Ontogenetic variation in the diet of the southern copperhead, Agkistrodon contortrix, in northeastern Texas.
Subject:
Snakes (Observations)
Snakes (Food and nutrition)
Authors:
Lagesse, Leon A.
Ford, Neil B.
Pub Date:
02/01/1996
Publication:
Name: The Texas Journal of Science Publisher: Texas Academy of Science Audience: Academic; General Format: Magazine/Journal Subject: Science and technology Copyright: COPYRIGHT 1996 Texas Academy of Science ISSN: 0040-4403
Issue:
Date: Feb, 1996 Source Volume: 48 Source Issue: 1
Geographic:
Geographic Scope: Texas Geographic Code: 1U7TX Texas

Accession Number:
128678771
Full Text:
Abstract. -- The stomach contents of a series of 96 specimens of Agkistrodon contortrix from two adjoining counties in northeastern Texas were examined. Arthropods comprized the majority of prey items (53%) but lizards, snakes and a few rodents were also consumed by southern copperheads. Neonate and small copperheads (less than 35 cm SVL) were found to contain more squamata, whereas midsize (35 to 45 cm SVL) and adult snakes (greater than 45 cm SVL) were found to contain primarily cicadas and lepidopteran larva. Only adult specimens were found to contain rodents as prey items. The small gape of neonate and young copperheads appears to restrict them from consuming cicadas and rodents. Large copperheads did continue to consume the small snakes and lizards that constituted the primary prey of small copperheads.

**********

Although data on the stomach contents of snakes are abundant in the literature, there are few species for which food habits have been studied in any detail other than just as lists of prey taken (Mushinsky 1987). These anecdotal data have been used to review the foraging ecology and morphological adaptations for feeding among species of snakes both in evolutionary terms and as resource partitioning mechanisms (Toft 1985; Mushinsky 1987). However, ontogenetic, seasonal and population differences within a species are important questions that because of the lack of information have rarely been addressed. Only recently has enough relevant data been gathered to begin examining these aspects of the foraging strategies of snakes. Arnold (1993), for example, did a provisional evaluation of the prey size to predator size relationships of snakes and encouraged further study. Because of the dramatic size differences of neonate and adult snakes, ontogenetic shifts in diet are particularly important to this group of organisms. An understanding of the occurrence of the variation in diet of snakes is complicated by the interaction of innate changes in behavior and the role of experience (Halloy & Burghardt 1990). Several recent natural history studies of crotalids have included analysis of diet in relation to snake size and some discussion of this problem (Campbell & Solorzano 1992; Sazima 1992) but data from additional species is still needed.

Prey items from series of the southern copperhead, Agkistrodon contortrix, collected at single localities have been documented from Georgia, North and South Carolina, Mississippi and Eastern Texas (reviewed in Fitch 1960; Gloyd & Conant 1990). A large variety of organisms are consumed by this species which ranges from various invertebrates to reptiles, mammals and birds (Ernst & Barbour 1989). However, because limited information relative to the specimens from which the prey were removed are given, the causes of the variation are difficult to assess. Although geographic variation may be involved, such factors as the season when collected and size of the specimen would also be expected to have influences on the type of prey consumed (Greding 1964). Certain prey will only be abundant during some periods (i.e. frogs after a rain) and restriction of the gape of the jaw will affect the size of prey that small snakes can consume.

The southern copperhead is one of the most abundant snakes in both coniferous woodlands and upland deciduous forests in northeastern Texas (Ford et al. 1991). A collection of A. contortrix contortrix in the vertebrate collection at the University of Texas at Tyler were primarily from one area and collected during the same season, but exhibited diversity in body size. These specimens were examined to evaluate size class variation in the diet of this species and also to determine whether differences found reflect ontogenetic changes in prey selection in this species.

