magnetic stirrer. By maintaining a constant inflow, the depth and velocity of the riffle and pool treatments were essentially invariant across replicates. For this reason I did not quantify these parameters during the experiment but instead calculated the mean depth and velocity of nine points (7, 15, and 21 cm width along three transects 5, 25, and 45 cm from the front of the observation chamber) for five riffles and five pools; between each habitat replicate I emptied the tank and reset the dam to simulate experimental conditions. For each of the 10 replicates, I took a single estimate of surface velocity by recording the time it took a small piece of foam rubber to drift the length of the observation chamber (Table 2). Water temperature was maintained at 11°C throughout the experiment.
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Experimental procedure
The experimental design consisted of two habitats (pool and riffle) crossed with three size treatments (salmon larger than trout, size matched, and salmon smaller than trout) for a total of six treatments. The six treatments were replicated in random order on 10 nonconsecutive days (blocks) between 10July and 25 July. Size-matched fish were of equal length. Inthe size asymmetry treatments fish differed by approximately 6 mm (Table 1), similar to the size advantage of salmon during mid-summer in local populations (Hartman, 1965
), and similar to the size difference that influenced the outcome ofdyadic contests between yearling coho and cutthroat trout (O. clarkii; Sabo and Pauley, 1997). Approximately 24 h before each block of trials, I lightly anesthetized two fish from each ofthe six holding troughs with tricaine methanesulfate andrecorded their standard length (to 1 mm) and weight (to 0.01 g). Fish were then held individually in flow-through buckets until trials were run the following afternoon. During the experiment, fish remaining in the holding troughs were fed after the next day's test fish were removed, so all fish were starved for 2 days to ensure active foraging during the trials.
Before each trial, 0.05 g of the hatchery feed (same as above) was mixed into the gravity feed bucket with the dis charge tube pinched shut. I placed the two test fish in the observation chamber and gave them 5 min to acclimate. The feeding tube was then opened and the trial began with thefirstforaging attempt by either fish. After the trial test, Imarked fish with an adipose fin clip to ensure they were not reused and returned them to the appropriate holding trough. I was the sole observer for all of the trials. Each trial lasted 10min, during which I recorded the number of seconds eachfish spent in the dominant, upstream position closer to the food source; the number of drift items entering the observation tank; the number of drift items captured by each fish; and the total number of fighting behaviors initiated by each fish. I divided fighting behaviors into three classes of decreasing aggressiveness (Chapman, 1962; Hartman, 1965; Sabo and Pauley, 1997; Taylor and Larkin, 1986): chases—an individual actively attacked and displaced (at least momentarily) the heterospecific competitor; nips—an individual bit the heterospecific competitor; displays—an individual wiggled or stiffened its body and/or stiffened its dorsal and anal fins. Studies of juvenile salmonid behavior generally score more than three behavior types. I placed behaviors into only three easily distinguished categories to reduce the number of judgment calls and potential bias.
Statistical analyses
I analyzed six response variables. The proportion of time a fish spent in the dominant, upstream position and the proportion of total drift items captured were used to quantify competitive ability. I used the total number of fighting behaviors initiated and the proportion of those behaviors that were chases, nips,and displays to characterize fighting behavior. Because the proportions of behaviors that were chases, nips, and dis plays are necessarily dependent, these data were first ana lyzedtogether using MANOVA to test for the effects of each experimental factor on fighting behavior. This model was thendecomposed into separate ANOVAs for each of the three behavior types. All statistical analyses were conducted using PROC GLM (Type III SS) of SAS software (SAS Institute, 1990).
Analyses were conducted using a split-plot design in which the different treatment effects and their interactions are tested over different error terms depending on whether they are varied as main-plot or subplot factors (Snedecor and Cochran, 1980). In this experiment, habitat (pool or riffle) and fish size (smaller, matched, or larger) were main-plot factors, and species (coho or steelhead) was the subplot factor. Block (random), habitat, and size and their two way interaction were tested over the main-plot error term. I tested species and all two- and three-way interactions involving species over the subplot error term (Table 3). To meet assump tions of the analyses, total number of fighting behaviors initiated was log10(x + 1) transformed, and all proportional data were arcsine transformed. For clarity, I present the untransformed data in the figures. Of the 120 fish observed, 17 did not initiate any fighting behaviors; these fish were not included in the analyses of behavior type.
