Food resources vary in terms of digestibility and constraints in food processing are an essential factor driving the evolution of adaptations to cope with them, for example, a complex morphology of gastric tract, symbiosis with microorganisms, enzymatic specialization (McNab 2002). Pellet egestion is another important adaptation that enables to remove indigestible food particles and is observed in several vertebrate taxa. Pellets are most commonly reported in sauropsids, particularly birds, but published records indicate pellet formation also in the 2nd groups of sauropsids, that is, nonavian reptiles, including lizards and snakes (Myhrvold 2012). In snakes, pellet formation and egestion may, however, seem counterintuitive due to extreme digestive efficiency of snakes that allows them to process almost all prey tissues (Skoczylas 1970). In fact, pellets in snakes are reported mostly in fossil material (Myhrvold 2012), whereas in modern snakes pellets seem to be documented exclusively in ovivorous species (e.g., Dasypeltis spp. and Elachistodon spp.; Gans 1952). In carnivorous snakes, pellet egestion was only invoked by Myhrvold (2012) as a reference to Gans (1952). Surprisingly, the cited study of Gans (1952) does not mention nor document physiological pellet formation and egestion by carnivorous, but only ovivorous snakes. Therefore, observations of pellets in modern carnivorous snakes appear entirely lacking. Such observations could, however, provide substantial contribution to the comparative digestive physiology of vertebrates. Here, we document probably the first empirical evidence for the pellet formation and egestion in modern carnivorous snakes. Observations were performed on captive groups of banded water snakes Nerodia fasciata (N1⁄430) obtained from private collection as juveniles (<6 months). All snakes were kept in standardized conditions, solitarily, with water container, photoperiod set at 12:12 h light:dark and ambient temperature on the level of 26 C–27 C (temperature preferred by N. fasciata; S. B. personal observation and Hopkins et al. 2004). Snakes were provided with dead hairless rodent pups once per week. Food was changed from pups to subadult rodents covered with fur as snakes were growing to size making them enable to ingest larger food particles. In one female (body mass: 23.4 g), 7 days after ingestion of a vole, prior next feeding, a compact mass of distinguishable shape was palpated. The location of the object in the mid-body at approximately half of the snout– vent length indicated its location in the stomach, which was further confirmed by the X-ray examination (performed in MedicaVet Veterinary Clinic, Cracow, Poland; Figure 1A). One day later the specimen was observed to eject orally a structure resembling avian pellet, that is, containing bones and fur, being dry and lacking any soft tissues (Figure 1B). Similar behavior was observed afterward in 7 other specimens. In total, pellet egestion was observed in 8 among 681 feeding events ( 1%) within 6 months. These feeding events concerned 30 individuals of N. fasciata, among which 8 (3 males and 5 females; 26% of snakes) have egested pellets. In all cases, pellets occurred after 1st or 2nd feeding with vole covered with fur and were observed only once per individual. Each time pellets were egested after 5th-day post-feeding. Simultaneously, a group of fieldcaught European grass snakes Natrix natrix (N1⁄451) was maintained under similar captive conditions as banded water snakes. All specimens were adults and also fed with rodents, but due to larger size not with pups but subadult or adult voles. Among more than 1,200 feeding events over 6 months only once a structure resembling a pellet was found, however, it was not observed whether snake egested it orally or removed with feces. We have controlled snakes’ health, condition, and the environment, because egestion of food particles is commonly reported in association with digestive disorders, stress, and suboptimal temperature (Divers and Mader 2005). However, signs of such disorders, for example, gross swelling of the abdomen or diarrhea, were not observed. Snake that produced pellets did not express any symptoms of pain or stress, for example, ceased food intake (James et al. 2017) and fecal samples did not exhibit any signs of parasitic infestation. Furthermore, egestion of pellets seemed not to result from regurgitation or vomiting, both outcomes of incomplete digestion (Divers & Mader 2005). Vomited or regurgitated particles are nonor partially digested, usually floppy, wet, and still containing soft tissues contrary to those here reported (see above and