MATERIALS AND METHODS

Dietary information was obtained from specimens collected during the summers of 1988-89 at a 515 ha nature camp in Anderson County and from road-killed specimens collected in adjoining Smith County, Texas. This area of northeastern Texas is characterized by predominantly second-growth oak-hickory and short-leaf pine forests with sandy loam soil. Specimens were frozen until size measurements (snout-vent length, SVL) could be taken. The venter of each was then cut longitudinally and the specimen fixed in a 10% solution of formaldehyde. After one week, specimens were washed and transferred to 70% ethyl alcohol. All specimens were placed in the vertebrate collection at the University of Texas at Tyler. Digestive contents were removed from each specimen and identified to as low a taxonomic level as possible. Records of direction of ingestion were also recorded for each item when it was possible to determine. For a general comparison to dietary preferences, the presence of invertebrates and small vertebrates at the resident camp were sampled by pit traps in the same general area and time that the specimens of A. contortrix were collected.

RESULTS

A total of 84 prey items were found in the gastrointestinal tract of 78% of the 96 copperheads examined (Table 1). Forty-eight specimens were found to contain one prey item, 15 contained two items, seven contained three prey, and one specimen had consumed four prey items. Although 62.5% of the prey were consumed head first, there was no apparent correlation of this factor to the type or size of food item. Specimens of A. contortrix were grouped into three size classes representing juveniles and subadults, small adults and large adults and the type of prey relative to the size of the snake is presented in Fig. 1. Cicadas were the most commonly eaten prey (36.9%) and invertebrates constituted over 63% of the diet of the specimens examined. Some specimens in all size classes were found to contain insects but small copperheads contained a larger proportion of lizards and snakes (Figure 1). Only the largest copperheads contained mammals and grasshoppers, but this size class also consumed small snakes. The pit traps were found to contain squamata (lizards and small snakes) and arthropoda (primarily aranae, orthoptera, hymenoptera, coleoptera, diptera and isopoda).

[FIGURE 1 OMITTED]

DISCUSSION

The southern copperhead ranks as a top carnivore in the biotic community of northeastern Texas. Ford et al. (1991) found it to be ubiquitous in all habitats and dominant in all areas except the lowland floodplain where it was second in abundance to the cottonmouth, Agkistrodon piscivorus. Its frequency in northeastern Texas would suggest that it has an important role in the ecology of both the pine and hardwood forests of the area.

The present study supports both Curtis' (1949) and Werler's (1978) observations that in eastern Texas the most common prey items taken by this species are cicadas (37%, Table 1). It is important in analysis of prey selection by predators to have knowledge of the prey abundance for the species being studied. In this study, pit traps did not sample all possible copperhead prey equally (especially arboreal insects such as the cicada) and turned out not to be a completely adequate technique for evaluating prey abundance for copperheads. Because of this it is impossible to determine if these copperheads were preferentially foraging for cicadas or are just active in places where the cicadas occur. Since these insects are generally located in trees at least after emergence from the ground, either hypotheses would require the snakes to climb. Copperheads are known to climb bushes, vines and even high up in trees (Fitch 1960; Werler 1978; Gloyd & Conant 1990). It is suggested here that this arboreal activity most likely reflects this species searching for this prey because the other food items of copperheads are terrestrial.

Cicadas are very abundant in eastern Texas during mid to late summer when most of the specimens examined during this study were collected. The abundance of this prey item in these specimens is not surprising but points to a problem that can occur in dietary studies of snakes. Specimens often come primarily from one collecting trip or time period and if seasonal variation occurs in prey abundance, analysis of those snakes may give a mistaken view of the species diet. Copperheads collected in the spring or fall in east Texas would likely present a very different picture of the prey taken by this population because cicadas would not be available. A difference in when collections were made or the particular habitats collected might explain why frogs (Rana) predominated in Clark's (1949) study of this species in north central Louisiana. Indeed, Greding (1964) suggested that the ground skink, Scincella laterale was the most important prey for this species during the spring in eastern Texas. Studies from one locality in which seasons of the collection are noted are obviously important.