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RESULTS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
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Competitive ability
Size was the most important factor influencing position (F2,45= 31.3, p <.0001; Figure 1, Table 3). Large and small fish were in the dominant, upstream position 77% and 23% of the time, respectively. Species identity did not significantly effect position (F1,54 = 1.6, p =.22), though salmon were in the dominant, upstream position slightly more than trout (54% and 46%, respectively). There was a significant speciesx size interaction (F2,54 = 3.7, p <.05). Although both species were dominant when given a size advantage, the degree of dominance was greater for salmon experiencing their natural size advantage than for trout when the natural size asymmetry was reversed (Figure 1). Neither habitat nor its interaction effects influenced position.
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The results for foraging success were similar to those for position except that both species captured more drift items in pools than in riffles (F1,45 = 9.53, p <.005; Figure 1, Table 3). Size was again the most important factor, with large fish capturing the highest proportion of drift items (F2,45 = 29.6, p <.0001). Overall, salmon captured slightly more drift items than trout (45% and 38%, respectively; F1,54 = 4.27, p <.05). Salmon had higher foraging success under the natural size asymmetry than did trout when the size asymmetry was reversed, though this result was not significant (species x size: F2,54 = 2.3, p =.11).
In summary, competitive ability during dyadic contests depended primarily on the relative size of the two species. When individuals were size matched, salmon had no competitive advantage and were even slightly subordinate relative to trout. The slight overall competitive advantage of salmon resulted from being more dominant under the natural size asymmetry than were trout when the size asymmetry was reversed.
Fighting behavior
Of the 17 fish that initiated no fighting behaviors, 9 were small trout, all of which spent no (6 fish) or little (24, 45, and 267 s) time in the dominant, upstream position. Two small salmon (0 and 202 s in the upstream position) initiated no behaviors. Three large salmon (51, 600, 600 s in the upstream position) and one size matched salmon (0 s) initiated no behaviors. Two large trout initiated no behaviors and spent no time in the dominant, upstream position. Thus, there was a nonsignificant tendency for small fish and for trout (
2 = 4.58, df = 2, p =.10, for both) not to initiate any fighting behaviors.
Total number of fighting behaviors initiated depended mostly on the direction of the size asymmetry, with size matched (mean number of behaviors ± SE = 9.1 ± 0.88), and large fish (8.0 ± 1.19) initiating more behaviors than small fish (4.4 ± 0.89; F2,45 = 11.0, p <.0001; Figure 2, Table 3). However, the number of behaviors each species initiated depended on the direction of the size asymmetry. Salmon initiated more fighting behaviors than trout when competing against larger heterospecifics (species x size: F2,54 = 7.9, p <.001). When the combined behaviors of a dyad was analyzed using single-factor ANOVA, size matched (mean number of behaviors = 18.4 ± 1.54) and asymmetry reversed (16.1 ± 1.75) pairs initiated significantly more total behaviors than natural asymmetry pairs (8.7 ± 2.25; F2,48 = 12.59, p <.0001; posterior multiple comparisons, Tukey's HSD, = 0.05).
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When analyzed using MANOVA, fighting behavior depended primarily on species identity (Wilks's
= 0.72, F3,36= 4.5, p <.001), but also on size ( = 0.68, F6,84 = 2.96, p =.01) and the interaction between the two ( = 0.78, F3,36 =3.46, p <.05). The univariate ANOVAs revealed that the behavioral differences between the species were in the direction predicted by their natural size/competitive asymmetry (Figure 3, Table 3). Salmon were more likely than trout (27% and 11%,respectively) to initiate aggressive chases (F1,38 = 11.5, p =.002). Though both species were more likely to chase when given a size advantage (F2,44 = 3.25, p <.05), the trend was extreme for trout. In 20 trials under the natural size asymmetry, small trout did not initiate a single chase against larger salmon. In contrast, when the natural size asymmetry was reversed, 18% of small salmon behaviors were chases against larger trout. Trout were more likely than salmon to use passive displays (62% vs. 52%; F1,38 = 6.9, p =.01). Just aslargefish were more likely to chase, small fish were more likely to display (F2,44 = 6.98, p <.005). There also was a significant interaction between species and habitat (F1,38 = 7.85, p <.01); salmon displayed more frequently in riffles than in pools (62% vs. 41%), whereas steelhead displayed more frequently in pools than in riffles (71% vs. 52%). The proportion of fighting behaviors that were nips did not depend on any experimental factors.
