© 2003 by The Society for Integrative and Comparative Biology
Body Size, Performance and Fitness in Galapagos Marine Iguanas1
1 Department of Ecology and Evolutionary Biology, Guyot Hall 303, Princeton University, Princeton, New Jersey 08540
2 Department of Biology, Tufts University, Medford, Massachusetts 02155
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Complex organismal traits such as body size are influenced by innumerable selective pressures, making the prediction of evolutionary trajectories for those traits difficult. A potentially powerful way to predict fitness in natural systems is to study the composite response of individuals in terms of performance measures, such as foraging or reproductive performance. Once key performance measures are identified in this top-down approach, we can determine the underlying physiological mechanisms and gain predictive power over long-term evolutionary processes. Here we use marine iguanas as a model system where body size differs by more than one order of magnitude between island populations. We identified foraging efficiency as the main performance measure that constrains body size. Mechanistically, foraging performance is determined by food pasture height and the thermal environment, influencing intake and digestion. Stress hormones may be a flexible way of influencing an individual's response to low-food situations that may be caused by high population density, famines, or anthropogenic disturbances like oil spills. Reproductive performance, on the other hand, increases with body size and is mediated by higher survival of larger hatchlings from larger females and increased mating success of larger males. Reproductive performance of males may be adjusted via plastic hormonal feedback mechanisms that allow individuals to assess their social rank annually within the current population size structure. When integrated, these data suggest that reproductive performance favors increased body size (influenced by reproductive hormones), with an overall limit imposed by foraging performance (influenced by stress hormones). Based on our mechanistic understanding of individual performances we predicted an evolutionary increase in maximum body size caused by global warming trends. We support this prediction using specimens collected during 1905. We also show in a common-garden experiment that body size may have a genetic component in iguanids. This ‘performance paradigm’ allows predictions about adaptive evolution in natural populations.
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INTRODUCTION |
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One of the big challenges in integrative organismal biology is to explain—and ideally predict—the evolution of complex animal traits in nature (Darwin, 1883
; Bonner, 1984; West-Eberhard, 2003). A prime example for an integrative trait is body size (Peters, 1983; Schmidt-Nielsen, 1984; Reiss, 1989; McKinney, 1990; Perrin, 1998). We know for many animals that body size is highly heritable, with heritability coefficients (narrow-sense) ranging from 0.3 to 0.9 (Peters, 1983). Body size has also been suggested to be consistently influenced by natural selection in the wild: Mammals on islands generally decrease in body size towards small sizes (Brown and Maurer, 1989), while body sizes in all endotherms appear to increase towards higher latitudes (Bergmann's rule, e.g., Meiri and Dayan, 2003). At the same time it remains unresolved for many animal species why they attain a specific size in a specific habitat (Dobson and Headrick, 1995). The reason why we have so little power in predicting what body size animals ought to have under specific circumstances is that many environmental and internal factors potentially influence body size. Thus it is difficult to determine how body size might evolve (Schmidt-Nielsen, 1984).
Performance measures integrate selective processes
Arnold (1983) suggested a simple yet powerful method to help resolve the multitude of selective processes influencing complex traits. By studying the performance of individuals at the organismal level we can simultaneously integrate and quantify a large number of potential selective agents (e.g., Tsuji et al., 1989). One example is locomotor performance. Sprint performance, for example, can have a direct impact on fitness (i.e., escaping from predators), yet is the net result of many processes, including limb morphology, muscle fiber composition, neuronal (signal processing) capabilities and the natural environment (e.g., altitude, ambient temperature, humidity; Huey and Hertz, 1982; Waldschmidt and Tracy, 1983; Sinervo et al., 1991; Sorci et al., 1994; Bauwens et al., 1995; Farley, 1997). An obvious drawback of such composite performance measures is that the exact mechanistic base for selection on traits may remain unclear (Garland et al., 1991). However, we can determine which performance measures most critically influence fitness (Sorci et al., 1994). Subsequently we can exclude less important performances (e.g., walking performance) or factors (e.g., limb morphology) based on the ranking of their influence on fitness (Swoap et al., 1993). Extending Arnold's (1983) approach, we could then start to analyze the physiological mechanisms underlying specific critical performances (Klukowski et al., 1998).
