Color and morphological differentiation in the Sinaloa Wren (Thryophilus sinaloa) in the tropical dry forests of Mexico: The role of environment and geographic isolation

Abstract

The role and the degree to which environment and geographic isolation contribute to phenotypic diversity has been widely debated. Here, we studied phenotypic variation (morphology and plumage reflectance) in the Sinaloa Wren, an endemic bird distributed throughout the tropical dry forest (TDF) on the Mexican pacific slope where a pronounced variability in environmental conditions has been reported. In particular, we aimed: 1) to characterize phenotypic variation between subspecies; 2) to analyze the relationship between phenotypic and environmental variation in the context of classic ecogeographic rules, such as Bergmann’s, Allen’s, Gloger’s, and Bogert’s, and to quantify the relative roles of environment and geographic isolation and their interaction in shaping phenotypic variation; and 3) to test for niche conservatism between subspecies. Our data revealed significant differences among subspecies morphology and plumage reflectance. The environment explained a higher proportion of the morphological variation, while geography explained a smaller proportion. However, variation in plumage reflectance was mainly explained by the joint effect of geography and environment. Our data did not support for Bergmann´s and Allen´s rule. However, longer tails and wings were positively associated with higher elevations, larger tarsus and culmens were positively related to higher latitudes and to greater tree cover, respectively. Our data partially supported Gloger´s rule, where darker plumages were associated with more humid environments. The effects of temperature on plumage coloration were more consistent with Bogert´s rule. In addition, we found darker plumages related to higher levels of UV-B radiation. Finally, niche divergence was detected between T. s. cinereus and T. s. sinaloa vs. T. s. russeus. In a continuously distributed ecosystem such as the TDF on the pacific slope of Mexico, the environmental conditions and geographic isolation have played an important role in promoting phenotypic differentiation in the Sinaloa Wren.

Ecological factors drive the divergence of morphological, colour and behavioural traits in cactus wrens (Aves, Troglodytidae)

“…For instance, male populations of the variable antshrike ( Thamnophilus caerulescens ) with a ventral rufous coloration are found in warm regions, while males with white, black and grey are found in humid regions [9]. These studies provide evidence supporting the hypothesis that humidity and temperature play significant roles in driving the evolution of phenotypic traits in animals [9,11,18].…”

“…Evidence for Bergmann's rule is contradictory in birds, mammals and insects (e.g. [7,11,24,83,84]). A review study that considered the body geographical variation in 92 species of North American birds revealed that 42% of these species supported Bergmann's rule, 12% provided weak or ambiguous support, 16% showed no association between latitude or latitude, and in 29% of the species the patterns observed contradicted the rule [85].…”

“…In birds, studies that show divergent ecologies acting on colour, morphology and song [5,6] refer to Gloger's, Bergmann's and Allen's rules for morphological traits or the acoustic adaptation hypothesis for vocalizations to explain geographical patterns of phenotypic variation (e.g. [7][8][9][10][11]).…”

“…However, Allen's rule does not explain appendage size in all species (e.g. [11]) because in multifunctional traits (e.g. beak length), other selective forces, such as feeding ecology, may be involved in morphological evolution [29,31].…”

“…Many studies explain patterns of divergence in phenotypic traits using some ecological rules for morphology and colour or the acoustic adaptation hypothesis for vocalizations (e.g. [11,30]). However, to our knowledge, no study considers all these rules and hypotheses to explain variation simultaneously in multiple traits.…”