Diets, foreging and predation herpetology

DIETS, FORAGING, AND PREDATION

Chapter 15: Diets of A and R

  • Associated with:

    • Morphological factors

    • Physiological factors

    • Behavioral factors

  • These facilitate:

    • Location of food

    • Identification of food

    • Capture of food

    • Ingestion of food

    • Digestion of food

Diets

  • Most herpetofauna (herps) are carnivores.

    • Carnivorous reptiles assimilate approximately 90% of energy from vertebrate prey.

    • Characterized by short digestive tracts.

    • Herbivorous reptiles assimilate 30-60% of energy from plants.

    • Characterized by long digestive tracts.

Carnivory

  • Categorized by size of prey relative to predator:

    • In general, smaller species consume invertebrates while larger species consume vertebrates.

    • Large salamanders and frogs may occasionally consume small vertebrates.

    • Juvenile crocodiles and snakes often start their diet with invertebrates.

    • Size of feeding structures (e.g., mouth size) typically correlates with prey size (e.g., adult frogs swallow food whole, necessitating larger mouths).

    • Anurans (frogs and toads) that consume larger prey display broad heads and wide mouths (e.g., bullfrogs), while those feeding on smaller prey have smaller heads and narrower mouths.

    • Most herps exhibit generalist feeding practices, particularly in invertebrate predation.

    • Salamanders preferentially consume soft-bodied invertebrates but may also eat more hard-bodied invertebrates during dry periods to avoid dehydration.

Specialization in Prey Use

  • Specialization in prey is relatively uncommon among herps:

    • May exhibit morphological, physiological, and behavioral adaptations for feeding.

    • Example: Phrynosoma (horny devil lizards) primarily eat ants and have distinct adaptations:

    • Spiny, tank-like bodies for protection

    • Peg-like teeth to efficiently capture and consume ants

    • Cryptic coloration to avoid predation

    • Limited locomotion leading to adaptation for remaining still and out of sight, hence the need for spiny protection.

  • Further examples of specialists:

    • Snake-shaped lizards focus on small invertebrates like termites and ants, possessing less kinetic skulls than snakes.

    • The majority of marine snakes are specialists divided into five primary categories:

    • Eels

    • Gobies

    • Burrowing gobies

    • Catfish

    • Fish eggs

    • Turtle-headed sea snakes exemplify egg specialization, lacking teeth except for medial fangs designed for scraping eggs off of substrates.

    • Dendrobatid tadpoles showcase oophagy by consuming unfertilized eggs.

More Specialists in Carnivory

  • Sea turtles and sea snakes exhibit tendencies towards specialization:

    • Hawksbill turtles predominantly consume sponges.

    • Leatherback turtles primarily feed on jellyfish, equipped with backward-pointing spines in their mouths and throats to assist in capturing slippery prey.

  • Considerations for mollusk-eaters:

    • Tomierella possess a strongly braced upper jaw that aids in pulling snails off attachments and consumes them whole.

    • The genus Dipsas can efficiently remove snails from their shells.

    • Some female Graptemys (box turtles) display larger sizes than males and possess strong jaws suited for crushing mollusks, while males tend to feed on softer-bodied organisms.

  • Carrion-eating practices, while uncommon, include various species:

    • Certain pit vipers

    • Some piscivorous snakes (those that eat fish)

    • Komodo dragons

    • Aquatic turtles

    • Blue-tongued skinks

    • Shingleback skinks

    • Tadpoles also demonstrate carrion consumption.

Cannibalism

  • Cannibalism is widespread among amphibians, particularly common in larvae as a survival mechanism under resource-limited conditions.

    • Particularly prevalent among tiger salamanders and spadefoot frogs:

    • Some larvae develop enlarged jaw muscles and exhibit aggressive foraging behaviors.

    • Triggers for cannibalistic behavior vary:

      • For tiger salamanders, crowding serves as a significant trigger.

      • For Spea frogs, ingestion of freshwater shrimp or other tadpoles can initiate predatory behavior.

    • Benefits of cannibalism include obtaining nutrition and reducing competition.

    • Costs of this practice include:

    • Detriment to genetic fitness due to consuming close relatives.

    • Increased risk of disease and parasite transmission.

    • Cannibalism is relatively rare in reptiles, with tuataras notably eating hatchlings.

Herbivory

  • Specialized herbivores include:

    • Anuran tadpoles

    • Tortoises

    • Green sea turtles

    • Various members of iguanidae

    • Some agamids and skinks

  • Such reptiles possess specific adaptations:

    • Symbiotic gut flora for fermentation processes.

    • High bite force relative to body size.

    • Robust, blade-like teeth.

    • Enlarged colon to aid in digestion, achieving efficiency levels comparable to mammals utilizing hind-gut fermentation.

    • Young green iguanas acquire gut flora through consumption of adult feces within a week after hatching.

  • Specialized herbivorous lizards generally tend to be larger and inhabit warmer climates, adapting to their environment to process large volumes of plant material effectively.

Omnivory and Herbivory

  • Lizards and turtles often display omnivorous behaviors instead of strict herbivory:

    • Statistic indication: approximately 12% of lizard species are omnivorous while only around 1% are categorized as herbivorous.

    • Omnivorous lizards share various adaptations common to herbivorous species.

