Mammals Obtained by Dr. Curt von Wedel from the Barrier Beach of Tamaulipas Part 1

Mammals Obtained by Dr. Curt von Wedel from the Barrier Beach of Tamaulipas, Mexico.

by E. Raymond Hall.

What species of mammals occur on the "coastal island", barrier beach, of Tamaulipas? Are the closest relatives of these mammals on Padre and Mustang islands of Texas, instead of on the mainland of Tamaulipas, or are the mammals on the barrier beach distinct from all others? These were questions that Dr. von Wedel of Oklahoma City and I asked ourselves in March of 1950 when we were in southern Texas. With the aim in mind of answering these questions, Dr. von Wedel arranged round-trip transportation, by air, for the two of us between Brownsville, Texas, and Boca Jesus Maria. The latter place is a "pa.s.s", tidal inlet, through the long barrier beach. The waters of the Gulf of Mexico and of the lagoon behind the beach flow back and forth with the changing tides through the inlet.

We arrived at Boca Jesus Maria on March 18, 1950, and left on March 22, 1950. Our headquarters there were in one of the four one-story buildings immediately north of the inlet. This place is approximately 89-1/2 miles south, and 10 miles west, of Matamoros, Mexico. Most of our collecting was done on the sand dunes one and one-half miles north of the buildings but on the evening of March 20 we made a round-trip, by boat of course, to the sand dunes on the south side of the inlet to set traps; these traps, and the _Dipodomys_ that were caught in them, were picked up the following morning.

At the time of our visit, the part of the barrier beach south of the tidal inlet was connected with the mainland. The connection was far to the southward, according to our pilot, Mr. Kagy of Brownsville, and also according to the testimony of the Mexicans at the fishing camp where we stayed on the north side of the inlet. The barrier beach which lay to the north of the inlet extended sixty-odd miles northward to the delta of the Rio Grande and had, we were told, eight "pa.s.ses," including Paso Jesus Maria. At the time of our visit, however, only three of these tidal inlets were open, it was said; the five others were thought to be filled in with sand, which permitted terrestrial animals to move from one part of the beach to another. Dr. von Wedel and I saw two tidal inlets that were open when we were being flown back to Brownsville.

[Ill.u.s.tration: FIG. 1. Diagram of physiographic features of the barrier beach of Tamaulipas. Top view looking down, as from an airplane, on the beach. Bottom view is profile.]

The long, low, sandy island, technically a barrier beach, irrespective of tide varied in width from a quarter of a mile to as much as a mile and was separated from the mainland by the Laguna Madre, which was four miles wide opposite our trapping station. To the northward the width of the lagoon gradually increased until, at a place thirty miles north of our trapping station, the lagoon was almost 20 miles wide.

The island was perhaps four feet above high tide. Superimposed on this, in places, there were sand dunes, technically barchans, so arranged that the end of one touched the end of the next. The tops of some were as much as 20 feet above high tides and the chain of these connected-dunes on which we trapped was approximately a mile long. Incipient tidal inlets were frequent; they were where storm-driven waves of high tides had broken across the island between the adjacent ends of two dunes. The windward side of a dune was toward the Gulf and the slope of that side was gentler than that on the leeward side. According to the cycle described by Davis (Proc. Amer. Acad. Arts and Sciences, 22:303-332, 1896) and recently figured on page 364 by Lobeck (Geomorphology, 1st ed., xii + 731 pp., 1939, McGraw Hill Book Co., Inc., New York) the barrier beach concerned was in the early part of the "Middle Youth Stage".

Typically, on the center of the area in the lee of a dune there was a patch of plum brush, almost five feet tall and so dense that a person could not penetrate it. A belt of gra.s.s, 20 to 100 feet wide, surrounded the plum brush. The gra.s.s was approximately 20 inches high. Outside the area of gra.s.s, there were widely-s.p.a.ced xerophitic shrubs which grew also on the dunes. The diagram (fig. 1) shows these prominent features as a person might see them if he looked directly down from an airplane.

We obtained specimens of the spotted ground squirrel (_Citellus spilosoma_), Ord kangaroo rat (_Dipodomys ordii_), hispid cotton rat (_Sigmodon hispidus_) and black-tailed jack rabbit (_Lepus californicus_). Tracks and other sign of the coyote (_Canis latrans_) were seen. So far as we could ascertain, by our own investigations and from our Mexican hosts at the fishing camp, no other kinds of native mammals lived on the island. The ground squirrel and kangaroo rat were found by us on only the sandy areas where there were xerophitic shrubs.

The cotton rat was found only in the gra.s.s. The jack rabbit and coyote ranged over the whole of the island excepting the areas of plum brush in which we saw no sign of any mammal.

