Occurrence of the Garter Snake, Thamnophis sirtalis, in the Great Plains and Rocky Mountains

In Short

This scientific monograph investigates the geographic distribution and taxonomy of the common garter snake, Thamnophis sirtalis, across the American West. By reviewing historical records and analyzing freshly collected specimens, the authors clarify the boundaries between established subspecies and identify a distinct population in New Mexico. It serves as a rigorous correction to decades of misidentification and generalized mapping, establishing the Continental Divide as a primary biogeographic marker. It remains a vital reference for herpetologists and naturalists interested in the precise classification of North American wildlife.

The Story

The narrative begins with a recognition that the common garter snake—while ubiquitous across the continent—has been poorly understood in the central and western United States. Since the early 20th century, researchers had relied on outdated revisions, leaving the taxonomy of western populations in a state of confusion. The authors set out to bridge this gap by defining the range of the subspecies parietalis, traditionally associated with the region between the Mississippi River and the Rockies, and its relationship to the western subspecies fitchi.

The authors argue that previous maps of the snake’s distribution were overly optimistic, assuming a continuous range that simply does not exist. Through a meticulous review of museum collections, they expose a history of error, showing how isolated specimens were misidentified and subsequently used to create misleading range maps. They contend that the arid climate and the intense competition from other snake species create natural barriers, resulting in a fractured, discontinuous distribution rather than a uniform one.

A significant portion of the work is dedicated to resolving the status of populations in the Rio Grande Valley. The authors identify these snakes as a separate subspecies, T. s. ornata, distinguishing them from parietalis by their paler olive ground color and specific pattern of black ventral markings. This re-classification involves a deep dive into historical taxonomic records, including the problematic provenance of lost type specimens from the 1850s. The authors navigate these historical murky waters with caution, ultimately justifying the resurrection of the name ornata to bring order to the New Mexican population.

The final act of the study focuses on the comparison between parietalis and the western fitchi. By examining physical characteristics such as scale patterns, the presence of red skin markings, and ventral spotting, the authors conclude that the Continental Divide serves as a logical, if imperfect, boundary between these two forms. They demonstrate that while individual snakes may show variations that bridge the gap, the populations remain geographically and biologically distinct. The study concludes by acknowledging the existence of relict colonies—pockets of snakes surviving in isolated, hospitable environments—that offer a glimpse into a time when the species was more widely distributed across the High Plains. It closes not with a finality of all questions, but with a foundational framework that encourages future field workers to look closer at the specific drainage basins and local ecologies that dictate where these animals can—and cannot—survive.

How It Unfolds

The diagnostic challenge The authors introduce the massive geographic range of the garter snake, noting that despite its abundance, little systematic work had been done for fifty years. They identify the confusion surrounding subspecific boundaries as the primary problem to be solved.

Mapping the gaps By analyzing museum records and field data, they demonstrate that the species is largely absent from the arid High Plains. They identify historical misidentifications of other species, such as T. radix, as the root cause of inaccurate, overly inclusive range maps.

Defining the New Mexican population The authors isolate the snakes of the Rio Grande drainage as a unique group. They provide a formal re-description of T. s. ornata, justifying its distinction through detailed measurements of scale counts and color patterns.

The Continental Divide as a boundary The authors compare the eastern parietalis with the western fitchi through a series of pattern analyses. They determine that the Continental Divide acts as a natural separator, effectively partitioning the populations into their respective subspecies.

The People

Henry S. Fitch and T. Paul Maslin are the primary architects of this study, acting as both detectives and taxonomists. They seek to bring professional order to a chaotic field of study where previous researchers—such as Ruthven, Van Denburgh, and Fox—left behind a confusing legacy of shifting names and misunderstood specimens. They are hindered by the "scrap basket" nature of historical records, where poorly preserved specimens and vague geographic labeling make clear classification difficult.

The authors rely on the past work of figures like Colonel J. D. Graham, whose 1850s collection provides the basis for the ornata classification, even though his original documentation is frustratingly incomplete. They also rely on the contributions of various museum curators and fellow researchers who provide locality data and loan critical specimens. Ultimately, the authors emerge as the voices of scientific precision, replacing the broad, inaccurate strokes of their predecessors with a detailed, geographically grounded understanding of how these snakes are distributed across the West.

In Its Own Voice

The range of the common garter snake has never been adequately mapped in the Rocky Mountain and Great Basin states.

This statement sets the stage for the book’s central argument: that current scientific knowledge of the species’ geography is flawed and requires immediate correction.

In view of this situation some plausibility could be claimed for any of several dividing lines between the subspecies.

This reflects the authors' nuanced approach, acknowledging that nature does not always adhere to the neat, absolute boundaries that human taxonomy tries to impose.

What It's Really About

At its core, this book is an exercise in biological cartography and taxonomic rigor. It probes the tension between the fluid reality of nature and the rigid requirements of human naming systems. The authors are obsessed with the "why" and "where" of animal distribution, questioning how climate change and geographic barriers like the Continental Divide shape the evolution of a species. It asks readers to consider that a "common" animal is often only as well-known as the quality of the records we keep. It is a defense of the scientific method, emphasizing that even a widely recognized creature can remain a mystery if researchers stop asking the right questions and relying on outdated assumptions.

Why Read It Today

Readers with an interest in natural history, biogeography, or the quiet, meticulous work of museum science will find this a rewarding study. It captures the essence of a mid-20th-century scientific expedition, where discovery was often a matter of chasing down obscure records and enduring "mosquito-infested" fieldwork. The prose is clean and unadorned, reflecting the authors’ dedication to precision over flair.

The book is not a narrative in the traditional sense; it is a technical report. Modern readers should be prepared for the specialized language of herpetology, including frequent references to ventral scales and specific river drainages. The value here is not in a fast-moving plot, but in the slow, satisfying process of scientific correction. You will walk away with a profound appreciation for how much effort goes into establishing the truth about even the most "common" things in our environment. It serves as a reminder that science is an iterative process—one that requires looking at the same world through a sharper lens to see what has been missed. For anyone who enjoys the intersection of history and biology, it is a testament to the fact that the most important work is often that which cleans up the mistakes of the past.