Part 7

=Geomys personatus personatus= True: Texas Pocket Gopher.--This pocket gopher was abundant on low, stabilized dunes on the barrier island from four to 73 miles south of Washington Beach. One of us (Wilks) made a trip down the beach on May 20 and 21, 1961, and collected specimens at localities four miles south and 33 miles south of Washington Beach; additional specimens were taken at both Camp 1 and Camp 2 from July 6 to 10. At these localities the gophers seemed to maintain population densities approximating those of G. personatus on Padre and Mustang islands on the Texan coast.

There is but one other record of the Texas Pocket Gopher from Mexico. Goldman (1915) described G. p. tropicalis from Altamira on the basis of specimens collected in 1898. Since that time, the species has not been reported as occurring south of Cameron County, Texas (Kennerly, 1954), some 50 miles northwest of the closest station of occurrence of the gophers on the barrier beach of Tamaulipas.

Our specimens are slightly smaller than G. p. personatus and slightly larger than G. p. megapotamus, the subspecies of nearest geographic occurrence to the barrier island. The degree to which our specimens differ in other respects, such as configuration of the pterygoid, is being studied further by Wilks. For the present, reference of our material to the nominate subspecies best expresses the relationships of these coastal gophers.

The fact that pocket gophers from the Tamaulipan barrier island occupy a position geographically intermediate between present Texan populations and the isolated population in southern Tamaulipas (G. p. tropicalis) helps explain the origin of the latter. It is likely that G. p. tropicalis represents the southern remnant of a once continuously-distributed population of pocket gophers living in coastal Tamaulipas in mid-Wisconsin to late Wisconsin time. At that time, sea level is thought to have been considerably lower than at present, exposing a sandy strip 80 to 100 miles wide off the present coastline. Presumably this would have been an area suitable for gophers and for southward dispersal of individuals from Texas. The only conceivable barrier to dispersal, and thus to a panmictic population, would have been the Rio Grande, but over the wide, low and sandy coastal plain the river channel almost certainly shifted regularly, thus decreasing its effectiveness as a barrier to movement. With subsequent rise in sea level, the gophers at Altamira became isolated and have presumably remained so for a considerable time. To judge by the marked morphologic differentiation of G. p. tropicalis, its degree of isolation from other populations has been much greater than those of populations inhabiting the Tamaulipan barrier island and the barrier islands of the coast of Texas. Contact between the latter two populations was probably fairly regular before man's stabilization of the channel of the lowermost reaches of the Rio Grande.

At Camp 1 we found evidence of the former occurrence of gophers in an area now largely covered by active beach dunes. Numerous skeletal parts of gophers and "fossilized" burrows (Plate 8) were found on the surface where troughs between active dunes reached down to an older, darker, and more tightly cemented layer of sand underlying the present dunes. It is clear that these gophers were not transported there, because the bones were not damaged, some of the skeletons were almost complete, and many of the bones were found near the "fossilized" burrows. Weathered but well preserved skeletal remains of at least 12 gophers were picked up at this site.

Specimens (17): , 89023, Camp 1, May 20. 4 , 89024-026, 89029; 3 , 89027, 89028, 89030; Camp 1, May 21. Male, 89031, Camp 1, July 6. Three , 89032, 89035, 89038; 4 , 89033, 89034, 89036, 89037; Camp 2, July 9. Female, 89039, Camp 2, July 10.

=Perognathus merriami merriami= Allen: Merriam Pocket Mouse.--An individual taken in a trap in the dunes near Camp 2 constitutes the first record of this species from the barrier island of Tamaulipas. This pocket mouse seems to be uncommon on other barrier islands of the western Gulf of Mexico, for there is only one published report of its occurrence on Padre Island, Texas (Bailey, 1905:141). Other nearby stations of occurrence are Altamira, Tamaulipas (Hall and Kelson, 1960:477), Brownsville, Texas (Bailey, loc. cit.), and 17 miles northwest of Edinburg, Texas (Blair, 1952:240).

Specimen: sex?, 89040, skull only, Camp 2, July 10.

=Dipodomys ordii parvabullatus= Hall: Ord Kangaroo Rat.--We found this species uncommon and confined in distribution to dunes, in which it was recorded as follows: an adult female was shot and two other individuals were seen at night on July 6 at Camp 1; three were trapped near Camp 1 on July 7; two were trapped at Camp 2 on July 10.

