Estimating Prey Size and Number in Crayfish-Eating Snakes, Genus Regina
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Herpetologíca, 40(1), 1984, 82-88 © 1984 by The Herpetologists' League, Inc. ESTIMATING PREY SIZE AND NUMBER IN CRAYFISH-EATING SNAKES, GENUS REGINA J. S. GODLEY, R. W. McDiARMID, AND N. N. ROJAS ABSTRACT: Snakes of the genus Regina feed almost exclusively on crayfish. The paired, sym- metrical gastroliths of crayfish are not digested and are detectable from x-rays of the snake. Gastrolith length is directly proportional to carapace length and can be obtained from x-rays. Carapace length can be converted to kcal of ingested energy. Using these relationships and repeated captures of radio-telemetered Regina, estimates of food consumption and energy intake by free- living snakes are feasible. New information on prey selectivity, feeding behavior, and predator- prey size relations in Regina grahami and R. septemvittata are presented and compared with similar data for other snakes. Key words: Reptilia; Serpentes; Colubridae; Regina; Food habits; Predator-prey size relation- ships; Crayfish THE foraging ecology of snakes is less ing of specific events in the crayfish molt often studied than many aspects of snake cycle and the relationship of these events biology. Some species are rare and others to prey selection in R. grahami and R. are secretive; consequently, obtaining ad- septemvittata. Briefly, individuals of var- equate samples is a common problem ious species of crayfish molt one to 12 times (Turner, 1977). In addition, most snakes per year depending upon sex, age and en- eat relatively large prey at infrequent in- vironment. Usually less than 15% of a tervals (e.g.. Beavers, 1976; Fitch, 1960, population is molting at any one time (Ca- 1965; Godley, 1980; Greene, 1983a,fe; pelli and Magnuson, 1975; Drach, 1939; Schoener, 1977; Seib, 1981), and their Prins, 1968; Stein, 1977; Stevenson, 1975). digestive tracts seldom contain food. Be- Prior to molt, calcium is extracted from cause of the prey's advanced state of the exoskeleton and stored in paired, sym- digestion in the hindgut (Brown, 1958; metrical gastroliths located in the cardiac Godley, 1980; Henderson, 1970; Skoczy- stomach (McWhinnie, 1962; Richards, las, 1970), positive identification of prey 1951; Stein and Murphy, 1976; Travis, species and important measures of prey 1960). The gastroliths are resorbed shortly size often are obtainable only while the after molt and contribute to the formation prey is in the snake's stomach. of a new exoskeleton. Gastroliths reappear Here we describe an accurate, non-in- upon initiation of the next molt cycle. vasive technique for estimating the num- Gastrolith growth and résorption are dis- ber and sizes of prey in the entire diges- tinct events and are tightly synchronized tive tracts of crayfish-eating snakes, genus with other identifiable stages in the molt Regina. The technique is designed specif- cycle (McWhinnie, 1962; Stevenson, 1975). ically for dietary studies of R. grahami As we shall show, these gastroliths provide and R. septemvittata, two species that feed a convenient, quantitative "marker" for almost exclusively on soft, freshly molted estimating rates of feeding and energy in- crayfish (Branson and Baker, 1974; Burg- take in R. grahami and R. septemvittata, hardt, 1968; Hall, 1969; Kofron, 1978; and perhaps in other species that feed ex- Mushinsky and Hebrard, 1977; Strecker, tensively on molting crayfish (Neill, 1951; 1926; Wood, 1949). We also provide new Penn, 1950). information on prey selectivity, feeding behavior, and predator-prey size relations MATERIALS AND METHODS in these species and summarize similar We estimated the number, size and data for other species of snakes. feeding sequence of crayfish ingested by The technique requires an understand- 36 R. grahami (USNM 12864-65, 12891, 82 March 1984] HERPETOLOGICA 83 12940-70, 13038-39) from New Orleans, were examined after dissection measure- Orleans Parish, Louisiana, and two ments were recorded. Because methods (197638) from Badger Lake, Monona varied between the snake species, results County, Iowa, by exposing identifiable are presented separately. prey with a single ventro-medial incision through the gut wall. The orientation RESULTS (head or abdomen first) and the ingestion position (dorsal, ventral or lateral) of the Regina grahami crayfish relative to the snake's skull were Only the remains of crayfish, repre- recorded prior to removal of prey from senting at least three species, Orconectes the snake's stomach. After removal, the palmeri croelanus (Creaser), Procamba- crayfish's carapace length (CL = maxi- rus clarkii (Girard) and P. vioscai (Penn), mum dorsal midline distance from tip of were recovered from the stomachs of 38 rostrum to posterior edge of carapace) and R. grahami. Of these prey, measurements gastrolith length (GL = maximum