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DISCUSSION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
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I used a phenotypic manipulation approach to test the prediction that large-dominant and small-subordinant species engaging in asymmetric interference competition should use aggressive and passive fighting behaviors, respectively. Coho salmon, which under natural conditions are larger and competitively dominant, used a higher proportion of aggressive chase behaviors. Alternatively, steelhead trout used a higher proportion of passive display behaviors. Although competitive ability during dyadic contests depended mostly on relative size, fighting behavior was determined mainly by species identity. That salmon and trout used aggressive and passive behaviors, and that these behaviors persisted independently of competitive ability, supports the prediction that asymmetric competition is associated with evolutionary divergence in fighting behavior.
The phenotypic manipulations were largely successful at removing and reversing the natural competitive asymmetry. Competitive ability was influenced mainly by size; each species was dominant when given a 6-mm size advantage, and neither species dominated when size matched. Salmon tended to be more dominant relative to trout under the natural and reversed size asymmetry treatments. This pattern suggests an intrinsic difference in competitive ability but may also be related to the relative masses of the species. Coho salmon have a more laterally compressed body shape and are heavier than steelhead trout of the same length (Bisson et al., 1988). If mass and length both influenced competitive ability, then the phenotypic manipulations would have failed to completely remove and reverse the natural competitive asymmetry. This explanation is consistent with the relative advantage of salmon in the two size asymmetry treatments. However, in the size matched treatments, where lengths were equal but salmon had a mass advantage, salmon had no competitive advantage. Still, the importance of relative mass may have differed across the size treatments and cannot be ruled out as the cause of the small species effect I observed.
Salmon and trout initiated the same number of fighting behaviors overall but initiated different numbers of fighting behaviors across the three size treatments. Small salmon initiated more fighting behaviors than small trout, suggestingsmall-subordinate salmon were more willing than small-subordinate trout to escalate against a larger heterospecific. This result is consistent with the predicted difference in fighting behavior and the size-dependent difference in competitive ability. Salmon used more displays in riffles, and trout used more displays in pools. The habitat-dependent increase in passive fighting behavior observed in both species suggests their perception of competitive ability varied with habitat, though I found no evidence that habitat affected the species' relative competitive abilities. Size matched fish and large fish initiated more behaviors than small fish, and when a dyad's cumulative number of behaviors was considered, reversed asymmetry and size matched pairs initiated more total behaviors than natural asymmetry pairs. The second result is consistent with the widely supported prediction (Parker, 1974; Maynard Smith and Parker, 1976) that equal competitors are more likely than asymmetric competitors to escalate during fighting contests.
The key result of the experiment is that coho salmon and steelhead trout use fighting behaviors that differ in the direction predicted by the size/competitive asymmetry they experience under natural conditions. Salmon used a higher proportion of aggressive chases, and trout used a higher proportion of passive displays. Though differences in fighting behavior between asymmetric competitors have been documented in various taxa (Alatalo and Moreno, 1987; Bleich and Price, 1995; Marvin, 1998; Nishikawa, 1985), until now it was not known that such differences persist independently of competitive ability.
