Behavioral Ecology Advance Access published online on February 27, 2008
Behavioral Ecology, doi:10.1093/beheco/arn014
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A growth cost of experimentally induced conspicuous coloration in first-year collared lizard males
Department of Biology, University of Central Oklahoma, 100 North University Drive, Edmond, OK 73034, USA
Address correspondence to T.A. Baird. E-mail: tbaird{at}ucok.edu.
Received 26 May 2007; revised 13 December 2007; accepted 30 December 2007.
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ABSTRACT |
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I used painted first-year collared lizard males in the field to test the hypothesis that conspicuous coloration imposes a growth cost, either because it makes lizards less cryptic to their prey or because conspicuous color attracts predators forcing increased refuging by lizards. To make males more conspicuous, I painted them green and yellow to match hues of older territorial males, made another group inconspicuous by painting males brown like females, and painted a control group with water. I then compared rates of travel, the frequency, and the percentage of strikes on prey that were successful. I recaptured males periodically to record cloacal and substrate temperatures, to measure growth rate, and to retouch paint. Time spent refuging, travel rate, and both cloacal and substrate temperatures were similar in the 3 groups. Males painted conspicuously had slower growth than males in the other treatment groups. Inconspicuous males initiated strikes on prey more than 4 times more frequently than conspicuous males, and the number of strikes that resulted in prey capture was higher in inconspicuous males, suggesting that conspicuous coloration reduces foraging opportunities because it makes first-year males more visible to their prey. Rapid growth during the first season is necessary for collared lizard males to become large enough to compete for breeding territories as 2 year olds. Therefore, conspicuous coloration may develop gradually in first-year collared lizard males to maintain crypsis, which promotes food intake sufficient for rapid growth to large size that has important consequences for future reproductive success.
Key words: aggressive crypsis, costs of conspicuous coloration, foraging, predation risk.
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONMETHODSRESULTSDISCUSSIONFUNDINGREFERENCES |
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A variety of selective pressures interact to shape the coloration patterns of animals (Endler 1980
, 1992; Macedonia et al. 2004; Husak et al. 2006). These sometimes include natural selection for effective thermoregulation (Christian 1996; Bittner et al. 2002; Garcia et al. 2003) and interactions with both predators and prey (Grether and Grey 1996). Sexual selection may also favor coloration that functions in social signaling (Cooper and Greenberg 1992; Whiting et al. 2003). Predator avoidance and foraging efficiency are usually thought to be enhanced by cryptic coloration (Grether and Grey 1996; Huhta et al. 2003; Stuart-Fox et al. 2003, 2004). By contrast, especially in males, sexual selection may favor conspicuous color signals through female choice (Vitt and Cooper 1985; Andersson 1994; Andersson and Iwasa 1996; Kwiatkowski and Sullivan 2002; Whiting et al. 2003), male competition (Vitt and Cooper 1985), or both of these mechanisms (Berglund et al. 1996; Baird et al. 1997).
Although conspicuous male coloration may confer mating advantages, it is also hypothesized to impose one or more fitness costs. In species where males use visual signals to advertise resource-holding potential to competitors (Husak 2004; Lappin et al. 2006), conspicuous coloration may be costly if it prompts increased aggression from conspecifics (Kotiaho 2001; Macedonia et al. 2004). Conspicuous coloration is also hypothesized to increase predation risk, and some correlational evidence between interpopulation variation in color intensity and predation pressure is at least consistent with this hypothesis (McPhail 1969; Barlow and Ballin 1976; Huhta et al. 2003). Higher rates of attack on conspicuously colored inanimate models (Stuart-Fox et al. 2003; Husak et al. 2006) also support a proposed predation cost of conspicuous coloration; however, such model studies do not mimic predation on conspicuously colored live prey (Slagsvold et al. 1995; Götmark and Olsson 1997), and evidence of higher predation on live prey that are conspicuously colored is limited (Slagsvold et al. 1995; Götmark and Olsson 1997; Kotiaho 2001).
Conspicuous coloration may also be costly because it reduces foraging effectiveness in 2 different ways. If conspicuous coloration attracts the attention of potential predators, then prey may be forced to spend more time taking refuge, which likely reduces their ability to forage effectively (Martín and López 2001). Conspicuously colored individuals may also be less effective predators because they are visually conspicuous to their prey (reduced aggressive crypsis; Grether and Grey 1996; Ortolani 1999; Macedonia et al. 2002). Such increased conspicuousness may be particularly costly in diurnal sit-and-wait predators that rely on aggressive crypsis while they scan for prey that are approaching within striking distance.