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MARINE IGUANAS AS MODEL SYSTEM FOR THE EVOLUTION OF BODY SIZE |
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To understand the complex trait body size, we selected the Galapagos marine iguana as a model system: Island populations differ by more than one order of magnitude in body mass. The maximum body mass on Genovesa island in the NE of the Galapagos archipelago is
1,000 g while the largest animals in SW Isabela island reach >12,000 grams (Wikelski et al., 1997). At the same time two confounding factors that often influence body size in unpredictable ways, predation and competition, are largely absent for adult iguanas (Curio, 1965). There are no major predators to adult marine iguanas except the Galapagos hawks (Buteo galapagoensis) that kill a minor percentage of females during the nesting periods on some islands, independent of their body size (unpublished data, M. Wikelski). Competition with other vertebrates is similarly unimportant for marine iguanas, which gather all of their food in the intertidal algae flats that are exposed at low tide, or by diving for submerged algae pastures (Wikelski and Trillmich, 1994). Marine iguanas possess specialized hindgut fermenting microbes that help them to digest algae cell walls (Mackie et al., 2003). Furthermore, intraspecific competition is minimal since marine iguanas forage on red and green algae in scramble competition, i.e., can not exclude each other from foraging sites but simply try to maximize individual intake during the time-constrained foraging hours (Trillmich and Trillmich, 1986). Most individuals of each population (95%) forage approximately every second day in the intertidal areas as soon as high tides recede, while only the largest individuals (mostly males, 5% of each population) also go diving for food (Hobson, 1965; Wikelski and Hau, 1995). Diving animals emerge from underwater foraging bouts during the hottest time of the day and thus maximize their re-warming rate (Buttemer and Dawson, 1993).
The key natural history observation in marine iguanas that allowed for an evolutionary analysis of body size is that the largest animals of each island die first during adverse environmental condition such as El Niño periods (Fig. 1, upper panel; Laurie, 1989, 1990; Laurie and Brown, 1990a, b). During El Niños, sea surface water temperature can rise from as low as 11°C to as high as 32°C when the upwelling of cool, nutrient-rich water ceases. Post-mortem analysis suggested that death occurred as a result of starvation induced by a lack of food in the intertidal areas (Cooper and Laurie, 1987). Indeed, the largest animals per island were unable to reach high body condition values, suggesting that food supply limits body size in some way (Fig. 1, lower panel; Wikelski and Trillmich, 1997).
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Performance measures that are not important in marine iguanas
Initially we predicted that locomotor performance should be of great importance for marine iguanas as they forage actively by grazing in the intertidal zone. Marine iguanas have to run towards the intertidal areas between huge breaking waves, need to sprint to crevices or behind rocks to avoid being swept away, and have to grip the intertidal rocks as strongly as possible to resist wave drag. We therefore first measured—as do many lizard researchers—sprint performance, as well as the iguana's force of gripping wire mesh (Wikelski and Trillmich, 1994
). Small animals had higher sprint speeds compared to large individuals at high body temperatures (35°C), but not at lower body temperatures (25°C; see also Bartholomew and Lasiewski, 1965; Bartholomew, 1966; White, 1973). We suggest the ultimate reason for high sprint speeds in small iguanas may be predation by hawks, or the need to run from huge waves that could wash them out into the ocean. Small animals could not grab a wire mesh as forcefully as large animals (2 N versus 25 N). Those data initially encouraged us to predict that sprint speed and grabbing force would strongly influence foraging performance. However, when we observed foraging durations of animals of all sizes we found that there was no difference in average daily foraging time according to body size: All individuals on Santa Fe island foraged for about 30 minutes in the intertidal areas (Wikelski and Trillmich, 1984). Thus although sprint speed and grabbing force are likely important per se, they appear to be unrelated to foraging performance (Wikelski and Trillmich, 1997).
Based on the scramble competition in the foraging areas we then tested whether the timing of arrival in the foraging areas significantly contributed to differential survival of large and small animals. We found that early arrival at intertidal foraging areas was associated with improved survival during El Niño conditions (Wikelski and Hau, 1995). However, the differences in arrival time only explained survival differences within each size class of animals, not the large differences in survival between small and large animals during El Niños. Although the discovery of presumably endogenous circatidal foraging rhythms in a terrestrial vertebrate was interesting (Woodley et al., 2003), it did not contribute significantly to the understanding of the evolution of body size.