longi- of both GL and CL were obtainable from tudinal length of gastroliths) were mea- 17 crayfish (all were in molt stages Ai or sured with vernier calipers to the nearest early Aj), CL only from two crayfish (one 0.1 mm. Only GL was obtained from in- in stage B, the other in stage Aj or As but testinal contents; paired, symmetrical gas- gastroliths broken during processing), GL troliths were assumed to represent the di- only from six crayfish (all in stages Ai or gested remains of a single crayfish. The As but carapace partially digested), and molt stage of the crayfish (Drach, 1939; two crayfish were represented only by sin- McWhinnie, 1962; Stevenson, 1975) also gle, very large (57 and 63 mm total length) was recorded. Molt stages pertinent to this chaela, each in a different snake. The study are as follows. A;•Exoskeleton is largest chaela from an intact crayfish soft, gelatinous and easily torn; gastrolith (CL = 31.5 mm) in the sample was 36.0 is fully formed; duration is of a few hours. mm, suggesting that the latter two snakes Aj•Exoskeleton is leathery; gastrolith is encountered crayfish too large to consume reduced to about 50% of former size by whole and instead removed the only large end of stage; duration is ca. 24 h. B• portion (chaela) that could be disarticu- Rostrum and cephalic carapace are firmer lated. In addition, GL measurements from but flexible; gastrolith is very small to ab- 20 crayfish were obtained from intestinal sent at end of stage; duration is of one to contents. All of these crayfish appeared to three days. have been in molt stage Ai or early A2. Similar data were obtained from a se- Although most of the exoskeletons of ries of 36 R. septemvittata collected in crayfish were partially digested, the gas- Montgomery Co., Ohio (USNM 128974- troliths seemed normal and unaffected by 98, 129016-27). However, before dissec- passage through the snake's digestive tract. tion, each specimen was radiographed in In fact, captives of all species of Regina dorso-ventral view. The radiographs were defecate gastroliths which macroscopical- exposed for 30 s at instrument readings of ly are indistinguishable from those dis- 5 mamp and between 20 and 23 kV de- sected from live crayfish of a similar molt pending on snake size. Gastrolith mea- stage (J. S. Godley, personal observation). surements were taken from the radio- As is typical of freshly molted crayfish graphs with a dial caliper. Because (McWhinnie, 1962 for Orconectes virilis), gastroliths are slightly oblong in shape and GL is a linear function of CL (Fig. 1) and the x-rays provided only a single plane of provides an excellent estimate of crayfish view, the longest dimension of either gas- size. Most of the variance about this line trolith of the pair was chosen to represent probably is caused by three sources of ex- the GL. To avoid possible bias, GL mea- perimental error: (1) differences in GL: surements taken from the radiographs CL ratios among the three species of cray- 84 HERPETOLOGICA [Vol. 40, No. 1 food, 14 contained one crayfish, six had two crayfish each, two contained three, Y.0.157X-0.5S2 • 5,0 and three had eaten four crayfish. SVL Ê r = 0.8 8 0 was positively correlated with the total E number of prey whether individuals with empty digestive tracts were included (Spearman rank correlation, r^ = 0.47; P = 0.0076, n = 38) or excluded (r, = 0.58, P = 0.0024, n = 25) from the analysis. Prey orientation could be determined for 26 of 27 crayfish recovered from the R. grahami stomachs; 12 were ingested head first and 14 abdomen first, suggest- 20 30 Carapace Length (mm) ing no orientation preference (x^ = 0.08, F > 0.75). Vertical position of prey could FIG. 1.•Relationship of gastrolith length and car- be determined for 22 of the 27 prey and apace length from 17 crayfish removed from the was not significantly different from ran- stomachs of Regina grahami. dom (x' = 4.0, 2 df, F > 0.10) with 12 crayfish ingested on their side with re- spect to the snake's skull, eight with ven- fish represented in the sample, (2) differ- ters up, and two with venters down. Anal- ences in crayfish molt stage, and (3) the ysis of covariance with GL as the difficulties of accurately measuring the CL dependent variable, SVL as the covariate, of soft-bodied crayfish taken from a and prey orientation and position as in- snake's stomach. In a sample of 18 Pro- dependent variables showed no significant cambarus fallax from Hillsborough Co., effects for the latter two variables. Florida, which were sacrificed within six h of molting, the correlation coefficient Regina septemvittata between GL and CL was 0.98 (J. S. God- In this species, specimens were x-rayed ley, unpublished data). The slope of this prior to dissection to determine if radio- line was not significantly different from graphs could provide reliable estimates of that shown in Fig. 1 (F^ai = 2.48, F = feeding activity.