Gharial Conservation Alliance

Gharial biology



Form and Function

The Gharial is one of the largest crocodiles, ranked closely behind the salt-water crocodile (Crocodylus porosus).

Males can grow 5 – 6 meters. Females can grow 3.5 – 4.5 meters. Adults weigh between 159 and 181 kg.

Gharials are the only crocodilian that are sexually dimorphic, meaning the adult males and females look distinctly different from each other.  Males are larger than females and only mature males have a bulbous growth at the end of their snouts.  Adult females are smaller than adult males and they do not have the bulbous growth on their snouts.

 




The Ghara

The gharial gets its name from the Hindi word “ghara” which means pot. The “ghara” is a bulbous blob begins growing on the nostrils at the tip of the snout as soon as the males reach maturity.  This strange form, called the ghara, turns the hissing noise commonly made by crocodilians into a buzzing noise, as air is forced through the nasal passage. It may also act as a sound amplifier, carrying the produced buzzing sound for great distances across water. The ghara may also help females identify mature males.

The ghara, a help and hindrance

The ghara is thought to play an important role in gharial reproduction by identifying mature males to females and as an instrument in courtship auditory communication. However this odd-looking anatomical accessory does not come without a cost. In building the ghara, the structure of the normal crocodilian nostril is completely disrupted such that it cannot function properly to close the airway when the animal submerges. Thus the ghara essentially causes a problem. However this problem is solved by special erectile tissue masses in the snout of that gharial that can swell to close the nostril. (L.Witmer. Per.comm.).

 




Skin and scales

Gharials have thick skin covered with smooth epidermal scales that do not overlap. They have bony-plates called osteoderms on their dorsal (top) side which serves as armor and play a role in thermoregulation. Unlike some other species of crocodilians they have none on their belly.

(Left to Right). Belly scales (photo: Saravana Kumar), Back scales (photo: Saravana Kumar), Side scales (photo: Laurel Converse), Intergumentary Sensory Organs (photo: Saravana Kumar),

Gharials have Integumentary Sensory Organs (ISOs) on the scales on the lower half of their sides, belly, and tail. The function of ISO’s in not fully understood, but they may act as salinity receptors.

Gharials have similar features on the scales along the jaw, called Dermal Pressure Receptors (DPRs) which detect vibrations, aiding the gharials in detecting prey.

Dentition

Gharial Skull (photo: Vince Smith).

Different species of crocodiles have different numbers and shapes of teeth. Gharials have sharp, slender teeth of the same shape along the length of their snout.

Crocodilian and mammals have what are called thecodont teeth, which are teeth rooted and set in sockets.

Other reptiles have rootless teeth, attached to the jaw, rather than set in sockets.

Gharials have

  1. 5 pre-maxillary (upper front)
  2. 23-24 maxillary (upper jaw)
  3. 25-26 mandibular (lower jaw)

 

Diet

ADULTS

Fish (primary food source)
Small crustaceans (mud crabs)

JUVENILES

Insects, tadpoles, small fish, frogs

Feeding

Gharials’ jaws are too thin and delicate to grab larger prey. Gharials sweep their head side to side to detect prey, and sensory receptors called ISOs along their head may aid in detecting movement of prey. The many needle-like teeth along their long jaws are efficient at keeping a firm hold on slippery fish, caught in several ways:

•          Lying in wait for fish to swim by, and then catching the fish by quickly whipping their head sideways and grabbing it in their jaws.

•          Herding fish with their body against the shore.

•          Stunning fish using their underwater jaw clap.

Gharials do not chew their prey– they swallow food whole. In order to swallow, Gharials must hold their heads out of the water. A valve at the back of their throat, called the palatal valve, keeps water from entering their lungs while they are underwater. In order to swallow, this valve must be opened. Therefore crocodiles must have their heads out of water or they will drown when the valve opens.

Gharial helpful to fishermen?

There is some evidence to suggest that, like the NIle Crocodile, gharials prefer to eat slower-moving, larger predatory fish, such as catfish. This would mean they are helpful to fisheries, since they are eating the predators that normally eat commercially valuable fish. Further study into the feeding ecology of gharials needs to be done to determine if this is indeed the case. If true, it could be important for improving gharials’ image with local fishermen.

 




Behavior

Despite their intimidating looks, gharials are shy and generally passive animals. They do not kill and eat humans.

Thermoregulation Behavior

Gharials, like all crocodilians, are “cold-blooded” or ectothermic, meaning that their body temperature is not internally regulated; they must depend on the external environment to regulate it.

Gharials lie in the sun to warm up (basking) or they go into the water or shade to cool down. They  must have sunny basking spots where they can lie undisturbed.

Social Behavior

Gharials are generally not aggressive towards each other except during the breeding and nesting season, when males are territorial and females defend their nests. Gharials will often bask in groups, though individuals may have favorite basking spots they will defend from other gharials or mugger crocodiles. There is sometimes aggression between mugger crocodiles and gharials where their ranges overlap.

Juveniles stay together in groups near the female for between a few weeks to a few months after hatching.




Movement

Gharials have short weak legs and are very clumsy on land and usually only come on to land to lay their eggs or to bask. They are not able to raise their bodies off the ground and “high walk” the way most other species of crocodiles can. Instead, they drag their bodies across the ground.

