UN>X HARVARD UNIVERSITY Library of the Museum of Comparative Zoology UNIVERSITY OF KANSAS MUSEUM OF NATURAL HISTORY PUBLICATIONS The University of Kansas Publications, Museum of Natural History, beginning with volume 1 in 1946, was discontinued with volume 20 in 197 1 . Shorter research papers formerly published in the above series are now published as The University of Kansas Museum of Natural History Occasional Papers. The University of Kansas Museum of Natural History Miscellaneous Publications began with number 1 in 1946. Longer research papers are published in that series. Monographs of the Museum of Natural History were initiated in 1970. Authors should contact the editor regarding style and submission procedures before manuscript submission. All manuscripts are subjected to critical review by intra- and extramural specialists; final acceptance is at the discretion of the Director. This publication is printed on acid-free paper. Occasional Papers and Miscellaneous Publications are typeset using Microsoft - Word and Aldus PageMaker' on a Macintosh computer. - Museum of Natural History, The University of Kansas, Lawrence. Institutional libraries interested in exchanging publications may obtain the Occasional Papers and Miscellaneous Publications by addressing the Exchange Librarian, The University of Kansas Library, Lawrence, Kansas 66045-2800. USA. Individuals may purchase separate numbers from the Office of Publications, Museum of Natural History, The University of Kansas, Lawrence, Kansas 66045-2454, USA. Editor: Linda Trueb Managing Editor: Joseph T. Collins Assistant Managing Editor: Kate Shaw Printed by University of Kansas Printing Service Lawrence. Kansas OCCASIONAL PAPERS MCZ LIBRARY MAY 2 4 1993 of the MUSEUM OF NATURAL HISTORY The University of KansasRSlTY Lawrence, Kansas NUMBER 156. PAGES 1-24 28 APRIL 1993 Life History Traits of the Western DiAMONDBACK RATTLESNAKE {CrOTALUS ATROX) Studied from Roundup Samples in Oklahoma Henry S. Fitch and George R. Pisani Fitch Natural Histon- Reservation. The University of Kansas. 2060 East 1600 Road. Lawrence, Kansas 66044-9460 (HSF): Division of Biological Sciences. Haworth Hall, The University of Kansas, Lawrence, Kansas 66045-2106 (GRP) ABSTRACT In 1987, 1988, and 1989, 1011 live Crotalus atrox were examined at the Waynoka. Okeene, Apache. Waurika. and Mangum rattlesnake roundups in Oklahoma. Males are the more numerous in each local sample. At birth, the average size of males exceeds that of females and the differential increases with age, with adult males being about 10% larger than females. Young thought to have been born in August or early September increased in si7e nearly 54% by the following April (the time of the roundups) and added another rattle segment to the natal button; those thought to be second-year young were 168% larger than neonates and had six rattle segments. Sexual maturity evidently is attained in the third year when most females longer than 900 mm were fecund, and breeding seems to be annual. There is an average of 1 3 (4-24) yolked follicles and the number is proportional to female size. The average diameter of the natal button is 6.8 mm. Although successive rattle segments may increase slightly in size, after a snake acquires 10 segments (usually in the fourth year), there seems to be little growth between successive sloughs. Occasional reduction in the size of the proximal segments of rattle strings of medium-sized to large adults may indicate undernourishment. The largest and oldest snakes were probably between 10 and 15 yr old and represented 4.2% of the sample. Slightly more than half of the snakes were second-year young and young adults thought to be 3 or 4 yr old. Annual adult mortality is estimated at 20%. Subtle differences in sizes and proportions distinguish each of the five roundup populations. Key words: Rattlesnake roundups, Crotalus atrox. basal rattle segments, Oklahoma. © Museum of Natural History. The University of Kansas, Lawrence. ISSN:009 