Little is known about cyclic ovarian events that occur during the reproductive season for tropical reptiles. The ability to determine the timing of ovarian follicular events provides insight into the reproductive cycle, including seasonality, stages of follicular development, fertilization and determination of gravidity or pregnancy. Follicular dynamics were studied in two closely related species of snakes, the ball python and Brazilian rainbow boa, having different reproductive modes, oviparity and viviparity, respectively.
Ultrasound, a noninvasive diagnostic tool,2 was used to assess, classify, and quantify follicular changes over a 2-year period. Ultrasound imaging (ALOKA 500V, Corometrics Medical Systems, Inc., Wallingford, CT) was conducted biweekly for eight adult females of both species. The females were anesthetized using cotton saturated with 3 ml isoflurane (IsoFlo, Abbott Laboratories, North Chicago, IL) in a 60-cc syringe case that fits as a nose cone.1 The animal’s body was immersed in water to enhance ultrasound transmission. A short linear array transducer (7.5 MHz, Corometrics) was placed right lateral to midline on the ventral surface. The ovoid non-echogenic gall bladder was used as an anatomic landmark located approximately 2/3 the distance from the tip of the snout. The ovaries, just caudal to the gall bladder, are positioned asymmetrically so that the left ovary is distal to the right.
Ovarian cycles were observed in both species. Follicular stages included: 1) nonvitellogenic, 2) early vitellogenic, 3) vitellogenic, and 4) ovulatory. Ultrasonographically, the process of follicular maturation appeared similar in the oviparous and viviparous species. Mean follicle length and width were consistently larger in boas for all pairwise contrasts (Fisher’s protected LSD, p<0.0001, Table 1), which could be attributed to differences in body size between pythons (0.68±0.23, x±SE) and boas (2.57±0.34; t-test, p=0.0414). Differences were also detected in the number of follicles during each follicular stage (Table 1). The number of follicles (y) was an exponential decay function of mean follicle size (x). Regression analysis revealed that the relationship for pythons was: y=331.4x-0.6142, R2=0.9481; and boas: y=1186x-0.6816; R2=0.9586. This pattern indicates that there is a trade-off between follicular size and number.
We greatly appreciate Dale DeNardo, DVM, PhD, University of California, Berkeley, for advice on the ultrasound technique.
1. Bennett, R.A. 1991. A review of anesthesia and chemical restraint in reptiles. Journal of Zoo and Wildlife Medicine. 22 (3): 282–303.
2. DeNardo, D. 1996. Reproductive biology. In: D.R. Mader (ed.). Reptile Medicine and Surgery. W.B. Saunders Co., Philadelphia. Pp. 212–224.