I used paint manipulations of first-year collared lizard males in the field to test the hypothesis that conspicuous coloration imposes a growth cost because foraging effectiveness is reduced either through reduced aggressive crypsis or increased time spent refuging. I manipulated lizard color by enhancing the green and yellow of some males (hereafter conspicuous males), obscuring these colors in other males by applying brown paint similar to the color of females (hereafter inconspicuous males), and painting a control group with water. First-year male collared lizards at Arcadia Lake (hereafter AL), Oklahoma, are a good model system for this study. During the first year, males are only in the process of developing the conspicuous hues that characterize 2y+ territorial males (McCoy et al. 1997). Spectrophotometric studies on both the rock substrata and lizards in the AL study population showed that females are the most cryptically colored, whereas the coloration of 2y+ territorial males gives them higher contrast against the rock substrata (Macedonia et al. 2004). Because they are only developing conspicuous coloration during the first activity season (McCoy et al. 1997), the degree to which coloration can be altered in both directions within the natural range of this population is highest in first-year males.
Because collared lizards are stealthy, sit-and-wait predators primarily on insects (Best and Pfaffenberger 1987; Husak and McCoy 2000; Baird and Sloan 2003) and first-year male collared lizards grow 10 times faster than 2y+ males (Baird et al. 2003; Baird and Hews 2007), potential foraging consequences of conspicuous coloration should be pronounced in first-year males. Collared lizards capture arthropods by scanning surrounding vegetated areas from elevated rock perches until prey move within striking distance (1–2 m), at which point the lizards dash toward and strike the prey item (hereafter, prey strike; Baird and Sloan 2003). If increased predator attacks force conspicuously colored individuals to spend more time taking refuge (Martín and López 2001) which for AL collared lizards are abundant rock crevices (Baird and Sloan 2003), then the ability of lizards to scan for prey should be compromised, reducing their food intake. There is also considerable evidence that orthopterans (the major prey of collared lizards at AL) discriminate spectra ranging from 525 to 570 nm and respond strongly to wavelengths that appear green to humans (Kong et al. 1980, Wasserman and Kong 1982, Bailey and Harris 1991). Green is the primary hue developed by 2y+ male collared lizards (McCoy et al. 1997) that spectral data have shown are the most conspicuous against the background at the AL site (Macedonia et al. 2004). Therefore, painting first-year males green and yellow is expected to increase their degree of contrast against the rock substrata that these lizards use as perches while waiting for prey to approach within striking distance. Increased contrast would make it more difficult for perched males to remain inconspicuous until prey approach closely. Alternatively, painting males with hues that are similar to females that spectral data show are more cryptic against the background at AL (Macedonia et al. 2004) is expected to make lizard predators less conspicuous to their prey.
If foraging effectiveness is compromised because conspicuously colored lizards spend more time refuging in response to increased attention from predators, then the amount of time that subject males are emergent, travel rate, the frequency of prey strikes, and growth rates should all be inversely related to color conspicuousness (Table 1, predictions 1a–d). Alternatively, if foraging effectiveness is compromised because enhanced green/yellow coloration makes collared lizards more conspicuous to their arthropod prey, whereas brown coloration makes them more cryptic, then both the amount of time emergent and travel rate should be independent of coloration (Table 1 predictions 2a,b), but the frequency of prey strikes, the percentage of successful strikes on prey, and growth rates should be inversely related to color conspicuousness (Table 1, predictions 2c–e). I tested the predictions of these 2 reduced foraging effectiveness hypotheses by recording the percentage of censuses when lizards were emergent, rates of travel, the frequency of foraging strikes, and the percentage of strikes on prey that were successful during focal observations and the growth rates (change in body mass and snout-to-vent [SVL]) of first-year males in the 3 paint treatment groups.