Critical performance measures in marine iguanas: I) Foraging performance
Because some of the performance measures that often explain lizard fitness turned out to be relatively unimportant in marine iguanas, we concentrated on foraging performances that could potentially be related to survival (Illius and Gordon, 1992). We measured and marked individual iguanas and observed them continuously during their daily foraging trips, counting every bite they made when they foraged in the intertidal or while diving. We then captured those individuals immediately after their daily foraging periods and flushed their stomachs by lavage (Wikelski et al., 1997). Because marine iguanas graze on pastures with very short algae sward lengths, we are confident that stomach washing represented all food gathered by individuals during a daily foraging trip. Subsequently we determined the energy content of the lavaged food (30 kJ/dry mass) and the digestive efficiency of the animals (70%; Wikelski et al., 1993; Rubenstein and Wikelski, 2003). Neither algae quality nor digestive efficiency changed with body size, but total intake of algae dry mass was higher in larger animals. However, when we compared food intake with energy expenditure of marine iguanas of specific sizes, we found that large animals could barely gather enough food to sustain their body size while small animals had a surplus of energy beyond maintenance needs (Fig. 2; Drent et al., 1999; Wikelski et al., 1997). Field metabolic rates were measured on Fernandina Island (Nagy and Shoemaker, 1984) as well as on Santa Fe and Genovesa islands (Drent et al., 1999) via the doubly labeled water technique. Large animals in each population were presumably at their energetic limits, while small animals could support tissue growth or fattening with the excess energy they had available. Interestingly, the absolute body sizes at which energy intake equaled energy expenditure were different between populations. On Genovesa Island, where animals only reach a maximum body mass of 1,000 grams, the energy intake line crossed the energy expenditure line at a much lower body size than on Santa Fe Island, where marine iguanas reach a maximum body mass of 4,500 grams. This result suggests that foraging performance was key to the differences in body size between island populations (see also Illius and Gordon, 1992; Naganuma and Roughgarden, 1990; Carbone et al., 1999). We suggest that restrictions on foraging time do not influence metabolizable energy intake because the small Genovesa iguanas had slightly longer foraging times compared to the large Santa Fe iguanas (M. Wikelski, unpublished data).
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To determine the mechanistic basis for differences in foraging performance we first analyzed the foraging efficiency in terms of intake per bite in relation to each metabolic gram of body mass (field metabolic rate was found to scale to body mass to the power of
0.8 in marine iguanas, Nagy and Shoemaker, 1984; Drent et al., 1999; see also Butler et al., 2002). Foraging efficiency was much higher in populations where marine iguanas reached higher body masses (Santa Fe Island) compared to islands where maximum sizes are smaller (Genovesa Island; Fig. 3). In addition, we also detected a generally decreasing trend of foraging efficiency with body mass, larger animals being overall less efficient in their foraging efforts. However, our analysis failed to detect a decrease in foraging efficiency among the largest animals per island, i.e., among those individuals that show the strongest decrease in survival during adverse environmental conditions (El Niños; Fig. 3). Thus although differences in foraging efficiency are consistent with between-population differences in maximum body size, they are not sufficient in predicting differential survival of iguanas within each population.
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We therefore searched for additional environmental factors that could explain the remaining differences in survival among the largest animals in each population (Porter and Tracy, 1983
; Huey and Kingsolver, 1989; Bakken, 1992; Adolph and Porter, 1993; Porter et al., 2000). Marine iguana body temperature is directly related to ambient temperature that limits the rewarming intervals between feeding trips, as well as the warming-up phases before and after foraging in the cold intertidal areas. Sea surface temperatures in Galapagos can be as low as 11°C which can significantly limit foraging times (Bartholomew, 1966; Wikelski and Trillmich, 1994). Because we already knew (see above) that individuals of all body sizes within each population had similar total daily foraging time, we investigated the key parameter that might change with body temperature during foraging trips: bite rate. We implanted body temperature transmitters in marine iguanas on several islands and found that the maximum speed of biting depended linearly on body temperature. High body temperatures allow significantly faster bite rates than low body temperatures (Fig. 4). At the same time, we also detected a slowing of digestive rate, but not digestive efficiency, at lower body temperatures, with gut passage time having a Q10 of about 2.5 (Wikelski et al., 1993; cf., McWilliams et al., 1997; Secor and Phillips, 1997).
