CLINAL VARIATION IN VOCALIZATIONS OF AN ANTBIRD (THAMNOPHILIDAE) AND IMPLICATIONS FOR DEFINING SPECIES LIMITS

Auk, The, Apr 2005 by Isler, Morton L, Isler, Phyllis R, Brumfield, Robb T

ABSTRACT

In avian taxa in which vocalizations are considered innate, such as suboscine passerines, vocal characters are increasingly being used to help determine whether populations have achieved species status. In comparing vocal characteristics of distant populations, however, one must be concerned with the possibility of character gradation through intermediate populations. The first quantitative study of a species in a suboscine family to test for clinal vocal variation, our vocal study found clinal variation in the pace (number of notes per second) of male loudsongs, and revealed that the geographic pattern of the clines was consistent with genetic variation found in the companion molecular study (Brumfield 2005). The result underscores the necessity of searching for intermediacy when analyzing vocalizations of geographically distant populations. Furthermore, given that male loudsong pace was the only vocal character that varied across the iritergrading populations, the result also provides support to the guideline that one should expect thamnophilid species to differ in at least three vocal characters (Isler et al. 1998) and indicates that this degree of vocal character differences can be a valuable "yard stick" in determining which thamnophilid populations have achieved biological species status. Received 1 September 2003, accepted 21 December 2004.

Key words: clinal variation, hybrid zone, speciation, systematics, Thamnophilidae, Thamnophilus caerulescens, Variable Antshrike, vocalizations.

Variaci�n Clinal en las Vocalizaciones de un Hormiguero (Thamnophilidae) e Implicaciones para Definir los L�mites entre Especies

RESUMEN. - En las aves en que las vocalizaciones se consideran innatas, como en los Passeriformes suboscinos, los caracteres vocales han sido empleados cada vez m�s frecuentemente para ayudar a determinar si distintas poblaciones han alcanzado el estatus de especies diferentes. Sin embargo, al comparar las caracter�siticas vocales de poblaciones distantes, es importante prestar atenci�n a la posibilidad de que exista variaci�n gradual en los caracteres a trav�s de poblaciones intermedias. Este estudio es el primer trabajo cuantitativo que pone a prueba la posibilidad de que exista variaci�n clinal en vocalizaciones en una familia de suboscinos. Nuestro an�lisis de vocalizaciones indic� que existe variaci�n clinal en la cadencia (n�mero de notas por segundo) de los cantos fuertes de los machos, y que el patr�n geogr�fico de las clinas concuerda con la variaci�n gen�tica documentada en un estudio molecular conjunto (Brumfield 2005). Los resultados recalcan la necesidad de buscar rasgos intermedios al analizar las vocalizaciones de poblaciones geogr�ficamente distantes. M�s a�n, considerando que la cadencia de los cantos fuertes de los machos fue el �nico caracter que vari� a trav�s de las poblaciones, el resultado tambi�n apoya el lineamiento de que deber�a esperarse que las especies de Thamnophilidae difieran en por lo menos tres caracteres vocales (Isler et al. 1998), e indica que este grado de diferenciaci�n en vocalizaciones puede ser una "vara medidora" valiosa para determinar cu�les poblaciones de Thamnophilidae han alcanzado el estatus de especies biol�gicas.

INCREASINGLY, VOCAL CHARACTERS are considered important parameters in species-level taxonomic analysis of suboscines (Johnson et al. 1999, Helbig et al. 2002) and are being used as such (e.g. Isler et al. 1999, 2001, 2002; Whitney et al. 2000; Alvarez and Whitney 2001; Zimmer 2002). Vocalizations of suboscines are considered innate and reflective of genetic structure, though at least one species provides an exception (Baptista and Kroodsma 2001). An earlier paper developed guidelines for employing vocal characters in taxonomic studies of thamnophilid antbirds (Isler et al. 1998). However, when analyzing vocal data in that context, one must ask whether the data were obtained from populations known to be geographically isolated and thus currently incapable of interbreeding and, if not, whether vocal characters vary clinally through intermediate populations. Clinal variation has been found in thamnophilid plumage coloration (e.g. Thamnophilus atrinucha; Isler et al. 1997). Contemporary species concepts usually interpret broad geographic character dines as providing evidence of genetic integration and regard populations along a cline unworthy of taxonomic recognition as separately evolving lineages (Haffer 1998). In contrast, clines that are narrow as compared with the distributions of the interconnected populations may reflect selection against hybridization (i.e. reproductive isolation), and the populations on either side of the steep cline are often treated as biological species (Mayr 1996).

The geographic range of the Variable Antshrike (T. caerulescens) extends in a wide are from the Andes of north-central Peru southward through the Andes to central Bolivia and northern Argentina, where it spreads eastward through the lowlands of Bolivia, Paraguay, and Argentina to Uruguay and southeastern Brazil and thence north to northeastern Brazil (Zimmer and Isler 2003). The English common name reflects the substantial differences in plumage coloration among many of the twelve subspecies recognized by Peters (1951). A comprehensive review of the status of those taxa using modern analytic tools has yet to be done, but some taxa appear to intergrade morphologically. The plumage of the form found on the southern slope of the arm of the Andes in central Bolivia that extends east to the city of Santa Cruz, known as T. c. connectens, is intermediate between those of T. c. aspersiventer of the north slope of that arm of the Andes and T. c. dinellii of southern Bolivia and northwestern Argentina (Ridgely and Tudor 1994). To the east of the Andes in the Bolivian and Paraguayan chaco, a pale population is found that has been described as T. c. paraguayensis.