1-7958 2 UNIV. KANSAS MUS. NAT. HIST. OCC. PAP. No. 156 Since Klauber's (1956) landmark investigations, many excellent field studies of rattlesnakes have been made and knowledge of rattlesnake ecology has progressed significantly. The common and wide-ranging Cwtalus horhdus and C. viridis have been especially well studied at several sites in their extensive ranges (Brown, 1991; Brown et al., 1991; Diller and Wallace, 1984; Duval et al., 1985; Galligan and Dunson, 1979; Gannon and Secoy, 1984; Golan et al., 1982; Martin, 1982; McCartney, 1989; McCartney and Gregory, 1988; Reinert and Zappalorti, 1988; Wallace and Diller, 1990). No comparable studies have been made on Cwtalus atro.w although it also is wide-ranging and ecologically significant as a dominant predator. The "rattlesnake roundups" that are held annually at many places within the range of C. atrox yield harvests of thousands of snakes that are a potential source of demographic and life-history information. However, the opportunity to obtain such information largely has been neglected because conservationists and herpetologists generally deplore the roundups and tend to avoid them. We were contracted by the Non-Game Program of the Oklahoma Department of Conservation to investigate five annual rattlesnake roundups (Waynoka, Okeene, Apache, Waurika, and Mangum) to gather information concerning economics and sociology of the roundups and the effects of such roundups on local wildlife, including prey populations of rodent pests. With cooperation of the roundup leaders and organizers, we examined large samples of live snakes and snake viscera that were discarded at the roundup butcher shops. Our objective was to investigate growth, rattle development, longevity, reproduction, food habits, and population structure in the snakes examined. The formal roundups are held on spring weekends (mainly in April) when the snakes have recently emerged from nearby hibernacula. Many snakes are captured by local hunters on warm days in March and kept several weeks until the local roundup when they are sold for the "snake pit" displays or for their hides and meat. Data presented herein apply specifically to snakes at the beginning of their season of activity; the condition of their gonads, fat bodies, and rattle development differ at other times of year and are relatively poorly known. Those aspects of this study concerned with food habits and endoparasites were reported by Pisani and Stephenson (1991) and Stephenson and Pisani (1991), respectively. MATERIALS AND METHODS Eight roundups were attended, as follow: Waynoka (1987, GRP; 1988, HSF & GRP); Waurika ( 1988, HSF & GRP); Okeene (1988, HSF & GRP; 1989, HSF); Apache (1988, HSF; 1989, HSF); and Mangum (1988, HSF). Waynoka and Okeene are in northern Oklahoma in the Cimarron River drainage, whereas Waurika, Apache, and Mangum are in the southern part WESTERN DIAMONDBACK RATTLESNAKE LIFE HISTORY 3 of the state in the Red River drainage. Snakes were chilled by immersion in ice water for several minutes; when they were immobilized so that they could be handled with relative efficiency and safety, they were removed. After examination, the snakes were returned to holding boxes; usually, they recovered to full activity within 45 min. Data recorded from the live snakes include sex, snout-vent length (SVL), tail length, rattle-string length, number of rattle segments, vertical diameter of each segment, number of tail bands, and weight. All measurements are in millimeters and grams. Numbered plastic tags were inserted into the gullets of many live snakes ( 1988) or tied to their rattles (1989) and retrieved at the time of butchering; in this way. the viscera could be associated with individuals for which data had been recorded in life. At each roundup, snakes were collected mainly within a 40-km (25-mi) radius of the sponsoring town, but others that were captured earlier were brought from more remote areas. The Oklahoma roundups were held on