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METHODS |
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Study subjects and general methods
I conducted this study from 20 April to 25 June 2002–20004 at AL Dam located 9.6 km east of Edmond, OK. At AL, Crotaphytus collaris occupies 3 topographically homogeneous patches (1,505–19,850 m2) of boulders imported to construct flood-control spillways. This site allows effective behavioral studies because human access is restricted, lizards are undisturbed and readily observed, and the age of all males is known from mark-recapture studies conducted since 1990 (Baird et al. 1996
, 2003). The AL study site is mapped to scale using global positioning satellite accurate to the nearest meter (Baird and Timanus 1998; Baird et al. 2001). Beginning when they were hatchlings, lizards were noosed, the terminal phalanges of 3 digits were clipped for permanent identification, and unique combinations of nontoxic acrylic paint spots were applied to the dorsum for identification from a distance. Gender in hatchling collared lizards is readily determined by the presence (males) and absence (females) of enlarged postanal scales and later by development of dimorphic coloration (McCoy et al. 1997; Baird et al. 2003).
Painting-experiment protocol
For painting experiments, I used males (N = 42) that were in their first activity season (April–July). These males ranged from 73–93 in SVL at the beginning of painting experiments. From 1 to 20 May, subject males were captured by noose, measured (SVL to the nearest millimeter, mass to the nearest gram), and randomly assigned to 1 of 3 paint treatment groups (see below), and painted. Males in the conspicuous and inconspicuous treatment groups were painted on the dorsal and lateral surfaces of the torso, limbs, tail, head, and dewlap with nontoxic water-based acrylic paint. Control males were painted with water. Care was taken to keep paint (or water) away from the mouth, eyes, and external auditory meatus. Worn paint was retouched as necessary during routine recaptures (mean intercapture interval in days ± 1.0 standard error of the mean [SE] = 8.8 ± 0.45) to monitor growth rates (see below). Recapture interval for the 3 treatment groups was not different (F2,41 = 0.67, P = 0.51). To identify subject lizards from a distance, I painted a white number (3–4 mm high) on the dorsum. Paint was lost when lizards molted. Combined observations during quantitative censuses, focal observations, recaptures for growth rates, and other sightings when I simply noted whether or not males were painted revealed that subject males in paint treatment groups were without paint for no longer than 2 days during the study period (1 May–30 June).
The objective of the paint manipulation was to markedly increase the contrast against the natural background at AL of first-year males in the conspicuous group, to decrease contrast against the background for the inconspicuous group and not to change the contrast for the control group. The most conspicuously colored lizards against the background at the AL site are 2y+ males. Therefore, I used Munsell (1969) color charts to measure the coloration of unmanipulated 2y+ territorial males on the dorsal surface of the torso, limbs, tail (hue = 7.5 green-yellow, value = 4-6, chroma = 5–7), and the dewlap (hue = 2.5 yellow, value = 7, chroma = 10). For the conspicuous treatment group of first-year males (N = 14), I mixed paint to mimic this natural coloration, painted the dorsal surfaces of the torso, limbs, and tail green (hue = 7.5 green-yellow, value = 5, chroma = 6), and painted the dewlap, top and sides of the head yellow-orange (hue = 2.5 yellow, value = 7, chroma = 10). Spectral data show that females at AL are less conspicuous against their background than are adult males (Macedonia et al. 2004). Therefore, I used Munsell charts to measure the brown dorsal coloration of mature females (hue = 7.5 yellow-red, value = 5, chroma = 2), mixed paint to match, and painted the inconspicuous group (N = 13) on the dorsal surface of the torso, limbs, tail, dewlap, and sides and top of the head. Control males (N = 15) were painted on these surfaces with water. Once the paint (or water) had dried (
10 min), males were released at their precise capture locations. Males were first painted from 1 to 20 May which is the beginning of the reproductive season at AL. Date of first painting in the 3 treatment groups (mean date ± 1.0 SE in d; conspicuous = 7 May ± 1.4, inconspicuous = 8 May ± 1.5, water = 7 May ± 1.3) did not differ (F2,41 = 0.31, P = 0.74) among treatment groups. Painting did not appear to have any adverse affects on the behavior of lizards. Only 7 of 42 (16.7%) lizards disappeared abruptly during the study, which is well within the rate of disappearance of unmanipulated first-year males in this population (Baird TA, unpublished data).
To test if males in the 3 treatment groups differed in the amount of time spent refuging, I censused the entire study site at least every 2 days from 1 May to 30 June (at least 30 censuses/site). I quantified the frequency of prey strikes, whether or not prey strikes were successful, and the rate of travel by recording 5, 20-min focal observations (Baird et al. 2003) on each subject male. Focal observations were recorded on separate days distributed throughout the period 1 May to 30 June, which is the reproductive season for this species at AL (Baird et al. 1996, 2001, 2003). I measured the length of cumulative lizard movement paths using a digital planimeter (Planix 2000) and calculated the hourly rate of travel (meters/hour) by dividing the cumulative distance moved during all focal observations by the total observation time on each subject male (Baird et al. 1996, 2001). I used 1-way analysis of variance (ANOVA) to examine the influence of treatment group on rate of travel.