Empirical studies of asymmetric competition have traditionally focused on its role in structuring communities along various niche axes (e.g., Bleich and Price, 1995; Morse, 1974; Robinson and Terborgh, 1995; Thompson and Fox, 1993), not on its potential to promote evolutionary divergence in fighting behavior. Similarly, theoretical (Brown and Vincent, 1987; Law et al., 1997; Rummel and Roughgarden, 1985; Taper and Case, 1985, 1992) and empirical studies (Case and Bolger, 1991; Roughgarden and Pacala, 1989) of coevolution under asymmetric competition have focused on divergence in the physical trait (usually size) responsible for the competitive asymmetry, not divergence in fighting behavior. In as far as they apply to ethological traits, these models suggest asymmetric competition should facilitate evolutionary divergence in fighting behavior, particularly as the relative degree of inter- to intraspecific size asymmetries increases (Law et al., 1997). The model of Law et al. (1997) suggests the testable hypothesis that evolutionary divergence in fighting behavior between coho salmon and steelhead trout should mirror the latitudinal cline in juvenile size asymmetry. Behavioral differences should be greater at higher latitudes where the size/competitive asymmetry is greater and diminished at lower latitudes where emergence times and fry sizes converge.
A number of theoretical and empirical studies provide a broader context for the differences in fighting behavior documented here. Case and Gilpin (1974) incorporated interference and exploitation competition into a Lotka-Volterra model and made three predictions supported by the coho-steelhead system (see also Morse, 1974). Competitors that dominate heterospecific encounters should have similar fundamental and realized niches, while the subordinate species should experience niche shifts to the less favorable habitat in sympatry (Bugert and Bjornn, 1991). In sympatry, individuals of the subordinate species should use habitats where costly interference behaviors are less profitable for the dominant species. The nonsignificant tendency for coho salmon to initiate fewer energetically costly chases and more energetically conservative displays (Pucket and Dill, 1985) in the high-velocity riffle habitat supports this prediction. Finally, Case and Gilpin (1974) predicted there should be a trade-off between interference and exploitation abilities, with the inferior interference competitor being more efficient at using consumable resources. This prediction is supported by numerous experiments showing that steelhead trout have higher energy conversion efficiencies and growth rates than coho salmon (Everson, 1973; Fraser, 1969; Wurtsbaugh and Davis, 1977; Young, 2001).
My results are consistent with the prediction that asymmetric competition promotes evolutionary divergence in fighting behavior, but a number of other explanations cannot be ruled out. First, hydrological differences between pools and riffles may select for different fighting behaviors independently of interspecific competitive asymmetries. The nonsignificant tendency of salmon to use energetically costly chases in low-velocity pools and energetically conservative displays in high-velocity riffles is consistent with this interpretation. However, steelhead used a higher proportion of displays in the pool habitat, suggesting hydrological conditions are not directly correlated with fighting behavior in this system. Furthermore, large individuals of both species tended to chase and small individuals tended to display. Thus, the conditional fighting behaviors of large-dominant and small-subordinate individuals differed in the same direction predicted to evolve under the species' natural size/competitive asymmetry.
Second, interspecific competition per se, not asymmetric competition, may be responsible for the observed differences in fighting behavior. Individuals often respond more aggressively to conspecific or ecologically similar congeneric competitors (Ebersole, 1977; Myrberg and Thresher, 1974; but see Ambrose and Meehan, 1977; Minock, 1972, for contrary results), particularly when there is strong spatial segregation between species (Hairston et al., 1987; Nishikwa, 1985; Robinson and Terborgh, 1995). However, the one study to explore the relationship between interspecific competition and salmonid fighting behavior found that sympatry did not affect fighting behavior. Sabo and Pauley (1997) tested for character displacement in competitive ability and fighting behavior by observing dyadic encounters between juvenile coho salmon and cutthroat trout from two populations, one sympatric with coho salmon, the other isolated in allopatry above a waterfall. They found that allopatric cutthroat trout were more successful interspecific competitors than sympatric cutthroat trout, but that there was no difference in the types of fighting behavior used by individuals from the two populations.
