SEXUAL SIZE DIMORPHISM AND MORPHOLOGICAL EVIDENCE ...

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were identified between Genovesa and all the other islands (Genovesa 5 0.116 6 0.036, Santiago 5 0.083 6 0.034, Santa Cruz 5 0.095 6 0.030, Española 5 0.090 6 0.041, Santa Fe 5 0.098 6 0.042, Wolf 5 0.088 6 0.053). In the case of the tarsus index, differences were found between Santiago and Wolf Islands, and between Santa Cruz vs. Santa Fe, Genovesa, and Wolf Islands (Genovesa 5 0.120 6 0.028, Santiago 5 0.108 6 0.029, Santa Cruz 5 0.100 6 0.044, Española 5 0.115 6 0.042, Santa Fe 5 0.120 6 0.060, Wolf 5 0.129 6 0.040). We plotted sexual size dimorphism vs. body size for both males and females by island (Fig. 5) and found no positive correlations between the two measurements. The sample from Wolf Island, in particular, had low sexual size dimorphism relative to individual size. DISCUSSION Our finding of significant morphological differences between the sexes is in agreement with morphological records for other Zenaida species, in which males are generally larger than females (Baptista et al. 1997). The three discriminant models developed in this study supported the results obtained by the principal component analysis. Importantly, the efficiency of the discriminant model was reduced when samples from Wolf Island (northern subspecies) were included in the analysis, which supports observed morphological differences between FIGURE 5. Scatter plot showing the sexual size dimorphism (SSD) wing index against the average wing chord (body size surrogate) for both male and female Galápagos Doves. The lack of a positive relationship between SSD and body size might reflect differences in selection pressures for both males and females on the different islands. Santiago-Alarcon, Diego, 2008, UMSL, p. 34 SIZE DIMORPHISM IN THE GALÁPAGOS DOVE 139 individuals from Wolf and from the other islands. This is the first study to provide statistically significant morphological evidence that supports the separation of the Galápagos Dove into two subspecies, as has been previously suggested by other authors (Ridgway 1897, Gifford 1913, Swarth 1931). This result suggests that populations on Wolf and Darwin islands might be somewhat isolated from the other islands, which is supported by significant positive F ST values (based on five microsatellite loci; see Santiago-Alarcon et al. [2006] for an explanation of the molecular and genetic analyses used) and lower gene flow between northern and southern islands as estimated with program MIGRATE (DS-A and PGP, unpubl. data). In contrast, there is evidence of high gene flow among populations of the southern subspecies based on microsatellite markers (Santiago-Alarcon et al. 2006). The sexual size dimorphism wing chord index calculated for Genovesa doves was significantly higher than indices calculated for the other islands. Because of high gene flow among southern island populations of the Galápagos Dove (Santiago-Alarcon et. al. 2006), morphological differences between the sexes on this island might be the result of phenotypic plasticity in response to different local conditions (Wikelski and Trillmich 1997). Genovesa Island is unique in the Galápagos archipelago in that the top predator, the Galápagos Hawk, has been replaced by the Short-eared Owl (Asio flammeus galapagoensis). Thus, variation in predation regimes on Genovesa due to different hunting strategies and other life history traits of the Short-eared Owl compared to the Galápagos Hawk, coupled with abiotic conditions, might be generating different pressures on the dove population of Genovesa. In addition, we identified significant differences between some island pairs for the sexual size dimorphism tarsus index. It has been suggested that tarsus size depends on the foraging strategy of a species (Miles and Ricklefs 1984). If the two sexes of a species have different ecological roles, some of their morphological traits might vary. Thus, differences in this sexual size dimorphism index between some islands might be due to differences in habitat structure and ecological interactions. For example, inhabited islands such as Santa Cruz
