Comments on the Distribution and Abundance of the
Mexican Milksnake, (Lampropeltis triangulum annulata), in South Texas

By A. Rentfro


My first encounter with the Mexican Milksnake was in 1963 or 1964 on the outskirts of Brownsville, Texas at my great uncle’s farm. The animal, approximately two feet in length was laying in a rut of an unimproved road on a bright early Spring day. Beginning as early as 1971 I have actively searched for and observed Lampropeltis triangulum annulata in Texas and in adjacent Mexico, in the field and on the roads during every season of the year, in every type of weather and at any possible location, and have directly encountered in excess of 200 specimens in nature as of this writing.

Lampropeltis triangulum annulata is said to range in Texas from the very southern tip of the state near Brownsville in Cameron County, west to near Sanderson in Terrell County, up the Gulf coast to Refugio County and inland to Gillespie County south of the geographical center of the state. My experiences and those of my close associates indicate that annulata are most common in deep sandy soil associated with grass and brush habitat. Additional specific clues may be a relative absence of surface water (a clue to soil porosity) and an abundance of one of their preferred prey animals, Aspidoscelis (Cnemidophorus) sp., This habitat is best available in the southern half of the South Texas Plains and more importantly, the South Texas Sand Shield which begins at the western edge of the Gulf Coastal Plain in Willacy and Kenedy Counties and continues west and northwest through the Tamaulipan scrub and sandy grasslands to near Zapata, Texas and north before joining the Plains. Areas where the deep sandy soil meets caliche deposits or other rocky or gravelly strata may afford the best opportunity for field-collecting annulata resting or thermo-regulating near the surface or under debris. A good example is the vicinity of Delmita and La Reforma, Texas.

The Coastal Plain, with harder soils and more abundant surface water is also home to milksnakes. In February and March on some of the barrier islands in Nueces and Kleberg Counties Lampropeltis triangulum can be found in some numbers thermo-regulating under debris. These animals routinely display some of the attributes of L. t. amaura. Most of the coastal area roughly south of highway 100 in eastern Cameron County, including South Padre Island and Brazos Island is virtually devoid of annulata except in select areas where the Rio Grande Plain habitat is more dominant. Unlike some snake species, the Mexican Milksnake is not edificarious; development and modern agriculture have nearly extirpated annulata from much of the area adjacent to the Rio Grande River in deep South Texas.

In southwestern Madison County, Texas which is outside the accepted range of this subspecies, I encountered two specimens of triangulum in 1978 and 1979 indistinguishable at casual inspection from any annulata seen in Duval County or southward. Aside from two nearly entirely melanistic individuals from near Encinal, Texas the most odd-appearing Mexican Milksnake I’ve found was seen west of La Gloria, Texas in 1982, an adult male in excess of 1 meter in length, which had the wide red bands and reduced black and white imitating Lampropeltis triangulum sinaloae. Several specimens from Webb County have displayed the head and neck markings more common of the Jalisco Milksnake.

Mexican Milksnakes have been taken by practically every means across their range including road cruising (annulata may appear jet-black on the road in daytime viewed from a distance), searching beneath rocks and debris and the use of pit-traps. Unlike the syspila, gentilis, amaura and perhaps other sub-species no key or pattern of behaviour has been found to expose the dense populations of Mexican Milksnakes in the heart of their range to similar field observation or harvest in numbers. This has led some casual observers to believe annulata are not abundant. My experiences demonstrate that they are widespread and locally abundant only perhaps not as easily observed as some other closely related snakes.

Activity in Cameron and Willacy Counties may be year-round, excepting only the coldest and driest periods. Patiently searching the few remaining lightly traveled secondary and unimproved roads is the only reliable way to observe milksnakes in Cameron County. Elsewhere, annulata may not be observed with any predictability before mid-March most years and road sightings before Easter are not the norm. Yearly, the first surface activity of annulata has been observed to closely match two phenomena in south Texas; the first appearance coincides with the advent of bloom spikes on the Spanish Dagger or Yucca and road-sightings come at the time when the Aspidoscelis (Cemidophorus) sp. first become active. These natural markers undoubtedly give some clue to soil and sub-soil temperatures and possibly available moisture as well. Depending upon the weather these snakes may continue to be active until at least the first week of November across much of their range.

Annulata are not rare and occur over a very broad range. However, without chancing upon one or more individuals, accessible populations of annulata may remain undiscovered. For many years I drove through La Gloria, Texas stopping only for cold drink or fuel. Numerous attempts to find animals there under trash or debris had never yielded a milksnake. Running late one June evening I observed a large milksnake entering the road just at the western edge of town, before I was able to put it off the road, another came into view. Over the next few seasons it proved to be almost a surety that one or more annulata could be seen close to that point on most any warm, calm and humid evening if enough time was invested.

The very best opportunity to observe Mexican Milksnakes in number is during heavy Summer rainfall in select areas of their range. In southeastern Webb County, southern Duval County, northern Starr County as well as locations in Jim Hogg and Brooks Counties annulata can be extremely abundant during and after a hard, flooding rain, particularly in the months of June, July and August. It would not be unusual to observe multiples of these snakes in one night on a short section of road. And this same location may not have yielded a single sighting on previous visits. On three separate occasions I have observed as many as nine annulata in one night, twice in Starr County and once in Duval County. And on one rainy July night, Tim Workman and myself working in two vehicles observed twenty-one Mexican Milksnakes in Jim Hogg and Starr Counties. FYI, gravid females in this part of the range are most often encountered in late June or early July. Overall, approximately one-half of all seen are adult males, one-quarter or slightly more are sub-adults of either sex and the balance are mature females.

A. Rentfro
Brownsville, Texas January 2007
Date submitted 11/22/08