successive weekends over a 4-wk period; snakes from earlier roundups were shipped to. and displayed at. later roundups. Western Diamondback Rattlesnakes from Texas, which emerge earlier than those from Oklahoma, may have been represented at any or all of the Oklahoma events. However, we used only snakes from separately crated lots that were identified by hunters as having been collected locally. Although we attempted to obtain representative samples of each population, the snakes that we examined may already have been subject to selection, some of it subtle. For example, these collections lacked an adequate representation of first-year snakes. According to collectors' statements, young frequently were encountered in the field, but usually were "left for seed" because their monetary value was negligible. Dealers who purchased the snakes paid by the pound and the larger the snake, the greater its value. At the butcher shops, large snakes were preferred. Whenever feasible, we examined lots of snakes that were destined for immediate butchering so that we could collect their viscera; this may have resulted in some bias in the sex ratio of our sample. However, we found males to be more numerous even in the smaller size classes. Butchering was conducted sporadically at the roundups; its timing seemed to be correlated first, with the objective of providing fresh meat and skins for immediate sale and. second, with the aim of collecting admission fees from spectators. Statistical tests follow Sokal and Rohlf (1969). RESULTS AND DISCUSSION Sex Ratios and Sizes Males always were more numerous than females in the live samples; the ratio was 57.5% males to 42.5% females (1.4:1, /?= 1011. F < 0.05). The percentages of males at different roundups differed slightly, as follows: 4 UNIV. KANSAS MUS. NAT. HIST. OCC. PAP. No. 156 Waynoka, 52.1%; Waurika, 55.6%; Okeene, 57.4%; Mangum, 60.3%; and Apache, 60.4%. It is unclear whether males actually were more numerous than females or, instead, more easily found. Males may emerge from hibernacula a little earlier than females, but this possibility seems to be contraindicated by the fact that relatively fewer males were encountered in samples from early roundups (Waurika, Waynoka) than from later ones (Apache, Mangum). Perhaps males, motivated by sexual search, are more inclined to travel and to rattle when disturbed and, therefore, are found more easily. Our procedure of processing batches of snakes that were about to be butchered also may have biased sex ratios, because large snakes were preferred for butchering. As measured by snout-vent length (SVL) and weight, male Cwtalus atrox are substantially larger in average and maximum sizes than females (Figs. 1-2). Moreover, males have longer tails than females and there is scarcely any overlap between the sexes in relative tail length (Fig. 3). The wide range of variance in tail length in each sex reflects individual variation and allometric changes during growth. Mature female Cwtalus atrox possessing enlarged follicles range in size from 840 mm SVL (live weight, 440 g) to 1300 mm SVL (live weight, 1416 g). The longest male is 1520 mm SVL (live weight 2776 g). Our findings corroborate Klauber's (1956) report that male C. atrox are 10.3% longer than are females on an average. The sponsors of the Waynoka event have recorded the total lengths of the two longest snakes captured there each year since 1950. The approximate snout- vent lengths of these record snakes (presumed to be males) were estimated by subtracting the sum of 8.04% of the total length (= tail length) plus 49 mm (average length of a rattle string). The mean total length and snout-vent length of the 37 longest snakes are 1844 (range = 1473-2032) and 1647 (range = 1307-1820) mm, respectively. The means and ranges for the 37 runners-up are 1727 (1467- 2007) mm total length and 1540 (1300-1792) mm SVL. Growth and Rattle Development Each snake retaining an intact rattle string carries a record of its