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We then combined data for algae intake rates at various environmental temperatures with changes in digestion at different body temperatures in a model attempting to explain maximum body size throughout the Galapagos archipelago. In short, the model calculates the total number of bites per day for iguanas of various body sizes and islands under each island's typical environmental temperatures, using the formula:
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where B is the total number of bites obtained, t is time, Tb is body temperature, T0 is the initial body temperature (before entering the cold intertidal), c is an empirically determined negative variable that scales the rate of heat loss to the water and is dependent on the body mass of iguanas, and Tss is sea surface temperature. To arrive at total food intake per day, the total number of bites per day is then multiplied by the amount of food ingested per bite, which differs depending on the height of the algae pasture in the intertidal. These simple calculations, not yet based on first-principle heat transfer models (Porter et al., 2000), suggest that environmental temperatures and algae pasture heights are indeed sufficient to explain body size differences in marine iguanas (Fig. 5; Wikelski and Carbone, 2003; see also Porter and Tracy, 1983; Zimmermann and Tracy, 1989).
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Mechanistic regulation of body size and body mass
As our data suggest that foraging performance predicts body size we attempted to further understand the physiological mechanisms regulating selection on body size. The Galapagos archipelago offers a natural experimental situation for selection studies because recurring El Niño events lead to high mortality selection (Grant, 1986
; Snell, 1987; Wikelski and Trillmich, 1997). We found that the reptile-typical glucocorticoid stress hormone corticosterone (CORT) is elevated once body condition drops below a critical threshold level, determined as a significant change in the slope of a linear regression model (Fig. 6, and unpublished data; Fig. 6 shows data for Santa Fe only, but iguanas on five other islands showed the same trends). Glucocorticoids such as CORT are known to have dramatic influences on behavior and physiological systems in all vertebrates (Romero and Wikelski, 2002a). In general, the highest levels of CORT were found in animals that were almost completely emaciated, many of which died during the strong El Niño of 1997/98 (Fig. 7). However, not all iguanas in bad body condition had high CORT levels and we are presently investigating whether CORT levels are indeed causally related to the animals' death. Some large individuals survived long-term El Niño conditions while simultaneously shrinking in body mass and length (Fig. 8; Wikelski and Thom, 2000). Individuals that shrank between 21 to 60 mm in body length survived on average for 3 years, while individuals that did not shrink or that slightly grew only survived on average for one more year. In general, survival probabilities were higher for individuals that shrank more in body length. Although we are only beginning to unravel the mechanistic details of shrinkage in body length (unpublished data, T. Bromage, I. Smolyar and M. Wikelski), we hypothesize that high CORT levels may contribute to the remodeling of body structure, i.e., reversed growth or resorption of bones. It is so far unclear whether changes in a leptin-like hormone (unpublished data, R. Londraville and M. Wikelski) and/or growth hormone (cf., Gerwien and John-Alder, 1992) could be involved in body length shrinkage.
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The increased secretion of CORT is not restricted to large animals that are stressed by El Niño-induced food shortages. Marine iguanas of all sizes had high circulating levels of CORT in their blood after a man-made environmental disaster, an oil spill (Fig. 7; Wikelski et al., 2001a
). We assume that CORT is causally involved in supporting organisms during unpredictable but potentially disastrous events (Wingfield et al., 1998) such as oils spills. The population of Santa Fe iguanas affected by the oil spill dramatically declined, while an unaffected control population with low CORT levels had no mortalities (Wikelski et al., 2002). Thus in this case we could use the secretion of CORT—presumably intended to help the individual survive—as an indicator of environmental stress on the population level.
Critical performance measures in marine iguanas: II) Reproductive performance in females
While foraging performance appears to explain the limits to maximum body size, it does not predict that individuals should grow towards large sizes in the first place. Therefore we hypothesized that reproductive performance in females increases with body size because large females generally produce larger hatchlings that are better able to survive (Sinervo and Huey, 1990; Sinervo et al., 1992). So far we have not tested this prediction directly, but obtained supporting circumstantial evidence: First, we found that larger females produced larger clutches with larger eggs (Fig. 9), which in reptiles often produce larger hatchlings (e.g., Indenbosch, 1998; Angilletta, 1999; Angilletta et al., 2000). Egg size increases with body size in a stepwise fashion and marine iguana females allocate about 25–30% of their body mass into one clutch every second year (Fig. 9, inset). Females on Genovesa Island (maximum body mass 500 grams) produce only one egg per clutch, while females on Santa Fe produce two or three eggs per clutch. Large females on Fernandina Island (4,000 grams) can produce up to 6 eggs per clutch (Gus Angermeyer, personal communication). Second, we found that larger hatchlings survive longer (Fig. 9, lower panel). An increase in hatchling snout-to-vent length of 10% increases the average 1st year survival rate by about 30% (Fig. 9, lower panel). It is not yet clear whether large male size contributes to larger sizes of sons or daughters.
