The Natural History of Tassel-Eared Squirrels

1. Introduction

1 Introduction The Abert squirrel (Sciurus aberti aberti) is often described as one of the most beautiful squirrels in North America. Since the animal was first described by Woodhouse in 1853 it has held the fascination of the zoologist, the naturalist, and the tourist. —J. O. Keith, “The Abert Squirrel (Sciurus aberti aberti) and Its Relation to the Forests of Arizona,” 1956 T assel-eared squirrels were first described and collected near the San Francisco Peaks in northern Arizona by Dr. Samuel W. Woodhouse, a physician and naturalist with Captain Lorenzo Sitgreaves’ expedition in 1851. “Ears large and broad, tufted with long black gray hairs” began Woodhouse’s description of the squirrels he found in abundance during the month of October (ref. 1). With the conspicuous long, soft, blackish tufts of hair (tassels) extending from the tips of their ears accounting for the unique appearance of these squirrels, naturalists referred to them as tufted-eared squirrels, big-eared pine squirrels, mule-eared or donkey-eared squirrels , long-eared squirrels, saddle-backed gray squirrels, white-tailed squirrels, silver-tail squirrels, tassle-eared squirrels, and tassel-eared squirrels (ref. 1, 2, 3, 4, 5, 6). “Tassel-eared squirrel” is now the commonly used name. Woodhouse assigned the scientific name Sciurus dorsalis in 1853 to the specimens he had collected. However, the species name dorsalis had already been designated to another squirrel species, requiring Woodhouse to rename his specimen S. abertii in honor of Colonel John James Abert, chief of the Corps of Topological Engineers (later the U.S. Army Corps of Engineers), “to whose exertions science is so much indebted” (ref. 7). Later the second i was dropped for the more correct 1 2 C H A P T E R O N E Latin usage, aberti. Tassel-eared squirrels are now all referred to as Abert’s squirrels with one exception: the Kaibab squirrel. P Squirrels on the north side of the Colorado River on the Kaibab Plateau have been known as Kaibab squirrels since 1904 when that squirrel was identified and named as a separate species from the Abert’s squirrel living on the south side of the Colorado River (ref. 8). Geographical isolation of species can lead to allopatric speciation, a type of speciation that can occur if a barrier is present. Though the Grand Canyon and the Colorado River are such barriers, at present there is no strong evidence that a speciation event has occurred. The distinct species recognition of the Kaibab squirrel was later reexamined, resulting in both the Kaibab and the Abert’s squirrels being regarded as subspecies (ref. 9). A subspecies is defined as “a taxonomic category used by some systematists to designate a genetically distinct set of populations of a species that have a discrete range” (ref. 10). Taxonomists do not generally give subspecies distinct common names, but these two names have been allowed to stand since they have been widely accepted both in the lay and the scientific literature for FIGURE 1.1 First engraved image of Sciurus aberti by R. H. Kern. Taken from the Report of an Expedition down the Zuni and Colorado Rivers by Captain L. Sitgreaves. Senate Executive Document 59, 32nd Cong., 2nd sess., p. 53. Washington, DC, 1853. I n t r o d u c t i o n 3 well over a century. Appendix 1 gives a brief introduction to the Order Rodentia and the hierarchical system of nomenclature of the tasseleared squirrels. Over the years there have been as many as nine subspecies and two species of tassel-eared squirrels reported in the literature, but extensive revisions have resulted in a reduction to six subspecies (ref. 9). Appendix 2 provides more detail of their discovery and nomenclature history. Tassel-eared squirrels are indigenous to and dependent on ponderosa pine (Pinus ponderosa var. scopulorum) forest habitats that exist in the southwestern United States and Mexico (ref. 11, 12, 13, 14). Some rare exceptions of habitat are discussed in chapters 4 and 5 on food and nests. Within the United States, tassel-eared squirrels are found in Arizona, Colorado, New Mexico, Utah, and Wyoming. Arizona and New Mexico each have three of the subspecies existing within their borders. Two of the six subspecies live in the Sierra Madre Occidental in Mexico. This book describes every aspect of the tassel-eared squirrel from the DNA analysis of the six subspecies to the histology of its ovaries...