growth from the time of birth. There is a high correlation (± 94%) between the size of the basal rattle segment and SVL (Fig. 4). In the smallest snakes, the average diameter of the basal rattle segment is approximately 1.8% SVL, but owing to allometric growth, the relative size of the basal rattle segment decreases with respect to snout-vent length in larger and older individuals. Thus, it is about 1.6%- SVL in young adults, 1.55%- in 18 large adults 1200- 1299 mm SVL, 1.45% in four individuals 1300-1399 mm SVL, and 1.40% in four individuals that exceed 1400 mm SVL. In our composite pooled sample representing both sexes and all five localities, the average diameter of the natal button is 6.8 mm; the average sizes of successive rattles are 8.2, 9.5, 11.1, 12.3, 13.7, 14.3, 15.0. 15.6, 15.8, and 16.3. WESTERN DIAMONDBACK RATTLESNAKE LIFE HISTORY Sample Sizes 0.038 0.499 0.959 1.420 1.881 0.793 0.975 1.520 1.338 1.157 Live wt. X 103 2.341 0.612 2.802 0.430 SVLx103 1.300 1.156 1.012 0.731 0.962 0.723 0.868 1.193 0.579 Live Wt. X 1 o3 1.424 0.435 SVLx103 Fig. L The relationship between snout-vent length (SVL) and weight in samples of Cwtahis atrox from Oklahoma. At birth, the snout-vent length and rattle diameter of males exceed those of females and the disparity increases with age (Table 1). From the sizes of successive rattle segments, percentages of gains between successive sloughs (after the postnatal slough) are: 20.6, 15.9, 16.9, 10.8, 11.4, 4.4, 6 UNIV. KANSAS MUS. NAT. HIST. OCC. PAP. No. 156 4.9, 2.0, 2.0, and 1.3. These measurements demonstrate that growth slows markedly after the third slough and even more after the fifth. In all snakes having incomplete, but tapered, rattle strings, the number of missing segments was estimated from the size (diameter) of the terminal segment. For instance, if the diameter of the terminal segment is 13.7 mm or nearer this figure than to the means for the next smaller or larger segments (12.3 and 14.3, respectively), it was assumed that three more distal segments had been lost, because 13.7 mm is the mean for the fourth segment (counting inward from the tip toward the base) in intact strings. Thus, the growth rate postulated here and implied in Table 1 is inferred and is based on the following assumptions. ( 1 ) Large snakes produce larger rattles than smaller snakes and the diameter of the basal rattle segment is proportional to the snout-vent length of the snake. (2) Neonates of Crotaliis atrox from Oklahoma conform with Klauber's (1956) findings for the species; born in late summer, they have mean overall length of 330 mm (ca. 302 mm S VL). (3) The smallest snakes in our sample that are recognizable as a distinct cohort are first-year young, which are about 7 mo old. These snakes are 50% longer than neonates and possess one additional rattle segment. (4) Young too large to belong to the first-year cohort are concentrated in a second-year cohort; these individuals range between 800-900 mm SVL and usually possess strings of six rattle segments (having gained four segments in their first full growing season). (5) Under natural conditions during their season of activity, adult wild C. atrox shed with the same frequency as individuals thriving in captivity — i.e., about three times in 2 yr (with an 8-mo growing season). First-year young comprise a recognizable cohort, distinct from second- year young. There are only 20 first-year snakes in our entire sample, but the snake-catchers indicated that they were relatively numerous in the field. The average snout-vent length of snakes in our sample is 465 mm (range = 430-582). Seventeen of these snakes had added one rattle segment to the original button; two had added two segments and one still had only the button. Klauber (1956) reported the average total length of newborn Cwtalus atrox to be 330 mm; if one allows 28 mm for the tail and button, the average snout-vent length of these neonates, born primarily in August, would be 302 mm. We obtained no data