To answer the second of our initial questions: The affinities of the mammals of the barrier beach of Tamaulipas are approximately equally divided between those of the mainland and those of Padre Island. The ground squirrel is indistinguishable from the subspecies which occurs both on the mainland and Padre Island to the northward; the other three kinds of mammals of which we obtained specimens prove to be subspecifically distinct from any previously named kinds and seem to be confined to the off-sh.o.r.e beach. Accounts of these four mammals and of a previously unnamed subspecies of kangaroo rat on Mustang Island, Texas, follow.

Citellus spilosoma annectens (Merriam)

Spotted Ground Squirrel

1893. _Spermophilus spilosoma annectens_ Merriam, Proc. Biol. Soc.

Washington, 8:132, December 28, type from "The Tanks," 12 mi. from Point Isabel, Padre Island, Texas.

1904. [_Citellus spilosoma_] _annectens_, Trouessart, Catalogus Mammalium ..., p. 340.

Thirteen specimens (Nos. 35441-35453) were collected. All are from the north side of the tidal inlet. Although the ground squirrels were easily trapped, it was difficult to obtain a perfect skin because the gulls (_Larus_ sp.) pulled the skin off of the distal part of the tail as soon as a squirrel was secured in a trap. The specimens seem not to differ from Texan specimens from the type locality and Mustang Island.

Dipodomys ordii parvabullatus new subspecies

Ord Kangaroo Rat

_Type._--Male, adult, skull and skin, No. 35454, Mus. Nat. Hist. Univ.

Kansas, from island, 88 miles south and 10 miles west of Matamoros, Tamaulipas, Mexico; obtained 19 March 1950 by E.R. Hall and Curt von Wedel; original No. 6778 E.R. Hall.

_Range._--Islands along coast of Tamaulipas, Mexico.

_Diagnosis._--Size small (see measurements). Color pale; entire dorsal surface Light Ochraceous-Buff (Capitalized color terms according to Ridgway: Color Standards and Color Nomenclature, Washington, D.C., 1912), purest on sides and flanks, upper parts lightly suffused with black; cheeks white; plantar surfaces of hind feet, dorsal and ventral stripe of tail, and anterior face of ear brownish. Skull small; auditory bullae smaller (actually and relative to remainder of skull) than in any other known kind of _Dipodomys_, excepting the one from Mustang Island, Texas (named beyond) in which the breadth is approximately the same; rostrum and interorbital region narrow.

_Comparisons._--From _Dipodomys ordii sennetti_ (Allen), of the mainland of Texas, _D. o. parvabullatus_ differs in: Color paler on pigmented areas; white areas more extensive; skull smaller, in all parts measured, except the nasals which are slightly longer. From _Dipodomys ordii compactus_ of Padre Island, Texas, _D. o. parvabullatus_ differs in: Tail and hind foot shorter; skull smaller in all parts measured, especially so in breadth across maxillary processes of zygomatic arches.

_Remarks._--_D. o. parvabullatus_ resembles _D. o. sennetti_ in external proportions and _D. o. compactus_ in cranial proportions.

No difference was detected between specimens from the two sides of the tidal inlet 89 miles south of Matamoros. Only one of the 14 specimens is of the light color phase (upper parts Cartridge Buff). This pale specimen is from the north side of the inlet. The brownish stripe on the ventral side of the tail is absent on the distal two-fifths of the tail and the specimens are uniform in this respect. On the occlusal surfaces of the cheek-teeth, the enamel surrounding the dentine is incomplete on both the lingual and l.a.b.i.al sides of the teeth of five individuals and is incomplete on the l.a.b.i.al side of some of the teeth of a sixth specimen.

In the snap traps, all of which were baited with rolled oats, more than twice as many land crabs as kangaroo rats were taken. Judging from tracks in the sand, land crabs greatly outnumbered kangaroo rats. The parietal bones in two of the 13 skulls are much eroded by some parasite (seemingly nematode worms) and in one of these two specimens the roof of the left tympanic cavity is perforated. As regards life-zones, the occurrence of _Dipodomys ordii_ in the lower part of the Lower Sonoran Life-zone on the off-sh.o.r.e beach 88 and 90 miles south of Matamoros is low zonally and perhaps is at or near the zonal margin of the range of the species. The crabs and worms conceivably are two of the environmental features inhospitable to the rats.

_Specimens examined._--Total, 14, all from Tamaulipas, Mexico, as follows: 88 mi. S and 10 mi. W Matamoros, 7; 90 mi. S and 10 mi. W Matamoros, 7.