140 DIEGO SANTIAGO-ALARCON AND PATRICIA G. PARKER have been altered and are composed of habitats not found on other uninhabited islands. These differences among islands might represent areas of strong selection pressure for endemic doves. Nonetheless, because of high gene flow among southern island populations, observed differences are more likely the result of phenotypic plasticity. The ecological causes of sexual size dimorphism in the Galápagos Dove are difficult to identify because there are limited data for this species (Grant and Grant 1979). However, morphological differences are likely not the result of a single factor, but rather the result of multiple interacting factors (Moore 1990, Owens and Hartley 1998, Clegg and Owens 2002). ACKNOWLEDGMENTS We thank all the people who provided help during the different stages of the field season, particularly N. Whiteman, J. Bollmer, G. Jiménez, J. Merkel, J. Rabenold, and N. Gottdenker. We are grateful to the staff of the Charles Darwin Research Station for their invaluable help and logistical support during the course of this study, especially P. Robayo. We thank N. Cuevas and J. Freire who helped with dove sampling at Tortuga Bay, Santa Cruz. Permits for sample collection were provided by the Galápagos National Park authorities. B. Loiselle, R. Ricklefs, and two anonymous reviewers made helpful comments and suggestions on earlier versions of this manuscript. S. Siers helped in the preparation of the figures. Financial support was provided by The Whitney R. Harris World Ecology Center, The Saint Louis Zoo FRC# 05-2, and E. Des Lee Collaborative Vision in Zoological Research. LITERATURE CITED BAPTISTA, L. F., P. W. TRAIL, AND H. M. HORBLIT. 1997. Family Columbidae (pigeons and doves), p. 60–243. In J. del Hoyo, A. Elliot, and J. Sargatal [EDS.], Handbook of the birds of the world. Vol. 4. Sandgrouse to cuckoos. Lynx Edicions, Barcelona. BOAG, P. T. 1981. Morphological variation in the Darwin’s finches (Geospizinae) of Daphne Major Island, Galápagos. Ph.D. dissertation, McGill University, Montreal. BOAG, P. T. 1983. The heritability of external morphology in Darwin’s ground finches (Geospiza) on Isla Daphne Major, Galápagos. Evolution 37:877–894. BOLLMER, J. L., T. SANCHEZ, M.D.CANNON, D. SANCHEZ, B.CANNON, J.C.BEDNARZ, T. DE VRIES, M.S.STRUVE, AND P. G. PARKER. 2003. Variation in morphology and mating system among island populations of Galápagos Hawks. Condor 105:428–438. Santiago-Alarcon, Diego, 2008, UMSL, p. 35 BOLLMER, J. L., N. K. WHITEMAN, M.D.CANNON, J. C. BEDNARZ, T. DE VRIES, AND P. G. PARKER. 2005. Population genetics of the Galápagos Hawk: genetic monomorphism within isolated populations. Auk 122:1210–1224. BOWMAN, R. L. 1961. Morphological differentiation and adaptation in the Galápagos finches. University of California Publications in Zoology 58:1–302. BUTLER, M. A., T. W. SCHOENER, AND J. B. LOSOS. 2000. The relationship between sexual size dimorphism and habitat use in Greater Antillean Anolis lizards. Evolution 54:259–272. CLEGG, S. M., AND I. P. F. OWENS. 2002. The ‘island rule’ in birds: medium body size and its ecological explanation. Proceedings of the Royal Society of London Series B 269:1359–1365. COLEMAN, M., AND R. J. ABBOTT. 2003. Possible causes of morphological variation in an endemic Moroccan groundsel (Senecio leucanthemifolius var. casablancae): evidence from chloroplast DNA and random amplified polymorphic DNA markers. Molecular Ecology 12:423–434. CURRY, R. L. 1988. Group structure, within group conflict and reproductive tactics in cooperatively breeding Galápagos Mockingbirds, Nesomimus parvulus. Animal Behaviour 36:1708–1728. CURRY, R. L. 1989. Geographic variation in social organization of Galápagos mockingbirds: ecological correlates of group territoriality and cooperative breeding. Behavioral Ecology and Sociobiology 25:147–160. CURRY, R. L., AND P. R. GRANT. 1989. Demography of the cooperatively breeding Galápagos Mockingbird, Nesomimus parvulus, in a climatically variable environment. Journal of Animal Ecology 58:441–463. FAIRBAIRN, D. J. 1997. Allometry for sexual size dimorphism: pattern and process in the coevolution of body size in males and females. Annual Review of Ecology and Systematics 28:659–687. FRIDOLFSSON, A. K., AND H. ELLEGREN. 1999. A simple and universal method for molecular sexing of non-ratite birds. Journal of Avian Biology 30:116–121. GIFFORD, E. W. 1913. Expedition of the California Academy of Sciences to the Galápagos Islands, 1905–1906. Proceedings of the California Academy of Sciences 2:1–132. GOODWIN, D. 1977. Pigeons and doves of the world. Comstock, Cornell University Press, Ithaca, NY. GRANT, P. R. 2001. Reconstructing the evolution of birds on islands: 100 years of reseach. Oikos 92:385–403. GRANT, P. R., I. ABBOTT, D. SCHLUTER, R. L. CURRY, AND L. K. ABBOTT. 1985. Variation in the size and shape of Darwin’s finches. Biological Journal of the Linnean Society 25:1–39. GRANT, P. R., AND K. T. GRANT. 1979. Breeding and feeding ecology of the Galápagos Dove. Condor 81:397–403. HOOD, G. [ONLINE]. 2003. PopTools. 5www.cse. csiro.au/poptools/4 (30 May 2006).
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