Third, interspecific differences in fighting behavior may result from differences in the intensity of intraspecific competition. Because coho salmon and steelhead trout are spatially segregated, individuals should engage in more intra- than interspecific interactions (Gill, 1974; Hartman, 1965; Morse, 1976). Rosenau and McPhail (1987) found that coho salmon fry from a high-density population were dominant over, and initiated more fighting behaviors than, coho salmon from a low-density population. Again, however, there were no differences in the fighting behavior of fish from the two populations. Thus, the presence of heterospecific competitors and the intensity of intraspecific competition appear to affect competitive ability and the number of behaviors initiated, but not the types of fighting behaviors used.
Although my results provide the first evidence that asymmetric competition can drive evolutionary divergence in fighting behavior, future experiments should use multiple populations and combine phenotypic manipulations with variation in three factors (the degree of competitive asymmetry, the presence of a heterospecific competitor, and the intensity of intraspecific competition) to determine the relative importance of each in the evolution of fighting behavior.
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ACKNOWLEDGEMENTS |
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I thank the employees of the Department of Fisheries and Oceans for logistical support and use of the Cultus Lake Salmon Research Laboratory. A. Lotto provided valuable assistance during the fish collections. P. Arcese, M. Healey, S. Hinch, E. Parkinson, D. Schluter, E. Taylor, and two anonymous reviewers provided valuable discussions and/or comments on earlier versions of the manuscript. This research was supported by Forest Renewal BC (FRBC) grants to E.Parkinson and J. Rosenfeld, a National Science and Engineering Research grant to S. Hinch, and a University of British Columbia Graduate Fellowship to K.A.Y.
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REFERENCES |
|---|
|
TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
|---|
Alatalo RV, Moreno J, 1987. Body size, interspecific interactions, and use of foraging sites in tits (Paridae). Ecology 68:1773-1777.[CrossRef]
Ambrose RF, Meehan JA, 1977. Aggressive behavior of Perognathus parvus and Peromyscus maniculatus. J Mammal 58:665-668.[CrossRef]
Berijikian BA, Mathews SB, Quinn TP, 1996. Effects of hatchery and wild ancestry and environments on the development of agonistic behavior in steelhead trout (Oncorhynchus mykiss) fry. Can J Fish Aquat Sci 53:2004-2014.[CrossRef]
Bisson PA, Sullivan K, Nielson JL, 1988. Channel hydraulics, habitat use, and body form of juvenile coho salmon, steelhead, and cutthroat trout in streams. Trans Am Fish Soc 117:262-273.[CrossRef]
Bleich VC, Price MV, 1995. Aggressive behavior of Dipodomys stephensi, an endangered species, and Dipodomys agilis, a sympatric congener. JMammal 76:646-651.[CrossRef]
Brown JS, Vincent TL, 1987. Coevolution as an evolutionary game. Evolution 41:66-79.[CrossRef]
Brown WLJ, Wilson EO, 1956. Character displacement. Syst Zool 5:49-64.
Bugert RM, Bjornn TC, 1991. Habitat use by steelhead and coho salmon and their responses to predators and cover in the laboratory. Trans Am Fish Soc 120:486-493.[CrossRef]
Bugert RM, Bjornn TC, Meehan WR, 1991. Summer habitat use by salmonids and their responses to cover and predators in a small southeast Alaska stream. Trans Am Fish Soc 120:474-485.[CrossRef]
Burns JW, 1972. Some effects of logging and associated road construction on Northern California streams. Trans Am Fish Soc 101:1-17.[CrossRef]
Busby PJ, Wainwright TC, Bryant GJ, Lierheimer LJ, Waples RS, Waknitz FW, Lagomarsino IV, 1996. Status review of west coast steelhead from Washington, Idaho, Oregon, and California. NOAA Technical Memorandum NMFS-NWFSC-27. Seattle,. Washington: National Oceanic and Atmospheric Administration.
Case TJ, Bolger TD, 1991. The role of interspecific competition in the biogeography of island lizards. Trends Ecol Evol 6:135-139.
Case TJ, Gilpin ME, 1974. Interference competition in niche theory. Proc Nat Acad Sci USA 71:3073-3077.
Chapman DW, 1962. Aggressive behavior in juvenile coho salmon as a cause of emigration. J Fish Res Board Can 19:1047-1080.