Because the copperheads in this study were collected during the same season from similar habitats in a small area, differences in prey taken by different size classes most likely represent ontogenetic variation in diet. Smaller copperheads contained a larger proportion of lizards and snakes (Figure 1). This may reflect difficulty that smaller snakes would have attempting to eat cicadas. Only the largest snakes consumed mammals, even though in captivity rodents are readily eaten by medium-sized copperheads. Peromyscus, Reithrodontomys and Blarina are relatively common in the same habitats as copperheads. In other localities, small mammals apparently are quite important prey items. For example, in southern copperheads from North and South Carolina mammals ranked second (37.1%) in the number of prey items consumed (Brown 1979). Lepidopteran larvae and cicadas were commonly found in the larger copperheads examined in this study. Why more rodents were not taken by these east Texas copperheads is unknown. One observation that can be made is that the larger snakes appear to have a wider dietary niche than do the young copperheads. As copperheads grow it might be expected that larger prey items (i.e. cicadas and mammals) would be added to their diet but small snakes and lizards were also taken by the largest copperheads. In most species that have been studied, larger snakes tend to not eat the same small prey taken as juveniles; but the change from juvenile prey items is not universal in snakes nor even well documented in many species (Shine 1991; Arnold 1993). For example, Concho water snakes, Nerodia harteri, continue to take small fish as adult snakes (Greene et al. 1994). When ontogenetic shifts in diet in snakes do occur, these changes can be related to increased motor skills and alterations of habitat use in addition to simple structural enlargement of the animal (Lind & Welsh 1994).

Gloyd & Conant (1990) reviewed the diet of Agkistrodon contortrix from a number of localities and suggested that geographical variation in prey taken occurs. Although this conclusion is likely correct, the various collections involved may have bias samples, i.e. larger specimens, only those dead on roadways, or animals collected only from certain habitats. Differences observed among populations may also reflect seasonal variation in prey availability or ontogenetic changes in the foraging activities of young and adult copperheads. This study suggests that the latter can be an important factor that should be considered.

ACKNOWLEDGMENTS

We thank R. A. Seigel and two anonomous reviewers for their review of this manuscript. The Texas Nature Conservancy contributed support to NBF for research in northeastern Texas.

LITERATURE CITED

Arnold, S. J. 1993. Foraging Theory and Prey-size-Predator-size Relations in snakes. Pp. 87-115 in Snakes: Ecology and Behavior. (R. A. Seigel and J. T. Collins, Eds.). McGraw Hill, Inc. New York, 414 pp.

Brown, E. E. 1979. Some snake food records from the Carolinas. Brimleyana 1:113-124.

Campbell, J. A. & A. Solorzano. 1992. The distribution, variation, and natural history of the Middle American montane pitviper, Porthidium godmani. Pp. 223-250 in Biology of the Pitvipers. (J. A. Campbell and E. D. Brodie, Jr., Eds.). Selva, Tyler, Texas, 467 pp.

Clark, R. F. 1949. Snakes of the hill parishes of Louisiana. J. Tennessee Acad. Sci., 24:244-261.

Curtis, L. 1949. The snakes of Dallas county, Texas. Field and Lab, 17:5-13.

Dunham, A. E., D. B. Miles & D. N. Reznick. 1988. Life history patterns in squamate reptiles. Pp. 441-522 in Biology of the Reptilia, Vol. 16. (C. Gans and R. Huey, eds.). Alan R. Liss, Inc. New York, 659 pp.

Ernst, C. H. & R. W. Barbour. 1989. Snakes of Eastern North America. George Mason Univ. Press, Fairfax, Virginia, 282 pp.

Fitch, H. S. 1960. Autecology of the copperhead. Univ. Kansas Mus. Nat. Hist. Misc. Publ., 13:85-288.

Fitch, H. S. 1982. Resources of a snake community in a prairie-woodland habitat of northeastern Kansas. Pp. 83-99 in Herpetological communities. (N. J. Scott, Jr. ed.). U. S. Fish Wildl. Serv. Wildlife Report, 13, 239 pp.

Ford, N. B., V. A. Cobb & J. Stout. 1991. Species diversity and seasonal abundance of snakes in a mixed pine-hardwood forest of east Texas. Southwest. Nat., 36(2):171-177.

Gloyd, H. K. & R. Conant. 1990. Snakes of the Agkistrodon Complex: a monographic review. 615 pp. Soc. Stud. Amphib. Rept. Cont. Herpetol., 6.