To test the influence of coloration on first-year male foraging opportunities, I calculated the cumulative frequency of prey strikes during the total focal observation time on males in each treatment group. Prey strikes consist of lizards making sudden dashes or lunges from rock perches into vegetated areas immediately after which they return to their perches (Baird and Sloan 2003; Baird et al. 2003). Prey strikes were obviously different than any other behavior patterns because they involved very sudden short movements, and in most (> 95%) cases arthropod prey were either seen jumping/flying to escape pursuing lizards, or lizards were observed holding prey in their mouths. Consequently, I was also able to determine and compare the number of strikes on prey that were successful in the 3 treatment groups using a Chi-square Test of Association.
I monitored growth rates (mm/d; g/d) in the 3 treatment groups, by dividing the changes in SVL and mass between the date of first painting and date of last capture, by this interval in days. I measured SVL using a transparent ruler held firmly against the lizard's ventral midline, and mass (±0.5 g) using a spring scale (Pesola, Kupuskasing, Ontario) that was calibrated daily. At each capture, I made five measurements of both variables, and recorded the mean of these. On the day of first painting, both male SVL and body mass were similar in the 3 treatment groups (mean SVL in mm ± 1.0 SE; conspicuous = 86.3 ± 2.0; inconspicuous = 84.3 ± 1.5; water = 88.2 ± 1.0; F2,41 = 1.58, P = 0.22; mean mass in g ± 1.0 SE; conspicuous, 23.90 ± 4.22; inconspicuous, 21.43 ± 4.71; water, 21.04 ± 4.01; F2,41 = 1.75, P = 0.188). The number of days that I monitored growth rate (mean days ± 1.0; conspicuous = 34.1 days ± 2.0; inconspicuous = 36.2 days ± 3.1; control = 35.5 days ± 4.1), was also similar in the 3 treatment groups (F 2, 41 = 0.23, P = 0.79). Because the data for both change in SVL and mass, as well as the frequency of prey strikes did not meet parametric assumptions, I used Kruskal–Wallis tests to examine the effects of paint treatment on each of these dependent variables.
If painting males conspicuously versus inconspicuously differentially altered the ability of lizards to heat and cool their bodies, it is possible that behavior and/or growth rates could vary as a consequence of altered thermal parameters. To test this, I used a quick-reading thermometer (Avinet Inc., Dryden, NY) to measure male cloacal temperature and substrate temperature at the locations where lizards were captured. I recorded 4–6 body and substrate temperature measurements on different days in each treatment group (conspicuous, N = 12; inconspicuous, N = 9; water, N = 9) and used the means of these measurements in separate 1-way ANOVAs to test for effects of treatment. Because reduced growth could also result if conspicuously painted first-year males expended more energy fleeing territorial males, I recorded the frequency of aggression received by first-year males during focal observations and tested whether this differed by treatment.
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RESULTS |
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Neither the percentage of censuses that lizards were sighted (F2,41 = 0.53, P = 0.59) nor the rate of travel (F2,41 = 0.17, P = 0.85) by these first-year males were statistically different by treatment group (Table 2). There were no differences among treatment groups (F2,29 = 0.70, P = 0.50) in either cloacal temperature (mean °C ± 1.0 SE; conspicuous = 38.1 ± 0.40, inconspicuous = 37.5 ± 0.32, water = 37.7 ± 0.32) or substrate temperatures where these males were captured (F2,29 = 1.08, P = 0.35; mean ± 1.0 SE; conspicuous = 32.5 ± 0.69, inconspicuous = 31.3 ± 0.59, control = 32.2 ± 0.59). Aggression received from 2y+ territorial males was infrequent in this study (mean hourly frequency of aggression received by first-year males ± 1.0 SE = 0.17 ± 0.05) and did not differ (F1,41 = 0.47, P = 0.63) by treatment group.
