Introduction
Water snakes, belonging to the subfamily Natricinae, encompass around 200 species within 38 genera of the Colubridae family. These semiaquatic snakes are primarily found in the Northern Hemisphere and exhibit diverse feeding habits, primarily revolving around aquatic prey. This article delves into the diet and feeding behaviors of various water snake species, highlighting their adaptations and ecological roles.
General Characteristics of Water Snakes
Water snakes are typically characterized by stout bodies, strongly keeled scales (giving them a rough feel), and triangular heads. They are commonly found in or near water, only leaving these environments to bask in the sun or breed. While New World species are viviparous (giving birth to live young), most Old World species are oviparous (laying eggs). When handled, water snakes often defecate or excrete a foul-smelling substance from their anal scent glands. Despite being nonvenomous, they can be ill-tempered and bite freely.
Dietary Preferences
The principal diet of water snakes consists of fish and amphibians. However, their exact prey varies depending on species, location, and availability.
Fish
Fish form a significant part of the diet for many water snake species. Northern water snakes (Nerodia sipedon) are known to primarily feed on small fish. They have strong jaws and sharp teeth that enable them to grasp slippery fish effectively. Game fish are generally too agile for water snakes to catch, unless the fish is injured or diseased. Northern water snakes will also readily consume discarded fish carcasses left by anglers.
Amphibians
Amphibians, including frogs, tadpoles, toads, and salamanders, are also common prey items for water snakes. Plainbelly water snakes (Nerodia erythrogaster) consume amphibians. The common grass snake (Natrix natrix) in Europe and western Asia preys mainly upon frogs and toads, but will also eat salamanders, tadpoles, or fish.
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Other Prey
While fish and amphibians constitute the primary diet, some water snakes also consume other prey items depending on availability. These can include:
- Small mammals
- Birds
- Crayfish
- Insects
Feeding Strategies
Water snakes employ various feeding strategies depending on their species and the type of prey they target.
Ambush Predation
Some water snakes are ambush predators, waiting for prey to come within striking distance. Salt marsh snakes (Nerodia clarkii) hide among marsh grasses to ambush fish and crustaceans. Rattlesnakes typically set up in ambush along a rodent trail, with a characteristic S-shape in its neck.
Active Foraging
Other water snakes are active foragers, actively searching for prey in their environment. Striped whipsnakes (Masticophis taeniatus) are active foragers that use their quickness and binocular vision to catch lizards.
Aquatic Hunting
Northern water snakes are excellent swimmers and can feed both above and below the surface of the water. They are highly adaptable and can thrive in a variety of wetland habitats.
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Regional Examples
North America
In North America, the genus Nerodia is the most abundant, comprising 11 species. The northern water snake (N. sipedon) is the most common, inhabiting the eastern half of the United States, southern Ontario, and southern Quebec. It primarily feeds on fish and is active during the day in the fall and spring, and nocturnal during the summer. Salt marsh snakes (N. clarkii) live in brackish water habitats of the southeastern United States, ambushing fish and crustaceans among the marsh grasses.
Eurasia
The genus Natrix in Eurasia includes the common grass snake (N. natrix), which inhabits Europe and western Asia. It is more terrestrial than other water snakes and preys mainly on frogs and toads.
Asia
The genus Sinonatrix, found throughout Southeast Asia, southern China, and parts of Indonesia, is more aquatic than Natrix and primarily feeds upon fish. The checkered keelback (Xenochrophis piscator), a common species in southern Asia, preys upon fish and mice. Opisthotropis, a genus of 20 species, lives beneath stones in mountain streams in China and Southeast Asia, feeding upon fish, adult frogs, tadpoles, and invertebrates.
Specific Species and Their Diets
- Plainbelly Water Snake (Nerodia erythrogaster): Diet includes amphibians.
- Northern Water Snake (Nerodia sipedon): Primarily eats fish and amphibians, but also consumes small mammals, birds, crayfish, and insects.
- Salt Marsh Snake (Nerodia clarkii): Feeds on fish and crustaceans in brackish water habitats.
- Common Grass Snake (Natrix natrix): Preys mainly upon frogs and toads, but also eats salamanders, tadpoles, or fish.
- Checkered Keelback (Xenochrophis piscator): Preys upon fish and mice.
- Opisthotropis: Feeds upon fish, adult frogs, tadpoles, and invertebrates.
Ontogenetic Dietary Shifts
Some snakes, like the Great Basin Rattlesnake (Crotalus oreganus lutosus), undergo an ontogenetic change in food preferences as they mature. Young rattlesnakes feed primarily on lizards and small mice, while adults feed primarily on rodents like mice, woodrats, and chipmunks.
Factors Influencing Water Snake Diets
Several factors influence the diet of water snakes:
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- Habitat: The availability of prey species in the snake's habitat is a primary determinant of its diet. Water snakes in aquatic environments will primarily feed on fish and amphibians, while those in marshy areas may consume crustaceans and insects.
- Geographic Location: The feeding habits of some snake species vary widely geographically.
- Age and Size: Younger snakes may feed on smaller prey items, while adults can consume larger prey.
- Prey Availability: The abundance and availability of different prey species can influence the diet of water snakes.
Conservation and Human Interactions
Water snakes are often mistaken for venomous species like the cottonmouth water moccasin (Agkistrodon piscivorus), leading to their unnecessary killing. It is important to accurately identify water snakes and understand that they are nonvenomous and play a vital role in controlling populations of their prey. In Missouri, for example, the northern and midland watersnakes are often confused with the northern cottonmouth, resulting in unwarranted fear and persecution.
Water Snakes as Pets
Water snakes can make excellent pets for beginner hobbyists due to their manageable size and diurnal activity. They are known for their easygoing personalities, though they may occasionally mistake human hands as a food source. A single water snake should be housed in a spacious enclosure with a thermal gradient and proper humidity levels. Their diet in captivity typically consists of appropriately sized frozen rodents, thawed and warmed.