Vertebrates from the Barrier Island of Tamaulipas, Mexico Part 4

=_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 p.r.i.c.kly-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): Racc.o.o.n.--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 racc.o.o.n skulls found in archaeological sites along the Balcones Escarpment of Texas.

Such skulls are larger than skulls of racc.o.o.ns occurring today in Texas (_P. l. fuscipes_) and closely resemble skulls of racc.o.o.ns (_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 Pa.s.s but did not take specimens.

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

=_Odocoileus virginia.n.u.s_= (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 Pa.s.s.

Our specimens probably pertain to _O. v. texa.n.u.s_ 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 Pa.s.s 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 pa.s.serine birds of the barrier island and the breeding pa.s.serine a.s.semblage at the same lat.i.tude 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 Wa.s.serman (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.

_Nonbreeding sh.o.r.ebirds in summer south of breeding ranges._--Certain aspects of this subject have already been discussed by Eisenmann (1951). As he notes, the phenomenon is more regular and widespread than generally has been appreciated. The old idea, that such oversummering individuals were "abnormal" or "senile," is totally inadequate, especially in view of the frequently large numbers of individuals involved.

Eisenmann's suggestion that nonbreeders are immature is probably valid, and it is supported by Pitelka's examination of dowitchers (1950:28, 51). For gulls, which can be aged by characters of plumage, there is no question that most nonbreeders are immature. Unfortunately, there are few criteria for determination of age in charadriiform birds.

With the possible exception of a specimen of _Limosa fedoa_, none of the presumed nonbreeding, oversummering sh.o.r.ebirds collected by us showed gonadal enlargement above expected minimal sizes for the species. Even so, the season was late at the time when we were on the island and most of the birds were molting; it is possible their gonads had been enlarged earlier in the season. Behle and Selander (1953) and Johnston (1956) have shown that nonbreeding first-, second-, and third-year California Gulls (_Larus californicus_) undergo gonadal enlargement in summer. Additionally, nonbreeding first-year males of certain pa.s.serine species (for example, the Brown Jay, _Psilorhinus morio_; Selander, 1959) are known to experience partial gonadal recrudescence in summer. It would be useful, and would facilitate discussion, to have data on gonadal condition of oversummering birds; any functional enlargement would be worth doc.u.menting.

Some species, notably the Semipalmated Sandpiper, Semipalmated Plover, and Black Tern, oversummer as nonbreeders in such large numbers that it is obvious that a significant fraction of the total population of the species does not breed in any one year. This raises questions concerning the possible ecologic situations that would select for delay in time of recruitment of young birds into the breeding segment of the population, a.s.suming that nonbreeders are immature birds. Delay in maturation, or slow rates of maturation, may show general relationship to paucity of sites of breeding, as Orians (1961:308) suggests, but the sh.o.r.ebirds with which we are dealing breed in regions or in habitat-types not characteristically imposing general restriction on sites of nesting; more than one answer is necessary for the question even at this level. Data on age and numbers of nonbreeders, as well as on the ecology of breeding populations, are critical and are badly needed for most species.

In any event, species for which we have data demonstrating that they regularly oversummer south of their breeding ranges are probably adapted to having a part of their populations refrain from breeding each year. Whether this phenomenon can be explained solely in terms of selection at the level of individual birds (Lack, 1954) or involves selection of an adaptive response of the population as a whole (Wynne-Edwards, 1955; see also Taylor, 1961, concerning _Rattus_) is a problem that cannot be resolved at this time. We may note that the species involved ordinarily breed in arctic and subarctic regions, and it would seem advantageous (as set forth below) for nonbreeders to remain well south of such high lat.i.tudes. The numbers of oversummering individuals may fluctuate with over-all population density, possibly as a result of crude density, but possibly also as a result of emigration of individuals in excess of optimal density on breeding grounds (see Wynne-Edwards, 1959). One aspect of this phenomenon not explicitly discussed by Wynne-Edwards is the possibility that some individuals never move north to breeding grounds at all, perhaps as a result of a behavioral character genetically-grounded and mediated by delayed maturation of the neurohumoral "clock." This certainly would be an economical means by which population numbers could be regulated, for there would be a saving of energy in that some individuals not only would not move north, but also would not partic.i.p.ate in the behavioral interactions involved in territorial s.p.a.cing. Occurrence of these birds throughout southern North America, Middle America, and northern South America may thus reasonably be understood.

LITERATURE CITED

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1954. The systematic relationships of certain lizards in two species groups of the genus _Holbrookia_. M. A. Thesis, Department of Zoology, Univ. Texas. 55 pp.

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1953. Interesting herpetological records from southern Texas and northern Mexico. Herpetologica, 9:1-6.

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1938. Notes on the mammals of Galveston and Mustang islands, Texas. Jour. Mammal., 19:505.

BEHLE, W. H., and SELANDER, R. K.

1953. The plumage cycle of the California gull (_Larus californicus_) with notes on color changes of soft parts.

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BLAIR, W. F.

1952. Mammals of the Tamaulipan Biotic Province in Texas.