systematics of snakes of the dipsas oreas complex - BioOne

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120 Bulletin Museum of Comparative Zoology, Vol. 158, No. 3 arboreal seclusion also occurs within Dipsas. Porto and Fernandes (1996) reported an inactive D. neivai found within a bromeliad on a tree branch close to the ground, and I collected an inactive D. vermiculata (MVZ 163259) from a bromeliad 2 m from the ground during the day. Parker (1926) reported that the type of D. variegata trinitatis was obtained from a bromeliad on a fallen tree. Charles W. Myers collected several inactive D. temporalis from bromeliads on the sides of trees 1.2– 3 m aboveground during the day (Cadle and Myers, 2003: 36). Eggs and Hatchlings. Cadle and Chuna (1995: 32–34) reported communal egg-laying in Dipsas oreas (21 total eggs), which in that instance was coincident with a large number of communally laid eggs of the gymnophthalmid lizard, Macropholidus ruthveni. The eggs, discovered 17 June 1987 at the lower edge of the humid forest (1,490 m) at the Río Zaña Study Site, were underground in a moist crevice within a road embankment. The eggs were 20–30 cm from the entrance to the crevice and 15 cm beneath the surface of the soil (the crevice ran somewhat parallel to the soil surface); dimensions of the Dipsas eggs were 28–30 20 mm. On the basis of their clustering within the crevice, the snake eggs probably represented three clutches of eight, seven, and six eggs. One egg was opened at the time from each of the two deepest clutches and contained advanced embryos of slightly different stages. Five of the eggs were transported in plastic bags containing moist paper towels back to an air-conditioned office, where they were kept in similar fashion on a table. These eggs hatched between 23 September and 1 October. Hatchlings (FMNH 232570–72, MUSM 16751–52) were 214–225 mm total length (172–176 mm SVL) and 2.8–3.2 g. A hatchling from this clutch is illustrated in Figure 17, and another was illustrated by Cadle and Myers (2003: fig. 11). Two hatchlings or near-hatchlings of Dipsas oreas (MUSM 5530, 5532; total lengths 230–243 mm) were collected 13– 15 January 1989 near the site where the above eggs were discovered. It seems likely that these hatchlings derived from eggs laid the previous dry season, as indicated by the eggs discovered in June and their slightly larger size than the above series of hatchlings. Two other females of Dipsas oreas from Ecuador were gravid, and clutch sizes were estimated by palpation through the ventral body wall. ANSP 18117 (421 mm SVL) contained four eggs. BMNH 60.6.16.56 (566 mm SVL) contained about 8 eggs. No collection dates are available for these specimens. Aggregation Behavior in Dipsas oreas A peculiar aggregation of Dipsas oreas was discovered on 25 January 1989 at 1,800 m at the Río Zaña Study Site. At 10: 15 a.m., a field assistant found a ball of four snakes on the surface of the leaf litter adjacent to a small log (10–15 cm diameter, 2.5 m long) in a somewhat disturbed patch of cloud forest; all of these snakes were intertwined in a tight ball. Another individual was on the leaf litter surface about 20 cm from the ball. After alerting me to the discovery we returned to the site and found another snake hidden under the leaf litter (15–20 cm deep) where the initial ball was found. At the opposite end of the log (2.5 m from the ball), another individual was hidden under the leaf litter. A search of the immediate area (surface and under logs, litter, etc.) turned up no more snakes. The night before finding the aggregation included a light drizzle for several hours; on the morning of the encounter, the leaf litter was wet and the day was overcast. Temperature at 11:00 a.m. was 15 C. All snakes were quiescent and loosely or tightly coiled. The four snakes in the initial ball were completely and tightly intertwined, and consisted of an adult female and three adult males. The three individuals not associated directly with the ball were adult males. The female is ANSP
Dipsas oreas Complex in Ecuador and Peru • Cadle 121 31784 and the males are ANSP 31779–81, 31783, 31785–86 (497–551 mm SVL; see Figs. 18, 19). Aggregations of snakes occur for a variety of reasons, including hibernation (in temperate snakes), response to unfavorable temperature or humidity conditions, feeding, mating, and oviposition (e.g., Amr et al., 1997; Ford and Burghardt, 1993: 139–140; Gillingham, 1987: 197–198; Gregory et al., 1987: 370–373; Halpern, 1992; Noble and Clausen, 1936). The aggregation of Dipsas oreas just described seems unlikely to be due to unfavorable environmental conditions, hibernation, or feeding. The rainy season at the Río Zaña Study Sites commences in December or early January (as in 1989) and corresponds to the most active and environmentally favorable period for D. oreas and its gastropod prey. On the other hand, several observations suggest an association with reproductive behavior for the aggregation: all individuals are adults and all except one are males. This suggests that the aggregation may have been for mating, as does the morphology of the reproductive tracts described below. Unfortunately, the aggregation was disrupted before specific behaviors such as copulation or combat could be assessed. The fact that the aggregation was discovered during the day in a snake typically nocturnally active prompts questions that only additional serendipitous field observations could answer: Was this aggregation simply a continuation of courtship activity that had begun the previous night Alternatively, do males locate females during the day Gregory et al. (1987: 370) defined aggregations as ‘‘any concentration of snakes in a relatively small area such that the density of snakes in the aggregation contrasts sharply with that in the surrounding area.’’ Thus, aggregations need not involve continuous physical contact among individuals. In some snakes, males in mating aggregations are not necessarily continuously associated with the female, such as three of the males in the aggregation of Dipsas oreas just described. For example, Feio et al. (1999) observed six male Chironius flavolineatus courting a female, but only two were ever directly associated with the female, the other males being nearby on the ground or low bushes. Thus, even though three males were not directly associated with the tight cluster of D. oreas, it seems very unlikely that they were only coincidentally close by. That the aggregation of Dipsas oreas described was associated with reproduction is also suggested by the morphology of the reproductive tracts of the female and males. The female (ANSP 31784) has small ovarian eggs (2–4 mm) in her left ovary. These are somewhat yellowish, perhaps indicating the initiation of yolk supply to the ova. Her left oviduct is extremely dilated (3.5–4 mm broad and thrown into many pleats), which is the condition observed in female snakes of many species prior to ovulation and fertilization (review in Fox, 1977). The middle portion and cranial end of the left oviduct, corresponding to the tube portion of the oviduct (Guillette et al., 1989), are particularly dilated and pleated. The right oviduct is less dilated than the left and not at all pleated. Except for a single ovum similar in size to those in the left oviduct, eggs in the right ovary are smaller and paler than those in the left ovary. These observations suggest that the left reproductive tract of the female was gearing up for reproduction. Moreover, the size of the ova in the left ovary correspond to follicle size classes II (‘‘early growth’’) to III (‘‘hydration and initial vitellogenesis’’) recognized for D. catesbyi by Zug et al. (1979: 2–3), which corroborates the other indications of reproductive competence of this female. The males in the aggregation have swollen and highly convoluted vasa deferentia, which are characteristic of male snakes during the breeding season. Because so little is known of reproductive cycles and behavior of tropical snakes, it is worth exploring the details known for Dipsas oreas at the Río Zaña Study Site
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SYSTEMATICS OF SNAKES OF THE DIPSAS
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