Greding, E. J., Jr. 1964. Food of Ancistrodon c. concortrix (sic) in Houston and Trinity Counties, Texas. Southwest. Nat. 9(2):105.

Greene, B. D., J. R. Dixon, J. M. Mueller, M. J. Whiting & O. W. Thornton, Jr. 1994. Feeding ecology of the Concho Water Snake, Nerodia harteri paucimaculatata. J. Herp., 28:165-172.

Halloy, M. & G. M. Burghardt. 1990. Ontogeny of fish capture and ingestion in four species of garter snakes (Thamnophis). Behaviour, 112:299-317.

Lind, A. M. & H. H. Welsh, Jr. 1994. Ontogenetic changes in foraging behavior and habitat use by the Oregon garter snake, Thamnophis atratus hydrophilus. Anim. Behav., 48:1261-1273.

Mushinsky, H. R. 1987. Foraging ecology. Pp. 302-334 in Snakes: ecology and evolutionary biology. (R. A. Seigel, J. T. Collins and S. S. Novak, eds.), Macmillan Publ. Co., New York, 529 pp.

Sazima, I. 1992. Natural history of the Jararaca pitviper, Bothrops jararaca in southeastern Brazil. Pp. 199-216 in Biology of the Pitvipers. (J. A. Campbell and E. D. Brodie, Jr., Eds.). Selva, Tyler, Texas, 467 pp.

Shine, R. 1991. Why do larger snakes eat larger prey items? Funct. Ecol., 5:493-502.

Toft, C. A. 1985. Resource partitioning in amphibians and reptiles. Copeia, 1985:1-21.

Werler, J. E. 1978. Poisonous snakes of Texas. Texas Parks Wildl. Dept. Bull. 31:1-53. Austin, Texas.

Leon A. Lagesse and Neil B. Ford

Department of Biology, University of Texas at Tyler

Tyler, Texas 75701

Direct reprint requests to NBF at: neil_ford%ut-tyler@notes.worldcom.com
Table 1. Prey items recovered from the digestive tracts of southern
copperheads, Agkistrodon contortrix (N = 71 with food). Prey that were
unidentifiable at the lowest level were included in numbers for all
higher levels as were the snakes with that item.

Prey Type              Range of SVL's    Number of         Number of A.
                       of A. contortrix  prey items        contortrix
                       specimens         recovered         w/prey item
                       w/prey item       (% of total prey  (% of total
                                         items)            specimens)

Invertebrates
  Cicadidae            28-45 cm          31 (36.9%)        24 (33.8%)
  Coleoptera           46 cm              1 (1.2%)          1 (1.4%)
  Orthoptera           50 cm              1 (1.2%)          1 (1.4%)
  Lepidoptera          29-84 cm          12 (14.3%)         8 (11.3%)
Total Insecta          28-84 cm          48 (57.1%)        36 (50.7%)
Total Arthropoda       28-84 cm          53 (63.1%)        42 (59.2%)
Vertebrates
  Eumeces fasciatus    47 cm              1 (1.2%)          1 (1.4%)
  Scincella laterale   86 cm              1 (1.2%)          1 (1.4%)
Scincidae tail only    28-32 cm           4 (4.8%)          4 (5.6%)
Total Scincidae        28-47 cm          10 (11.9%)        10 (14.1%)
  Anolis carolinensis  35 cm              1 (1.2%)          1 (1.4%)
Unidentified lizards   24-75 cm           2 (2.4%)          2 (2.8%)
  Virginia striatula   28 cm              1 (1.2%)          1 (1.4%)
Unidentified snakes    25-65 cm           2 (2.4%)          2 (2.8%)
Total Squamata         24-75 cm          18 (21.4%)        16 (22.5%)
Mammalia               64 cm              2 (2.4%)          2 (2.8%)
Unidentifiable         34-57 cm          11 (13.1%)        11 (15.5%)
No food                20-81 cm                            21
TOTAL                                    84                96
Gale Copyright:
Copyright 1996 Gale, Cengage Learning. All rights reserved.