for neonates because the youngest snakes were already about 7 mo old at the time of the roundups and had been active for at least 2.5 mo. Based on Klauber's (1956) figures for C. atrox neonates, we deduced that, on the average, the first-year young in our sample had increased 162 mm in SVL and gained one rattle segment in the interval since birth. Although the rate of development of first-year young provides clues as to the stage of development to be expected in second-year young, presumably there is progressive diminution in length gains and WESTERN DIAMONDBACK RATTLESNAKE LIFE HISTORY -1400 • -1200 •^ •.A O) -1000 - 800 ? I- 600 ^ 400 ^^ fcp..^ 1-200 ^^.m^®® 600 800 1000 1200 • -2500 • ••• O) -2000 :a. -1500 -1000 - 500 ' 1 — • . .**A ®*®* • ^4^®®^* Males ^j)^i^(/®®® 1 ^- 500 1000 1500 Snout vent length (mm) Fig. 2. The relationships between length and weight in female (upper) and male (lower) Cwtahis atwx from Oklahoma. Circles represent one record, whereas triangles represent two records; multiple records greater than two are indicated by numbers enclosed in open circles. rattle gains as growth proceeds. The first-year snakes in our sample probably had been active through September and most of October; thus, in only 8-10 wk, they became 54% longer and added one rattle segment. The average diameter of the new rattle segment is 1.3 mm (19%) larger than the natal button; probably most of the young snakes grew after the slough that produced this second rattle segment. UNIV. KANSAS MUS. NAT. HIST. OCC. PAP. No. 156 p40 c/) ■o o o 0) 30 females ii: o I- 0) n E 7^ -20 -10 rrrn ™ m rrr lijii: M M ES^ 4.25 ■o i_ o o CD JO E 13 t-30 20 10 males ^•■^^^^^•^•■■^^^^•' 5.63 7.00 issEsa- 6.00 7.50 9.00 Percentage of Tail to Total Length Fig. 3. Relative tail length in male and female C rota I us atrox from Oklahoma. Only four snakes in our sample possess two or three rattle segments in addition to the natal button: apparently inost reached these stages at times of year other than the spring, when roundups were held. In contrast, 39 WESTERN DIAMONDBACK RATTLESNAKE LIFE HISTORY I 22t c CD E 0) CO t: ca DC 15 w cfl m 0) E CO b 20-- 18- 16-- 14-- B 12-- A • • • ▲•■A ▲ 800 1000 1200 1400 Snout Vent Length (mm) Fig. 4. The correlation between the diameter of the basal rattle segment and snout-vent length in Crotalus atrox. Circles represent one record, whereas triangles represent two records; squares represent three records and inverted triangles four records. immature snakes have four segments plus button, whereas 55 others have five rattle segments plus the button and 51 possess six segments plus button. Each of the three latter groups is assumed to represent second-year young. The average snout-vent length of these snakes, 810 mm (813 in 82 males; 806 in 62 females), indicates that they are about 74% longer than their first-year counterparts. In the 1-yr period, these snakes gained an average of four rattle segments and the diameter of basal rattle segment increased from 8.16 to 13.7 mm, about 65%. Correlations among numbers of rattle segments (in intact strings), size of rattle (diameter of basal segment), and body size (SVL) of Crotalus atrox are shown in Table 1 and Figure 4. For each rattle segment, beginning with the button, there is a wide range in rattle diameter and body length. The body size and the diameter of the basal rattle segment increase relatively rapidly as the first six segments develop during the fall period after birth and the first full growing season following hibernation; however, growth diminishes abruptly when the snakes reach adolescent size. Body sizes and sizes of new rattle segments continue to increase gradually in medium- and large-sized adults. The snakes acquire rattle segments 9-14 as young adults, evidently in their third, fourth, and fifth full growing seasons (April-October). At these ages, males are notably larger than females, but this sexual dimorphism is not as extreme as it is in old adults. After a