中文翻译：

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现代食肉蛇的颗粒消化

食物资源在消化率方面各不相同，食物加工中的限制是推动适应进化以应对它们的重要因素，例如，复杂的胃道形态、与微生物的共生、酶促专业化 (McNab 2002)。颗粒消化是另一种重要的适应性，可以去除难以消化的食物颗粒，并在几种脊椎动物类群中观察到。颗粒最常见于蜥脚类动物，尤其是鸟类，但已发表的记录表明，第二类蜥脚类动物也有颗粒形成，即非鸟类爬行动物，包括蜥蜴和蛇（Myhrvold 2012）。然而，在蛇中，由于蛇的极高消化效率使它们能够处理几乎所有猎物组织，因此颗粒的形成和排出似乎有悖常理（Skoczylas 1970）。实际上，蛇中的颗粒主要在化石材料中报道（Myhrvold 2012），而在现代蛇中，颗粒似乎仅在卵形物种中被记录（例如，Dasypeltis spp. 和 Elachistodon spp.；Gans 1952）。在肉食性蛇中，只有 Myhrvold (2012) 引用了颗粒消化作为对 Gans (1952) 的参考。令人惊讶的是，Gans (1952) 引用的研究既没有提到也没有记录肉食性蛇的生理颗粒形成和消化，而只提到了卵形蛇。因此，对现代食肉蛇的颗粒的观察似乎完全缺乏。然而，这些观察结果可以为脊椎动物的比较消化生理学提供实质性的贡献。在这里，我们可能记录了现代食肉蛇中颗粒形成和排出的第一个经验证据。对从私人收藏中获得的幼年（<6 个月）带状水蛇 Nerodia fasciata (N1⁄430) 的圈养群体进行了观察。所有蛇都被单独饲养在标准化条件下，有水容器，光周期设置为 12:12 h 光：暗和环境温度在 26 C-27 C 的水平（N. fasciata 首选温度；SB 个人观察和 Hopkins et 2004 年）。每周一次为蛇提供死亡的无毛啮齿动物幼崽。随着蛇的体型越来越大，它们能够摄取更大的食物颗粒，食物从幼崽变成了覆盖着毛皮的亚成年啮齿动物。在一只雌性（体重：23.4 克）中，在摄入田鼠后 7 天，在下次进食之前，触诊到一个形状可区分的致密块。物体在身体中部大约一半的鼻子 - 排气口长度的位置表明它在胃中的位置，X 射线检查进一步证实了这一点（在 MedicaVet 兽医诊所进行，克拉科夫，波兰；图 1A） . 一天后，该标本被观察到口腔喷射出类似于鸟类颗粒的结构，即含有骨骼和毛皮，干燥且没有任何软组织（图 1B）。之后在其他 7 个样本中也观察到了类似的行为。在 6 个月内的 681 次喂养事件中，总共有 8 次观察到颗粒排出（1%）。这些摄食事件涉及 30 只 N. fasciata，其中 8 只（3 雄性和 5 雌性；26% 的蛇）吞食了颗粒。在所有情况下，在第一次或第二次喂食后，田鼠身上都覆盖着毛皮，并且每只个体仅观察到一次。每次在喂食后第 5 天后排出颗粒。同时，一群野外捕获的欧洲草蛇 Natrix natrix (N1⁄451) 被饲养在与带状水蛇相似的圈养条件下。所有的标本都是成年的，也用啮齿动物喂养，但由于体型较大，不是幼崽而是亚成年或成年田鼠。在超过 6 个月的 1,200 多次喂食事件中，只有一次发现类似颗粒的结构，然而，没有观察到蛇是通过口服吞食还是随粪便排出。我们已经控制了蛇的健康、状况和环境，因为食物颗粒的排出通常与消化功能紊乱、压力和温度不佳有关（Divers and Mader 2005）。然而，没有观察到此类疾病的迹象，例如腹部严重肿胀或腹泻。产生颗粒的蛇没有表现出任何疼痛或压力症状，例如，停止进食（James et al. 2017）并且粪便样本没有表现出任何寄生虫感染的迹象。此外，颗粒的排出似乎不是由反流或呕吐引起的，这两种结果都是消化不完全的结果（Divers & Mader 2005）。呕吐或反流的颗粒未被消化或部分消化，通常松软、潮湿，并且仍含有与此处报道的相反的软组织（见上文和