Juveniles can raise the front of their bodies up while walking and during threat displays.

Gharials’ bodies are well adapted for swimming, with powerful tails, streamlined bodies, and highly webbed back feet. They swim with their limbs tucked in against their body, using their strong tail to propel them forward.

Communication

The bulbous growth on the snout, the ghara, partially covers the nostrils, turning hissing vocalization into a buzzin noise when air is forced through this passage.

It may also act as a vocal resonator, carrying the buzzing noise for long distances.

Gharial males often hiss and buzz while advertising territory, especially during breeding season.

When young gharials are ready to hatch out of the egg they make a grunting noise, alerting the mother.

Both males and females will make jawclaps during the courtship and mating season. Sometimes the jawclaps may be performed above the surface. Gharials are also able to jawclap underwater; biologists have not been able to discover how they do this.

When distressed, for example when caught, juveniles and sometimes subadults will make a whine, groan, or “quack”-type vocalization. See www.crocodilian.com for more information and recordings of crocodilian vocalizations.




Life history

Reproduction

Age at maturity: 10 yrs (females), 13 years (males)

Courtship: Begins December

Mating: January and February

Nesting: March and April

Nest type: Holes dug in sandy banks

Egg Clutch Size: 40-80 eggs

Incubation time: 70 days (average)

Females mature at around 10 years old and are usually about 3 m long.

Males mature at around 13 years old, which is when the ghara begins to grow on the snout.

The ghara may play an important role in reproduction, for mature males to be identified by females and by acting as a resonator for courtship and territorial male vocalization.

Courtship/Mating

Gharials are most active during the courtship and mating season. Males will aggressively defend their territory and advertise for mates by making hissing and buzzing noises as they patrol their territory. One research has described this noise as a “buzz-snort”. Males may have a harem of females within  a territory that they defend from other males.

During courtship, males and females will follow prospective mates around until a suitable mate is decided upon. Courtship also involves head and snout rubbing and mounting by both males and females.

Males may have a harem of females, and defend their territory and females from rival males.

Courtship behaviour involves males and females following potential mates around, mounting, rubbing of snouts, and vocalizing by the males.

Female communicate their readiness to mate by raising their snout upwards.

Once mates are chosen gharials may submerge for up to 30 minutes during mating.

Nesting

Nesting occurs during March and April.

Females dig trial nest holes on sandy banks. They may make one every night until the night they actually lay the eggs in one of these nest holes.

Females dig pitcher-shaped nest holes, about 50 cm(30 in) deep with their hind feet, where they then lay their eggs.

Gharials lay the largest eggs of any crocodile, weighing 160 g on average.

After laying between 40 to 80 eggs, mother gharials cover the hole and remain close by to guard their nests against egg predators like pigs, jackals, monitor lizards, and mongooses.

Incubation and Sex-Determination

The nest hole is a natural self-heating incubator – the sand heats up from the sun and helps the embryos develop.

The sex of crocodilians is not determined by genes, it is determined by incubation temperature.

In most species studied to date incubation temperatures of 31 ºC or less and 33 ºC or greater produce females. Constant incubation temperature of 32 ºC produces males

The incubation temperature also affects the time it takes for eggs to develop. Higher temperatures cause faster development than lower temperatures. However the optimal temperature range for crocodilians is narrow (around 30-34ºC). Higher or lower temperatures can cause defects or death.

The Mystery of Temperature-Dependent Sex-Determination in Gharials

There is no definitive knowledge of the exact temperatures which produce male and female gharials. It is possible that gharials follow the same rules as most other species (32 ºC produces males, higher or lower temperatures produce females). However the Australian freshwater crocodile (Crocodylus johnstoni) has a different pattern of sex-determining temperatures, and like gharials they lay their eggs in holes in the sand. In freshwater crocodiles all constant temperatures tend to produce more females, but gradually increasing incubation temperature tends to produce more males. It has been suggested this may be due to the increasing temperatures eggs are subjected to within natural nests in the sand. Is the sex of gharials determined the same way as freshwater crocs in their sandy nests? More research needs to be done to solve this mystery.

(L – R) Gharial Nest [photo: Rom Whitaker]; Mother with Young [photo: Jeff Lang]; Juveniles on a sand bank [photo: Jeff Lang]

 

Hatching

Incubation time for gharials is about 70 days.

When the hatchling gharials are ready to emerge they call from inside the egg – a signal to their mother, who stands guard close by to dig them out.

At hatching gharials weigh 75-130 g (2.6 – 4.6 oz) and are 325-392 mm (12.8 – 15.4 in) long.

Because of their unique snout and teeth, gharials may not be able to help the babies hatch, as many other crocodile species do. Instead female gharials must wait for the young to hatch unaided and will lead the young to the water.

Female gharials will stay near their young and protect them for a period of several weeks to several months. During this time the hatchlings usually stay together in groups near the female.

Often the monsoon forces the separation of the mother from her young, as the rising waters often wash the young juveniles downriver away from the protection of their mother. This may be a significant source of mortality in young gharials.

In Katerniaghat Wildlife Sanctuary hatchlings have been observed sheltering from the swift monsoon currents in the grasses along the edge of the nesting banks.