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Critical performance measures in marine iguanas: III) Reproductive performance in males
Male marine iguanas have the highest mating success if they can occupy a central display territory (lek) in a mating aggregation along the shore (Trillmich, 1983
; Partecke et al., 2002). However, competition for such territories is intense and the outcome of fierce fights is generally determined by body size (Wikelski et al., 1996). Males therefore have to grow to large body sizes and only at an age of 12–15 years will they establish a territory for the first time. Smaller and/or younger males are relegated to secondary positions in the lek, or have to roam around territories as satellite males. Even smaller "sneaker" males can mimic female behavior and thus remain within the territories of larger males, which can not distinguish them from females based on their morphology.
We investigated what aspect of reproductive performance contributes most to mating success in males. Female marine iguanas choose males based on the persistence of male head-bob display while they patrol their tiny territories (Wikelski et al., 2001b). The amount of head-bobbing in turn can be influenced by the level of circulating testosterone (Fig. 10; Wikelski et al., 2003). As for foraging performance, we attempted to understand the physiological basis of how individual males adjust their reproductive performance to the prevailing social environment. Our previous investigations showed that El Niño-related environmental catastrophes can seriously change the maximum and average body size within a population of marine iguanas. Some populations may lose as many as 90% of all males during a strong El Niño event (Romero and Wikelski, 2002a, b). The remaining small males adjust their reproductive tactics (Wikelski and Bäurle, 1996) via social feedback that in turn changes endocrine secretions. Via repeated-measures sampling of the same individuals we found that males that fight and are defeated increase CORT and decrease testosterone secretions, while males that win fights increase testosterone levels (Wikelski et al., 2003). In territorial males, testosterone levels then regulate the amount of head-bob patrolling. If satellite or sneaker males increase testosterone levels, they change their reproductive performances towards the next higher social level, i.e., from sneaker to satellite or from satellite to territorial male. We tested the flexibility of behavioral reproductive phenotypes by implanting either pharmacological testosterone blockers in territorial males, or by administering exogenous testosterone levels (via injection) in satellite and sneaker males (Fig. 10, upper and middle panel). All males that were hormonally forced to change reproductive performance (sneaker to satellite, satellite to territorial, territorial to satellite) decreased their access to females, thus presumably decreased their fitness (Fig. 10, lower panel). Thus although the change in reproductive performance was not beneficial for males in experimental situations, all males nevertheless switched from one phenotype into the other, consistent with the predictions of Moore (1991) and colleagues' (1998) Relative Plasticity Hypothesis. This implies that hormones are powerful physiological agents that can constrain behavior pattern of animals in the wild even when induced socially (Wingfield et al., 1998).
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Prediction of long-term evolutionary changes in marine iguana body size
Based on our above results we predict that sexual selection should always push body size towards larger sizes due to the enhancement of reproductive performance in both males and females (Wikelski et al., 1997
; see also Case, 1976; Case and Schwaner, 1993; Losos et al., 1997). At the same time, natural selection should counteract sexual selection on body size due to a decrease in foraging performance in large individuals in each population (Arnold and Wade, 1984a, b). Because bite rate is causally dependent upon environmental temperatures in marine iguanas, we predict that a relaxation of constraints on foraging performance should result in an overall increase in body size on all islands. Environmental temperatures have increased globally and in particular in the Pacific region over the past century (Parmesan and Yohe, 2003). We therefore expected that body size of Galapagos marine iguanas should increase along with global warming trends and warmer sea surface temperatures. We tested this prediction using specimens collected during the 1905/06 California Academy of Sciences expedition to the Galapagos Islands. We measured the snout-to-vent and skull lengths of the largest animal per island for extant populations of marine iguanas and for specimens in the California Academy of Sciences collection (VanDenburgh and Slevin, 1913). We assume, based on notes from the 1905/06 expedition members (personal observation, M. Wikelski), that the collection contains many of the largest animals found per site. Similarly, during our population surveys in 2000, we attempted to capture and measure the largest animal per site, which is generally a good predictor of the largest body size per island (Stamps and Andrews, 1992; Gaines and Denny, 1993). The comparison of body sizes, based on head length measurements (Wikelski and Trillmich, 1997), between 1905/06 and 2000 shows that the maximum body size of marine iguanas has increased significantly throughout the archipelago (Fig. 11). Although we can not ascertain that shrinkage in preservation media is completely absent even for head lengths nor that sampling techniques were similar during both expeditions, we suggest that our data are consistent with an environmentally induced increase in maximum size of marine iguanas in Galapagos over the course of the past century.