Connell JH, 1983. On the prevalence and relative importance of interspecific competition: evidence from field experiments. Am Nat 122:661-696.[CrossRef]
Ebersole JP, 1977. The adaptive significance of interspecific territoriality in the reef fish Eupomacentrus leucostictus. Ecology 58:914-920.[CrossRef]
Enquist M, 1985. Communication during aggressive interactions with particular reference to variation in choice of behaviour. Anim Behav 33:1152-1161.[CrossRef]
Everson LB, 1973. Growth and food consumption of juvenile coho salmon exposed to natural and elevated fluctuating temperatures (MS thesis). Corvallis: Oregon State University.
Fausch KD, 1984. Profitable stream positions for salmonids: relating specific growth rate to net energy gain. Can J Zool 62:441-451.
Fleming IA, Einum S, 1997. Experimental tests of genetic divergence of farmed from wild Atlantic salmon due to domestication. ICES J Mar Sci 54:1051-1063.
Fraser FJ, 1969. Population density effects on survival and growth of juvenile coho salmon and steelhead trout in experimental stream channels. In: Symposium on salmon and trout in streams (Northcote TG, ed). Vancouver: University of British Columbia Press; 253–266.
Gill DE, 1974. Intrinsic rate of increase, saturation density, and competitive ability. II. The of evolution of competitive ability. Am Nat 108:103-116.[CrossRef]
Giller PS, Doube BM, 1989. Experimental analysis of inter- and intraspecific competition in dung beetle communities. J Anim Ecol 58:129-142.
Grant PR, 1972. Convergent and divergent character displacement. Bio J Lin Soc 4:39-68.
Hairston NG, Nishikawa KC, Stenhouse SL, 1987. The evolution of competing species of terrestrial salamanders: niche partitioning or interference? Evol Ecol 1:247-262.[CrossRef]
Hartman GF, 1965. The role of behavior in the ecology and interaction of underyearling coho salmon (Oncorhynchus kisutch) and steelhead trout (Salmo gairdneri). J Fish Res Board Canada 22:1035-1081.
Harvey BC, Nakamoto RJ, 1997. Habitat-dependent interactions between two size-classes of juvenile steelhead in a small stream. Can J Fish Aquat Sci 54:27-31.[CrossRef]
Law R, Marrow P, Dieckman U, 1997. On evolution under asymmetric competition. Evol Ecol 11:485-501.[CrossRef]
Lawton JH, Hassell MP, 1981. Asymmetrical competition in insects. Nature 289:793-795.[CrossRef]
Marvin GA, 1998. Interspecific aggression and spatial relationships inthe salamanders Plethodon kentucki and Plethodon glutinosus: evidence of interspecific interference competition. Can J Zool 76:94-103.[CrossRef]
Maynard Smith J, Parker GA, 1976. The logic of asymmetric contests. Anim Behav 24:159-175.[CrossRef]
Minock ML, 1972. Interspecific aggression between black-capped and mountain chickadees at winter feeding stations. Condor 74:454-461.
Morin PJ, Johnson EA, 1988. Experimental studies of asymmetric competition among anurans. Oikos 53:398-407.[CrossRef]
Morse DH, 1974. Niche breadth as a function of social dominance. Am Nat 108:818-830.[CrossRef]
Morse DH, 1976. Hostile encounters among spruce-woods warblers (Dendroica: parulidae). Anim Behav 24:764-771.[CrossRef]
Myrberg AA, Thresher RE, 1974. Interspecific aggression and its relevance to the concept of territoriality in reef fishes. Am Zool 14:81-96.
Nielsen JL, 1992. Microhabitat-specific foraging behavior, diet, and growth of juvenile coho salmon. Trans Am Fish Soc 121:617-634.[CrossRef]
Nishikawa KC, 1985. Competition and the evolution of aggressive behavior in two species of terrestrial salamanders. Evolution 39:1282-1294.[CrossRef]
Parker GA, 1974. Assessment strategy and the evolution of fighting behaviour. J Theor Biol 47:223-243.[CrossRef][Web of Science][Medline]
Persson L, 1985. Asymmetrical competition: are larger animals competitively superior. Am Nat 126:261-266.[CrossRef]
Pucket KJ, Dill LM, 1985. The energetics of feeding territoriality in juvenile coho salmon (Oncorhynchus kisutch). Behaviour 92:97-111.
