Phylogeny of social behavior in aphelocoma jays: A role for hybridization?

-The phylogeny of jays in the genus Aphelocoma describes the evolutionary appearance or disappearance of helping and other social behaviors that have been much studied in this genus. Using published allozyme data and new behavioral characters we reexamine the phylogeny of Aphelocoma. The best-fitting hypothesis is that the complex social system of the Mexican Jay (Aphelocoma ultramarina), including delayed maturation, helping, plural breeding, and loss of the rattle call, arose from a simpler state in a jay ancestor, perhaps one that already had occasional nonbreeding associates. The interpretation of phylogeny in eastern populations of the Mexican Jay may be complicated by hybridization with the Scrub Jay (A. coerulescens), suggesting the possibility of reticulate evolution in the genus. The Florida Scrub Jay (A. c. coerulescens) appears to be closely related to the population of Scrub Jays in southern Mexico, and together the two constitute a distinct clade within the Scrub Jay superspecies. Received 4 March 1994, accepted 28 January 1995. ALTHOUGH BEHAVIOR was used as a tool to study phylogeny in birds in the 1940s and 1950s (e.g. Lorenz 1941), the tables have turned recently; now phylogeny has become a tool to study the evolution of behavior (Brooks and McLennan 1991, Harvey and Pagel 1991, Harvey and Purvis 1991, Price 1991, Lanyon 1992, Richman and Price 1992). The evolution of social behavior in general and of helping behavior in particular can be studied in this way (Edwards and Naeem 1993). The genus Aphelocoma is a promising subject for such research because it spans the gamut from a complex, pluralbreeding system in the Mexican Jay (Aphelocoma ultramarina; Brown 1963), through a polyandrous system in the Unicolored Jay (A. unicolor; Webber and Brown 1994), to singular breeding in pairs with or without nonbreeding helpers in the Scrub Jay (A. coerulescens; Woolfenden 1975, Carmen 1989). The genus inhabits a wide range of habitats including Central American cloud forests (A. unicolor), pine-oak-juniper woodlands (A. ultramarina), and scrub vegetation (A. coerulescens). Some of the behavioral, phylogenetic, and taxonomic problems in this genus concern the populations of A. ultramarina that inhabit the Sierra Madre Oriental of eastern Mexico. This chain of montane populations includes two subspecies and extends from the Mexican state of Hidalgo (A. u. potosina) north to the Chisos Mountains on the Texas border (A. u. couchii). The Mexican Jay has two other groups of subspecies, a series of populations mainly in the mountains of western Mexico ranging from Arizona and New Mexico south through the Sierra Madre Occidental to Jalisco (arizonae, wollweberi, gracilis), and the populations crossing central Mexico in the Transvolcanic region (A. u. ultramarina, A. u. colimae). We refer to these geographic groups of subspecies as Orientalis, Occidentalis and Transvolcanic, respectively (Fig. 1). In this paper we survey the genus Aphelocoma with special reference to Orientalis. The Orientalis group may have been critical in the origin or loss of helping behavior in the genus (Brown and Horvath 1989, Peterson 1992b). Orientalis also has attracted the attention of behavioral ecologists because of the presence of certain Scrub-Jay-like behaviors and developmental patterns that are absent or weakly developed in other populations of the Mexican Jay. Three decades ago it was noticed that couchii, the population of Aphelocoma ultramarina in the border mountains of Texas and Coahuila, was in many respects somewhat intermediate between the Occidentalis group and the Scrub Jay (Brown 1963). Later, Orientalis was found to resemble the Scrub Jay in additional aspects of its social behavior (Brown and Horvath 1989). Northern populations of Orientalis possess a rattle call that is not found in the other groups (Strahl and Brown 1987 and earlier authors). In addition, sizes of territorial flocks in these populations are smaller, and the flocks appear to have only one breeding female each (singular