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Body size in iguanas may have a heritable component: a common garden experiment in green iguanas
Unfortunately, we were not allowed to directly conduct a common garden experiment in the Galapagos because marine iguanas from different populations can not be brought into captivity at one site. Therefore, we used the green iguana (Iguana iguana, Burghardt and Rand, 1992
) as a surrogate species in which to test potential genetic influences on growth rates and body size (for other iguanids, see Petren and Case, 1997; Tracy, 1999, 2001). We collected green iguana eggs from the island of Curacao in the Caribbean, where green iguanas attain much smaller maximum body size compared to conspecifics in Panama (Lichtenbelt, 1992, 1993; Lichtenbelt and Albers, 1993; Lichtenbelt et al., 1993, 1997). We transported the eggs from Curacao to Panama, incubated the eggs in similar environments, and subsequently held Panamanian and Curacao hatchlings under common garden conditions at the Tupper Research Center of the Smithsonian Tropical Research Institute in Panama City (unpublished data, M. Wikelski et al.). While Curacao green iguanas had smaller egg sizes and were smaller than Panama iguanas at hatch, they caught up with the larger Central American relatives within 4 months (Fig. 12). However, growth then stalled in Curacao iguanas during the supposed dry season in Curacao, while Panama iguanas grew rapidly. After about two years, Panama iguanas were substantially larger than Curacao iguanas, suggesting that growth and final size has at least some heritable component when environmental factors are largely controlled for (Niewiarowski and Roosenburg, 1993; Sugg et al., 1995). However, we do not yet know whether different populations of green iguanas have different yolk allocations, which could strongly affect growth rates and final sizes and thus be part of the mechanism determining body size in each population. It is interesting to note that green iguana growth patterns are similar to those found by Tracy, (1999) in chuckwallas (Sauromalus obesus), although on a different time scale.
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How useful is the "Performance paradigm"
We used individual performance tests to identify the factors that possibly contribute to individual fitness in the wild, and to exclude factors that are unlikely to be important for specific traits. We subsequently determined the causal link between the performance factors and physiological characteristics of individuals, allowing us to tentatively connect the life history characteristics of animals to their mechanistic causation. For several performance measures we could establish plausible endocrine mechanisms (corticosterone and testosterone secretions) that are possibly linked to the observed individual differences in selective regimens (Wikelski and Ricklefs, 2001
; Ricklefs and Wikelski, 2002). Our data suggest that the performance paradigm can be expanded to reveal more of the physiological underpinning of natural selection in the wild. We suggest that measuring morphological, behavioral and in particular, physiological performance in nature indeed allows us to predict evolutionary consequences of individual life histories in the wild. We further suggest that our analysis allows, for example, for the prediction of increased body sizes during the past 100 years in all Galapagos marine iguana populations. If confirmed by further experimental analysis, such a prediction would lend strong support for the continued application—and extension into the physiological and molecular field—of Arnold's (1983) "Performance paradigm."
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ACKNOWLEDGMENTS |
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We thank our friends and field assistants for invaluable help. We thank the Galápagos National Park Service, the Charles-Darwin Research Station, Metropolitan touring through the yacht ISABELA II, the Carmabi Foundation and TAME for enabling this work. E. Ott and S. Layer provided intercontinental communication. Andrew Laurie and Thomas Dellinger kindly permitted the use of their long-term population data. Special thanks to Wolfgang Wickler and Ebo Gwinner, Max-Planck Institute, and Fritz Trillmich for continuous support. This is contribution no. 771 of the Charles-Darwin-Foundation and was supported by the German Science Foundation, the Max Planck Society, the Humboldt Society, the Smithsonian Tropical Research Institute, the University of Illinois, Tufts University and Princeton University.
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FOOTNOTES |
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1 From the Symposium Selection and Evolution of Performance in Nature presented at the Annual Meeting of the Society for Integrative and Comparative Biology, 4–8 January 2003, at Toronto, Canada.
2 E-mail: Wikelski{at}princeton.edu
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