snake has gained its tenth segment, size gains between sloughs 10 UNIV. KANSAS MUS. NAT. HIST. OCC. PAP. No. 156 "3 c 1/2 c 1) c > 3 O C C/3 c c E t/5 c E 1/5 E 3 C C O C 00 c 1) 1) E 03 c w 00 +1 c t4- C O aj >- E E ^ 3 /-; r<-, — . o O CN ON (m' o r<-; ri r-^ ri in ^t o in +1 +1 +1 +1 +1 +1 +1 +1 +1 +1 +1 NO in, NO O t^ r<-, O ON o r~- NO (N — ON o o r«-j 00 O -* — ^ 00 O ^ ir, +1 P; in nO rn r) r- NO ^ — ri -xf o n-j O rj On r<-i On 1-^. r~- On mi (-<-, iTj r-oooooNONOOO"-^ — oo +1 +1 O ON ON NO — ON NO NO O ^ m, n ON (^ mi iTj Tf r~- — n r^-j r) NO On '^ oo t^ ON On — r~- rj n — — — — o) r-j r-i -^ — — ' m in ON -H -vf r^ n r<-i NO ri ^ -^ — -^ ri rj ^ONoooNoorioooo— 'O0t^r--r-o — NOr<", r--r-~o r-^ONOri-^NONdNdodoNodoooooNodr^-' I I I I i7777777777i O rn mi rj m, m r^i r-~ oo — NO r<-, — o 00 rj NO r-^ 00 o — ■ — • ri ri ri r<-] rt ^' m] no i^] no G^ ^ ri mi 1 1 1 1 ri -:j- o O r- 00 o — 1 r<-i r-i r~- ^ mi -^t r^ '^ rj ^ 00 r<~i "^ r<-| r<-, r- O <^i O m, ITi — ^ '^ mi r<-) in r-- O NO I^ ON O 't 00 — noooon — ^rir<-, -rtmNONONONor--r~--r--NOooONOri —" r-i rnr^inNOi^ooONO^- r)r<-, Ttm'-HrjmTtm) e PL, WESTERN DIAMONDBACK RATTLESNAKE LIFE HISTORY C c C > 3 O c t/3 C g « E (U c C/3 +1 (U c n (U (— u. c OJ f^fl J-i 1) r- y: 3 QJ z ■5 S3 00 r<", r>-, rf, r^, n ^ rr, CI — — ' O CI O O i^. i^. O i^. O ^C 00000-^ — — — o I I I I I I I I ON \c -j:) ri -* ON ri ir, r^ ■!+ ON — oc I/", r^ CC OC ON ON ON. ON ON oc I/"; r-~ IT", r-- ri O) OC — U-i r<-, 'Tt ri — — I o o ri ri r- c o 1^- NO o • — O >/". i/". i~n O vO On CI CO r~-r~-r-r-~DOococooONON — ^oc — ^o — cN^>/~ioq r<-] ON r^! — ^ r<-] ^ -^ Tj- c-l — — Tt- — — — — — — — +1 +1 +1 +1 +1 +1 +1 +1 +1 +1 sO oc >r-, ^ ^ ^ — r)'^oco<^iONr---r-~-— -o nc nC nc r-^ oc nc vd vd r-' r-~^ 1 1 1 r) 0 ri 1 1 1 1 1 1 1 0 oc r<-, x; —(-<-, oc — n — — ri r<-, ri -T -t u'. \0 U-, \C 3C 5C n-, 3C On. nC — vO i/~i i/~i On O r<-, O r<~, '^ O r<~] ^ -nJ- ^' i/S iri in in iri vD sOr^DCONO — r)i-<-, -xtm S (U [in 12 UNIV. KANSAS MUS. NAT. HIST. OCC. PAP. No. 156 Q. E 0) O) OS c CD O Q. -40 -30 Females -20 -10 WyM& WmM ■■■■■■■■■■■■■■■■■■■■■\ -30 -20 -10 Males :..:.::,.„.„.,..,: 13 14 15 16 17 18 19 20 21 Diameter of Basal Rattle Segement (mm) Fig. 5. Diameter of basal rattle segments in adult male and female Crotahis atrox. usually are relatively small. The diameters of basal rattle seginents in 645 fully adult snakes (413 males and 232 females) are shown in Figure 5. The basal rattle segment of most females is 15-16 inm in diameter, whereas the basal segments of most males are 16-17 mm in diaineter. The largest snake examined (SVL 1520 mm, weight 2.78 kg) had a string of eight rattle segments, each of which was 21.5 mm in diameter. Table 1 is based on the records of all snakes having complete or nearly complete strings that are tapered. Although some strings lack one or more WESTERN DIAMONDBACK RATTLESNAKE LIFE HISTORY L^ 30i CO CD ■o CD o £ 3 20 10- J^^K9% • A^«« 700 800 900 1000 1100 Snout Vent Length (mm) 1200 1300 Fig. 6. Numbers of yolked follicles and snout-vent length in female Cwtalus atrox. There is no reproductive activity in females with snout-vent lengths less than 800 mm. Reproductive activity is variable in females with snout-vent lengths between 800 and 1000 mm. whereas nearly all those longer than 1000 mm are reproductively active. Small closed circles represent one record and triangles represent two records. terminal segments, the number of missing segments could be extrapolated. The longest intact rattle strings have 1 1 segments plus the button, and each of 15 incomplete strings has 13 segments. Based on measurements of basal rattle segments of primarily immature or adolescent Cwtalus atrox that possessed complete strings, we calculated the average diameters of the button and each of the successive segments up to the tenth segment. Snakes having incomplete but tapered strings then were added. This permits projection of the pattern to adults that had produced as many as 16 rattle segments, even