When Setzer (Univ. Kansas Publ., Mus. Nat. Hist., 1:473-573, December 27, 1949) reviewed the subspecies of _Dipodomys ordii_ he lacked specimens of _Dipodomys ordii compactus_ from the type locality or from anywhere else on Padre Island. He used as representative of _D. o.

compactus_ specimens from Mustang Island, Texas, the island next northeast of Padre Island. Through the courtesy of Mr. Stanley P. Young, Dr. Hartley H.T. Jackson and Miss Viola S. Schantz, of the United States Biological Surveys Collection, I have examined topotypes of _D.

o. compactus_ from Padre Island. This examination discloses that the kangaroo rats on Padre Island and Mustang Island are significantly different. Those from Mustang Island may be named and described as follows:

MEASUREMENTS (IN MILLIMETERS) OF ADULT MALES OF FOUR SUBSPECIES OF DIPODOMYS ORDII

Key: A Total length F Breadth across maxillary arches B Length of tail G Width of rostrum C Length of hind foot H Length of nasals D Greatest length of skull I Least interorbital width E Greatest breadth across bullae J Basilar length ----------------------------------------------------------------------

_D. o. largus_, type locality (K.U.)

A B C D E F G H I J Mean (9) 226 117 35.8 36.3 21.5 19.2 3.6 13.7 12.6 23.2 Maximum 241 128 37 37.2 22.0 19.9 3.7 14.2 13.9 23.9 Minimum 212 105 35 35.2 20.7 18.6 3.5 13.5 11.6 22.3 ----------------------------------------------------------------------

_D. o. compactus_, type locality (U.S.N.M.)

A B C D E F G H I J Mean (10) 230 126 37.7 36.6 22.1 20.0 3.8 14.0 12.5 23.8 Maximum 241 135 40 37.8 23.2 21.4 4.0 14.5 13.1 24.4 Minimum 208 118 35 35.5 21.6 19.2 3.6 13.1 11.3 23.1 ----------------------------------------------------------------------

_D. o. parvabullatus_, type locality and 2 mi. S of same (K.U.)

A B C D E F G H I J Mean (7) 216 111 35.9 36.4 21.7 19.6 3.6 13.8 12.1 23.0 Maximum 222 113 37 36.9 22.1 20.7 3.8 14.2 12.5 23.5 Minimum 210 109 34 35.9 21.3 19.1 3.3 13.4 11.6 22.1 ----------------------------------------------------------------------

_D. o. sennetti_, 2 mi. S Riviera, Texas (after Setzer, op. cit. :565)

A B C D E F G H I J Mean (5) 218 112 35.8 37.2 23.4 20.1 4.0 13.6 13.1 24.2 Maximum 222 115 38 38.2 24.1 20.7 4.3 14.4 13.2 24.6 Minimum 208 104 34 36.3 23.0 19.4 3.8 13.0 12.6 23.8 ----------------------------------------------------------------------

Dipodomys ordii largus new subspecies

Ord Kangaroo Rat

_Type._--Female, adult, skull and skin, No. 27234, Mus. Nat. Hist., Univ. Kansas, from Mustang Island, 14 mi. SW Port Aransas, Aransas County, Texas; obtained 30 June 1948 by W.K. Clark; original No. 543.

_Range._--Known from Mustang Island only.

_Diagnosis._--Size medium (see measurements). Color pale, and as described for _D. o. parvabullatus_. Skull small; auditory bullae (actually and relative to remainder of skull) smaller than in any other known kind of _Dipodomys_, except _D. o. parvabullatus_ in which breadth across bullae is approximately the same; notably narrow across maxillary processes of zygomatic arches.

_Comparisons._--From _Dipodomys ordii sennetti_ (J.A. Allen) of the mainland, _D. o. largus_ differs in: Color paler on pigmented areas; white areas more extensive; skull averaging smaller except in basilar length and length of nasals which are approximately the same as in _D.

o. sennetti_. From _Dipodomys ordii compactus_ True of Padre Island, _D.

o. largus_ differs in: Body longer; tail shorter; skull narrower across tympanic bullae and across maxillary processes of zygomatic arches; nasals shorter. From _Dipodomys ordii parvabullatus_ of the coastal island south of Padre Island, along the gulf coast of Tamaulipas, _D. o.

largus_ differs in: Body and tail longer; basilar length of skull averaging less; breadth across maxillary processes of zygomatic arches greater; premaxillae not extending so far behind nasals.

_Remarks._--_D. o. largus_ resembles _D. o. compactus_ in external proportions and _D. o. parvabullatus_ in cranial proportions. The degree of difference between _D. o. compactus_ and _D. o. largus_ is less than between _D. o. compactus_ and _D. o. parvabullatus_. To me, the three subspecies mentioned in the preceding sentence are indistinguishable in color.