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The average frequency with which males painted inconspicuously made strikes on prey was 3.9 and 4.4 times higher than that of males painted conspicuously and with water, respectively, although this trend was not statistically significant (H2,41 = 3.76, P = 0.15, Table 2). However, paint treatment influenced the number of strikes on prey that were successful (X21,2 = 21.50, P < 0.01), with males that were painted inconspicuously having a higher rate of success relative to that of both males painted conspicuously and with water (Table 2). Paint treatment had a significant effect on both measures of growth rate (SVL; H2,41 = 11.46, P = 0.003; mass, H2,41 = 6.77, P = 0.034). The average change in SVL of inconspicuous males was 1.5 times higher (P < 0.05) than that of conspicuous males, whereas the change in SVL of inconspicuous and water painted males was not different (P > 0.05, Table 2). The change in mass in conspicuous males was lower (P < 0.05) than that of both inconspicuously painted and water-painted males (Table 2).
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DISCUSSION |
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Results indicate that males painted green/yellow, which likely increased their conspicuousness, had lower growth rates than males painted to match females that are less conspicuous in this population. There are several possible explanations for the reduction in growth of first-year males painted conspicuously. The trend for less-frequent strikes on prey by conspicuous males together with the higher rate of strikes that were successful by males painted inconspicuously support the hypothesis that conspicuous coloration reduces foraging efficiency through a reduction in aggressive crypsis. Because C. collaris is an ambush predator that primarily takes orthopterans capable of detecting wavelengths corresponding to the green body hues of males (Kong et al. 1980
; Wasserman and Kong 1982; Bailey and Harris 1991), green/yellow coloration may make lizards more visible to prey. The observation that males painted inconspicuously had a higher foraging success strongly suggests that brown coloration promoted their ability to remain inconspicuous until prey approached to within effective striking distance. Similarly, Grether and Grey (1996) showed that increased coloration (wing spots) decreased foraging efficiency in male damselflies, even though such pronounced coloration is favored by sexual selection. In wall lizards, although Martín and López (2001) attributed decreased growth of individuals painted conspicuously to increased refuging behavior, increased conspicuousness to the prey of these lizards may also have contributed to this result.
I cannot reject the hypothesis that painting males different hues altered their thermoregulatory abilities, and hence their growth and activity metabolism, because I did not measure the abilities of males in the 3 treatment groups to maintain preferred body temperatures. There were no differences in either the cloacal or substrate temperatures of subject males in the 3 treatment groups, however, which would be expected if paint treatments had differentially altered thermal physiology. Because nonterritorial first-year collared lizard males flee and hide when they are approached by older territorial males (Baird and Timanus 1998; Baird and Hews 2007), reduced growth could also result if conspicuously painted first-year males expended more energy fleeing territorial males, and/or more time spent hiding, which would reduce the ability to effectively scan for prey. Reduced growth in conspicuously painted males as a consequence of increased aggression from territorial males appears unlikely. Male–male aggression is infrequent in the AL population generally (Baird et al. 2003, 2007; Baird and Hews 2007), was infrequent during focal observations recorded for this study, and did not differ in the 3 paint treatment groups. Lastly, there was no indication that males in the 3 treatment groups in the present study differed in the amount of time that they spent taking refuge or moved throughout their home ranges because males in the 3 treatment groups were emergent during a similar number of censuses and had similar rates of travel during focal observations.
Conspicuous coloration as well as large size appear to be sexually selected traits in male collared lizards that probably promote the ability of 2y+ males to compete intrasexually for reproductive territories and advertise to female mates (McCoy et al. 1994, 2003; Baird et al. 1996, 1997; Husak et al. 2006). Such coloration develops gradually during the first season (McCoy et al. 1997) when males are becoming sexually mature but have not yet adopted territorial tactics and are growing at accelerated rates (Baird et al. 2003; Baird and Hews 2007). Reduced growth in first-year males painted conspicuously suggests that one cost of developing such coloration is reduced foraging effectiveness at a time when rapid growth is extremely important for future reproductive success. My results suggest, therefore, that the gradual development of conspicuous coloration during the first season may be advantageous to promote aggressive crypsis at a time when rapid growth is necessary to attain a body size allowing effective competition for a reproductive territory during the second season (Baird et al. 2007).
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FUNDING |
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Faculty Research Grants from the Joe C. Jackson College of Graduate Studies and Research at the University of Central Oklahoma in 2002–2004.
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ACKNOWLEDGEMENTS |
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I thank W. Parkerson of the United States Army Corps of Engineers for access to the Arcadia Lake Study Site and Teresa D. Baird and Jennifer L. Curtis for assistance in the field.
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