Quinn TP, Peterson NP, 1996. The influence of habitat complexity and fish size on over- winter survival and growth of individually marked juvenile coho salmon (Oncorhynchus kisutch) in Big Beef Creek,. Washington. Can J Fish Aquat Sci 53:1555-1564.
Robinson SK, Terborgh J, 1995. Interspecific aggression and habitat selection by Amazonian birds. J Anim Ecol 64:1-11.[CrossRef]
Rosenau ML, McPhail JD, 1987. Inherited differences in agonistic behavior between two populations of coho salmon. Trans Am Fish Soc 116:646-654.[CrossRef]
Roughgarden J, Pacala S, 1989. Taxon cycle among Anolis lizard populations: review of evidence. In: Speciation and its consequences (Otte D, Endler JA, eds.). Sunderland, Massachusetts: Sinauer Associates; 403–432.
Rummel JD, Roughgarden J, 1985. A theory of faunal buildup for competition communities. Evolution 39:1009-1033.[CrossRef]
Sabo JL, Pauley GB., 1997. Competition between stream-dwelling cutthroat trout (Oncorhynchus clarki) and coho salmon (Oncorhynchus kisutch): effects of relative size and population origin. Can JFishAquat Sci 54:2609-2617.
Sandercock FK, 1991. Life histories of coho salmon (Oncorhynchus kisutch). In: Pacific salmon life histories (Groot C, Margolis L, eds). Vancouver: University of British Columbia Press; 397–445.
SAS Institute,, 1990. SAS/STAT user's guide. Cary, North Carolina: SAS Institute.
Schoener TW, 1983. Field experiments on interspecific competition. Am Nat 122:240-285.[CrossRef]
Snedecor GW, Cochran WG, 1980. Statistical methods, 7th ed. Ames: The Iowa State University Press.
Swain DP, Riddell BE, 1990. Variation in agonistic behavior between newly emerged juveniles from hatchery and wild populations of coho salmon, Oncorhynchus kisutch. Can J Fish Aquat Sci 47:566-571.
Taper ML, Case TJ, 1985. Quantitative genetic models for the coevolution of character displacement. Ecology 66:355-371.[CrossRef]
Taper ML, Case TJ, 1993. Models of character displacement and the theoretical robustness of taxon cycles. Evolution 46:317-333.[CrossRef]
Taylor EB, Larkin PA, 1986. Current response and agonistic behavior in newly emerged fry of chinook salmon, Oncorhynchus tshawytscha, from ocean- and stream-type populations. Can J Fish Aquat Sci 43:565-573.
Thompson P, Fox BJ, 1993. Asymmetrical competition in Australian heathland rodents: a reciprocal removal experiment demonstrating the influence of size-class structure. Oikos 67:264-278.[CrossRef]
Tilman D, 1982. Resource competition and community structure. Princeton,. New Jersey: Princeton University Press.
Weitkamp LA, Wainwright TC, Bryant GJ, Milner GB, Teel DJ, KopeRG Waples RS, 1995. Status review of coho salmon from Washington, Oregon, and California. NOAA Technical Memorandum NMFS-NWFSC-24. Seattle, Washington: National Oceanic and Atmospheric Administration.
Withler IL, 1966. Variability in life history characteristics of steelhead trout (Salmo gairdneri) along the Pacific Coast of North America. JFish Res Board Can 23:365-393.
Wurtsbaugh WA, Davis GE, 1977. Effects of temperature and ration level on the growth and food conversion efficiency of Salmo gairdneri, Richardson. J Fish Biol 11:87-98.[CrossRef]
Young KA, 2001. The effects of size asymmetries and habitat diversity on interspecific interactions in juvenile salmonids (Oncorhycnhus sp.) (PhD dissertation). Vancouver: University of British Columbia.
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