Specimens (5): , 89041, 2 placental scars, 46 gm., Camp 1, July 6. Male, 89042, testes scrotal, 47 gm.; , 89044, 60 gm.; , 89043, 44 gm.; Camp 1, July 7. Sex?, 89045, skel. only, Camp 2, July 10.

Our material does not differ significantly from specimens obtained by Hall and von Wedel at Boca Jesus Maria in March, 1950, which formed the basis for Hall's description (1951:41) of D. o. parvabullatus. This subspecies is presumably confined in distribution to the barrier island of Tamaulipas. Two immature specimens from Bagdad, Tamaulipas, were tentatively assigned by Hall (1951:41) to D. o. compactus, a subspecies known otherwise only from Padre Island, Texas.

=Neotoma micropus micropus= Baird: Southern Plains Woodrat.--This species was noted only near Camp 1, where numerous houses were seen in stands of mesquite and prickly-pear cactus and an adult male (89046, 330 gm.) was taken on July 6. This species has not been reported previously from the barrier island of Tamaulipas. Our specimen is referable to the nominate subspecies and shows no approach to N. m. littoralis, a subspecies known only from the type locality at Altamira, Tamaulipas (see map, Hall and Kelson, 1960:684).

=Procyon lotor= (Linnaeus): Raccoon.--A weathered skull and a broken humerus were found at Camp 2. The skull is being studied by Dr. E. L. Lundelius, who informs us that it matches a number of raccoon skulls found in archaeological sites along the Balcones Escarpment of Texas. Such skulls are larger than skulls of raccoons occurring today in Texas (P. l. fuscipes) and closely resemble skulls of raccoons (P. l. excelsus) presently confined in distribution to Idaho, eastern Oregon, and eastern Washington. Further details of this situation are to be reported elsewhere by Lundelius.

=Taxidea taxus= (Schreber): Badger.--Two burrows were found in the stabilized dunes near Camp 1, tracks were noted on the alkaline flats, and a weathered skull (89047) was found on the flats west of Camp 1 on July 7. The skull appears to be of an immature animal, for the sutures are not well closed and the teeth show little wear.

Our records require an extension of known range of this species southeasterly by approximately 50 miles. The only previous record in coastal Tamaulipas is based on two skulls from Matamoros (Schantz, 1949:301). The skull from the barrier island cannot be determined to subspecies but on geographic grounds is referable to T. t. littoralis, with type locality at Corpus Christi, Texas.

=Canis= sp.--Numerous tracks made either by Coyotes (C. latrans Say) or by domestic dogs were seen in dunes and on the beach at both camps. A weathered, posterior part of a canid skull was found in dunes at Camp 2 on July 10, and a partial left mandible was taken on the beach at Camp 1 on July 6. Unfortunately, specific identification of the skull fragments is not possible, but the few reasonably good characters that we can use suggest that our material is of domestic dogs rather than of Coyotes. Hall (1951:37) found tracks and other signs of Coyotes at Eighth Pass but did not take specimens.

Most of the canid scats examined by us contained remains of crabs and fishes.

=Odocoileus virginianus= (Boddaert): White-tailed Deer.--A weathered Recent fragment of a mandible (89048) and part of a femur (89049) of this species were found near Camp 1 on July 7, and a metapodal was picked up in the dunes at Camp 2 on July 9. This species has not been reported previously on the barrier island of Tamaulipas and it probably no longer occurs there, for we saw no tracks or other signs of it. Hall (1951) did not find it at Eighth Pass.

Our specimens probably pertain to O. v. texanus but are possibly of O. v. veraecrucis, which has been reported from Soto la Marina (Goldman and Kellogg, 1940:89).

The only species of mammal known from the barrier island of Tamaulipas that we did not find is the Hispid Cotton Rat (Sigmodon hispidus). Two specimens of this species trapped near Eighth Pass in March, 1950, formed the basis for the description of S. h. solus (Hall, 1951:42), a subspecies known only from the type locality.