though they had lost one or more terminal segments. Examination of Table 1 reveals several irregularities — e.g., some females are larger than their male counterparts, and there is a cohort that is older but slightly smaller than a younger cohort. These anomalies doubtless result from the heterogeneity of the sample with respect to differing rates of development in different areas (notably between northern and southern Oklahoma) or between years. Forty-three of the largest and presumably the oldest snakes (4.7% of the total sample) have untapered strings of large rattle segments. Untapered rattle strings indicate loss of the tapered rattle segments acquired during growth and, in some, loss of additional segments that the snakes acquired as adults. In Table 1, snakes with one, two or three rattle segments are considered to be first-year young, whereas most of those with four (probably), five, six, or seven segments are considered to be in their second year. Klauber ( 1956) 14 UNIV. KANSAS MUS. NAT. HIST. OCC. PAP. No. 156 140- • /• _ 120- A / f O) • ^ X # / ^ 100- • • • / O) A ^ ;$ 80- y"^ ^ 60 - O 40- • • • • • • 20- — *--r^i^ • ••< 0 - • • •• I 1 1 800 1000 Snout Vent Length (mm) 1200 CT 2.0- • O) 1- 1.5- 1.0- 0.0- • -- '— 1 — 1 r • • • • 800 1000 1200 Snout Vent Lenath (mm) 1400 Fig. 7. Gonad weight and snout-vent length in Crotalus atrox. Upper: Ovary weight in female Crotalus atrox. Lower: Testes weight in male C. atrox. Circles represent one record, whereas triangles represent two records and squares represent three records. recorded a total of 118 sloughs in a combined total of 50 "snake-years" in four captive Crotalus atrox kept active year-round; this amounts to an average of 2.3 sloughs per snake per year. Given an activity season of about 8 mo in Oklahoma, an adult diamondback might be expected to slough about 1.5 times each growing season or three times in 2 yr; thus, the 15 sloughs tabulated in Table 1, might represent the tirst 4-5 yr of the snake's life. WESTERN DIAMONDBACK RATTLESNAKE LIFE HISTORY 15 _a) o o CO 0) E _5 o > c of 0) 10r .? 8 - ■o 0) o 6 4 - 2 ■■ :?*2* 800 900 1000 1100 1200 Snout Vent Length (mm) 1300 Fig. 8. Mean volume of yolked follicles correlated with snout-vent length in female Crotalus atrox. Circles represent one record, whereas triangles represent two records and squares represent three records Reproduction Most of the adult feinales that we examined had yolked follicles that were not fully grown, but that presumably would have matured to be ovulated later in the spring. In this respect, Crotalus atrox differs from species such as C. horridiis and C. viridis, both of which emerge from hibernation with fully mature follicles that are ready to be ovulated (Fitch, 1985). Most ovarian follicles of C. c//;o.v collected in April were about 30 x 10 mm, but some, especially those from large females, were considerably larger and others from relatively small females were smaller. Of the 59 female snakes examined that were longer than 900 mm (SVL), 54 (91.5%) were ovigerous: in contrast, of the 1 1 individuals that were less than 900 mm in length, only four (36.5%) were ovigerous (Fig. 6). Probably most females in the 800-899-mm size range and some of those in the 900-999- mm size range were immature second-year young. The correlation between ovary weight (and testis weights for males) and snout-vent length is graphically depicted in Figure 7. All ovaries of adult females {n = 25) longer than 1050 mm SVL had yolked follicles. We conclude that C. atrox bear litters annually in Oklahoma and that most individuals mature and produce their first litters as 3-year-olds. (See Growth and Rattle Development, above). Recent studies support the idea that rattlesnakes are highly efficient sit-and-wait predators that convert the prey into their own biomass (Derickson, 1976; Ford and Seigel, 1990: Gibson et al., 1990; Secor, 1990). 16 UNIV. KANSAS MUS. NAT. HIST. OCC. PAP. No. i.Se c o o CJ a (30 ^ 00 o o «N «N -^ m