  • Note: Plant material typically exhibits lower nutritional quality compared to animal material, and reptiles usually do not engage in chewing their food.

  • Most omnivorous species tend to consume softer plant parts such as fruits, flowers, and young leaves, demonstrating opportunistic feeding behaviors.

Nectivory

  • Observed in certain lizard species, particularly on specific islands.

    • Nectivory may play a significant role in pollination where there is a scarcity of nectarivorous birds.

    • Example: In the Canary Islands, lizards and a warbler species serve as the only pollinators for a specific species of foxglove, with lizards being more efficient due to their ability to generate higher seed sets, though warblers frequent the flowers more often.

    • Day geckos also represent key pollinators for two plant species on Mauritius, specifically adapted to nectar characteristics (colored rather than clear) that enhance their pollination efficiency.

Developmental Variation in Diet

  • Generally, as reptiles grow larger, they tend to consume larger and more variably-sized prey (e.g., Nerodia erythrogaster).

  • Some reptiles transition from a predominantly carnivorous diet to an omnivorous diet as they mature (e.g., Trachemys scripta).

Sexual Variation in Diet

  • Feeding habits can vary between sexes in dimorphic species.

    • Example: Female file snakes tend to exhibit larger sizes and consume larger fish species than their male counterparts.

Innate and Learned Responses to Prey

  • Innate responses refer to genetically-based behaviors:

    • Amphibians engage in foraging by seeking elongate prey that is in continuous movement, while ambush predators tend to focus on compact prey exhibiting jerky movements.

    • An example includes a Jamaican anole species that exhibits different prey preferences as hatchlings, with learning fostering greater food diversity in adults.

Learned Behaviors

  • Learned feeding behaviors can result from observing other individuals (even in non-social species), such as:

    • Red-footed tortoises that learned to navigate a fence to reach a strawberry patch through watching a trained tortoise.

    • Novel experiences inform juvenile/adult behavior concerning palatability and danger:

    • For instance, hatchling freshwater crocodiles may learn about the edibility of Cane Toad toadlets through direct consumption, continuing to avoid them as adults, which is critical since adult toadlets are toxic.

Sensory Modalities - Vomerolfaction

  • Vomerolfaction is an essential sensory mode for squamates.

    • This allows for the detection of non-volatile molecules using the tongue:

    • The ventral surface of the tongue delivers odor molecules to the Jacobson’s organ.

    • The speed of tongue flicking serves as an assay indicating stimulation of chemical receptors that trigger feeding responses, categorized as innate behavior.

Other Sensory Modes

  • Additional sensory modalities include:

    • Infrared Radiation: Detection of thermal cues, especially in pit vipers, boas, and pythons.

    • Lateral Line System: Comprises mechanoreceptors to detect changes in water currents and electroreceptors to sense weak, low-frequency electric stimuli.

    • Tympanic Membranes: Utilize airborne and substrate-born vibrations, highlighted in socially vocal frogs and crocodilians.

Modes of Foraging

  • Ambush (Sit-and-Wait) Foraging:

    • Utilizes vision; more inclined to target prey with chemical defenses (e.g., ants).

    • Adaptive physical characteristics include stout bodies, short tails, and relatively short limbs.

    • Females may exhibit high relative body mass of eggs or embryos.

    • Certain species use body parts as lures (e.g., baby copperheads draw attention with tail tips, alligator snapping turtles utilize worm-like tongues, and Ceratophrys frogs use toes as lures).

    • Common use of cryptic coloration.

    • Capture fewer prey when ambushing but require less energy.

  • Active Foraging:

    • Characterized by frequent movement and exploration for hidden prey.

    • Employs vomerolfaction; more likely to consume prey that lack chemical defenses (e.g., termites).

    • Physical adaptations include slim bodies, long tails, and relatively long legs.

    • Active foraging constrains the mass of eggs or embryos that females may carry due to energy allocations.

    • Oftentimes characterized by conspicuous patterns.

    • Capture more prey than ambush foragers but consume more energy.

Predator Interactions

  • Ambush predators can be susceptible to active foragers; by contrast, active foragers may also be at risk from ambush predators.

  • Various predators include invertebrates, fish, other herpetofauna, birds, and mammals.

    • Vulnerability varies across life stages.

Defensive Behavior in Amphibians

  • Defensive Strategies Include:

    • Cryptic coloration

    • Cryptic morphology

    • Disruptive patterns

    • Polymorphism

Chemical Defenses

  • Chemical defenses may include:

    • Aposematic (warning) coloration to signal toxicity.

    • Batesian mimicry, e.g., dangerous species like Phyllobates lugubris (poisonous) and Eleutherodactylus gaigeae (harmless mimic).

    • Müllerian mimicry and examples of object mimicry or crypsis for survival.

Additional Defensive Behaviors

  • General Strategies Include:

    • Hiding in holes or burrows

    • Parachuting for escape

    • Evertancing ribs as a last resort for defense (e.g., the strategy of Scaphiopus couchii)

    • Additional possibilities include curling into a ball or tail autotomy (breaking off the tail).

  • Other forms of defense may involve anatomical features such as osteoderms, reinforced skull structures, or simply possessing a pointed appearance (thus intimidating predators).