Discussion

The known vertebrate fauna of the barrier island of Tamaulipas consists of one species of tortoise, two species of lizards, at least one (unidentified) species of snake, 49 species of birds (48 recorded by us and the Semipalmated Sandpiper), and 12 species of mammals. This is clearly a depauperate fauna, such as is characteristic of islands generally, and indicates that the peninsular nature of the northern part of the barrier island is of relatively small consequence in determining presence or absence of species. It is likely that the restricted environmental spectrum is much more important in this regard than is the fact of semi-isolation.

Of the 49 species of birds, 10 are known to breed on the island and an additional 21 are suspected of breeding either on the island or on small islets in the adjacent Laguna Madre de Tamaulipas. Eleven species occur on the island as nonbreeding summer residents, about which we will have more to say below. Four species have been recorded on the island in summer but breed elsewhere, that is to say, they only wander over the island (Man-o'-war Bird, Turkey Vulture, etc.). Two species are known only as migrants, and the status of one, the Sora Rail, is uncertain. The number of migrant species doubtless will be greatly increased by field work at those times when birds migrate.

The avifauna is not depauperate owing to the exclusion of any one of the three major zoogeographic stocks thought to be important in the development of the present North American avifauna (Mayr, 1946). If we examine the breeding passerine birds of the barrier island and the breeding passerine assemblage at the same latitude in lowland Sonora (Mayr, loc. cit.) as to their ultimate evolutionary sources, we find that for both places somewhat more than half the birds have developed from indigenous, North American stocks, about one-third have been derived from South American stocks, and one-fifth to one-eighth are from Eurasian stocks. It is most unlikely that such close correspondence in relative composition of the two avifaunas would occur by chance. Thus, we can only conclude that each of the historical avian stocks is proportionately restricted in numbers on the barrier island.

Faunistically, the barrier island resembles Padre and Mustang islands and the adjacent mainland of Tamaulipas and southern Texas, reflecting the relative uniformity of environment in this region. It is apparent that there is a faunal "break" or region of transition in the vicinity of Tampico, in extreme southeastern Tamaulipas. On the coastal plain, many tropical species and subspecies occurring in Veracruz are found north to Tampico but fail to extend farther northward to the barrier island of northeastern Tamaulipas. Axtell and Wasserman (1953:4-5), have already commented on this situation, mentioning a number of snakes and lizards that have differentiated subspecifically on opposing sides of the Tampican region. They also note that large numbers of the lowland Neotropical floral and faunal elements reach their northern limits of distribution within the zone of transition around Tampico, and, also, many Nearctic elements find their southern distributional limits there.

Our small samples of birds and reptiles from the island show no detectable morphological differentiation from adjacent populations. However, several of the mammals are moderately-well differentiated, but the patterns and degrees of geographic variation are such that we can only speculate on the historical derivation of the insular populations. Lepus californicus curti is presently known only from the barrier island of Tamaulipas, but Hall (1951:43) has suggested that it may also occur on the adjacent mainland. A resemblance between individuals of this subspecies and specimens of L. c. merriami from Padre Island in smallness of the tympanic bullae is regarded, probably correctly, by Hall (1951:44) as independent development--that is, parallel adaptation to similar environmental conditions reaching fullest expression on the barrier island of Tamaulipas. As is also true with Geomys personatus and Neotoma micropus, the barrier island population of Lepus californicus shows relationships with animals from Texas and northern Tamaulipas (L. c. merriami) and no connection with (resemblance to) animals from the south (L. c. altamirae, known only from the type locality at Altamira, near Tampico).

In color and cranial proportions, Dipodomys ordii parvabullatus of the barrier island is closer to D. o. compactus of Padre Island than to D. o. sennetti of southern Texas and the Tamaulipan mainland. But, D. o. parvabullatus resembles D. o. sennetti in external measurements (Hall, 1951:39). Possibly D. o. parvabullatus and D. o. compactus are phylogentically closer to one another than is either to D. o. sennetti. It is also possible that each evolved independently from a mainland stock represented today by D. o. sennetti; the resemblance of the two insular populations would thus be a matter of convergence in response to like environmental conditions.

Sigmodon hispidus solus is an insular differentiate that probably reached the barrier island from the adjacent mainland of Tamaulipas, where its apparent closest relative, as judged by morphological similarity, now occurs.