Habit and habitat
Rana tigrina (the Indian bullfrog) is a cold-blooded (poikilothermic), amphibious vertebrate that can live both in water and on land, though it must return to water to breed and to keep its skin moist. It is commonly found in ponds, ditches, marshes, paddy fields and other damp places near fresh water across the Indian subcontinent, including Nepal's Tarai and mid hills. Frogs are mostly nocturnal, feeding at night on insects and other small invertebrates, and hiding in mud or vegetation by day. In cold winter months frogs undergo hibernation (burying themselves in mud, with metabolism greatly slowed), and in the hottest, driest part of summer some species undergo aestivation (summer dormancy) to avoid desiccation. Frogs also show pronounced seasonal breeding, with most mating and spawning taking place during the rainy season.
External features
- Body regions: the frog's body is divisible into just a head and a trunk; there is no distinct neck and adult frogs have no tail (the tail present in the larval tadpole is lost at metamorphosis).
- Skin: smooth, moist, and richly supplied with mucous glands that keep it slippery and permeable; the skin is a major respiratory surface (cutaneous respiration) and also carries pigment cells that allow limited colour change for camouflage.
- Head: bears a wide mouth, a pair of external nostrils (nares) near the tip of the snout used for breathing air, a pair of large bulging eyes each protected by upper and lower eyelids and a transparent nictitating membrane that can be drawn across the eye, and a pair of circular tympanic membranes (eardrums) just behind the eyes for hearing.
- Limbs: two pairs of limbs are present. The forelimbs are short with four unwebbed digits, used mainly to support the front of the body and cushion landing after a jump. The hindlimbs are much longer and more muscular, ending in five elongated, webbed digits, adapted for powerful jumping on land and swimming in water.
- Sexual dimorphism: males are usually smaller than females and develop a nuptial pad (a rough, dark, thickened pad) on the first digit of the forelimb during the breeding season, used to grip the female during mating; males also possess a vocal sac, an expandable skin pouch near the throat used to amplify their mating call.
Digestive system
Alimentary canal
The alimentary canal is a continuous tube running from the mouth to the cloaca:
Mouth -> buccal cavity -> pharynx -> oesophagus -> stomach -> small intestine (duodenum and ileum) -> large intestine (rectum) -> cloaca
- The mouth opens into a wide buccal cavity, the space between the upper and lower halves of the head.
- The upper jaw bears small, backward-pointing maxillary teeth used only to hold prey (not for chewing), along with a pair of vomerine teeth on the roof of the mouth, and paired internal nares (openings connecting to the nostrils).
- The floor of the buccal cavity bears a long, fleshy, bilobed (bifid) tongue attached at its front end, which can be flipped rapidly out of the mouth to capture insect prey; it also bears taste buds and mucous glands that keep it sticky and moist. The floor also shows the glottis (the opening to the windpipe) and, in males, openings to the vocal sacs.
- The buccal cavity leads to a short pharynx, then a short oesophagus, then the stomach, where digestion of proteins begins.
- The stomach leads into the small intestine, divided into the duodenum (which receives secretions from the liver and pancreas) and the longer, coiled ileum, whose internally folded lining increases the surface area for absorption of digested nutrients.
- The small intestine opens into a short, wide rectum (large intestine), which stores undigested waste before it is expelled through the cloaca, a common chamber that also receives the ducts of the excretory and reproductive systems.