Hybridization of Rufous-Naped Wrens in Chiapas, México

Abstract

TEN years ago I had an opportunity to study hybridization between two strongly differentiated subspecies of the Rufous-naped Wren (Campylorhynchus rufinucha) on the Pacific coastal plain of Chiapas, Mexico, near the town of Tonala and the Laguna de la Joya. My investigations in 1954 (Selander, Univ. California Pubis. Zool., 74: 77-116, 1964) showed that C. r. humilis, which occupies xeric habitats in western Mexico, extends southeast from the Isthmus of Tehuantepec to within a few miles of Tonal'a, while the form C. r. nigricaudatus, which is found in relatively mesic habitats on the coastal plain of southwestern Guatemala and southeastern Chiapas, ranges northwest to the southeastern edge of the Laguna de la Joya (Figure 1). Although the Laguna de la Joya forms a partial barrier between populations of these two wrens, they are in contact through the narrow Ocuilapa Valley, which lies inland from the laguna near the base of the Sierra Madre de Chiapas. Field work in the Ocuilapa Valley in 1954 revealed the existence of a linearly distributed series of partially isolated, variable, hybrid populations connecting the two parental forms (see Figure 2, below). On the basis of this finding, it is apparent that the two wrens are conspecific, notwithstanding their considerable morphological differences (Table 1). In March, 1963, nine years after my first study, I revisited the Tonala region while enroute to and from Nicaragua and was able to obtain additional information on the zone of hybridization. The chief purpose of my study in 1963 was to resample certain populations in the central part of the Ocuilapa Valley and, by comparing the samples with those collected at the same localities in 1954, to determine if any change in the character of the zone of hybridization had occurred. Additionally, I sought to determine the extent of introgression of genes of C. r. humilis into the population of C. r. nigricaudatus on the coastal plain southeast of the Laguna de la Joya. Finally, I was able to determine more precisely the distances between collecting stations in the Ocuilapa Valley. For a discussion of the physiographical and ecological setting of the zone of hybridization and an analysis of characters in the hybrids, the reader should consult the detailed account in my earlier paper (Selander, op. cit., pp. 80-105). This research was supported by the National Science Foundation (Grants 20873 and GB 1624).