Physical characteristics

A crocodile may be thought of as an elegant solution to the problem of catching prey, surviving unpredictable environments, conserving limited energy, and reproducing successfully. In appearance, crocodiles superficially resemble lizards, having scales, a long tail, and four limbs. But appearances can be deceptive, and a closer look reveals that crocodilians are unique.

All 23 species are broadly similar in appearance, varying mainly in size, scale patterns, color, and skull morphology. The smallest species is Cuvier's dwarf caiman (Paleosuchus palpebrosus); adult males rarely exceed 5 ft (1.6 m) in length and females 4 ft (1.2 m). Within the same family, the black caiman (Melanosuchus niger) and American alligator (Alligator mississippiensis) vie for largest size, yet rarely exceed 14 ft (4.3 m). Crocodylidae range from the diminutive dwarf crocodile (Osteolaemus tetraspis) at 6 ft (1.8 m) to the massive estuarine crocodile (Crocodylusporosus) that can exceed 16 ft (5 m). The

Crocodylia

Gavialoidea

I-Gavialis gangeticus

Alligatoroidea

— Leidyosuchus X

— Diplocynodon X

— Deinosuchus X Alligatoridae -Alligatorinae

Alligator

A. sinensis A. mississippiensis Caimaninae

Paleosuchus

-P. palpebrosus

-P. trigonatus

Caiman

C. crocodilus C. yacare C. latirostris Melanosuchus niger r; t:

Crocodyloidea

AsaitosuchusX Crocodylidae

Tomistominae

I-Tomistoma schlegelii

-Osteolaemus tetraspis

Crocodylus

C. cataphractus C. palustris -C. siamensis

-C. porosus -C. johnstoni

—I-C. novaeguineae

'-C. mindorensis

- C. niloticus

Phylogenetic tree of modern crocodilians. (Illustration by GGS. Courtesy of Gale.)

sole gavialid, the Indian gharial (Gavialis gangeticus) can also reach 16 ft (4.9 m). Females are always the smaller sex, and this is most apparent in estuarine crocodiles, where females of 10 ft (3 m) are considered to be very large.

As the largest living reptiles, males over 16 ft (4.9 m) may tip the scales at 1,100 lb (500 kg), but these are lightweights compared to rare individuals that exceed 18 ft (5.5 m) and 2,200 lb (1,000 kg). Although several species, including estu-

arine crocodiles and Indian gharials, are capable of attaining such sizes, evidence of these giants is scarce. The largest crocodile reliably measured and published in the literature, an es-tuarine crocodile from Papua New Guinea, was 20.7 ft (6.3 m) long. While unlikely to be the maximum possible size for this species, stories of even larger animals are difficult to verify. One fact is certain—crocodiles over 20 ft (6 m) are exceptionally rare.

Crocodilians undergo a dramatic increase in size from hatchling to adult. Over its lifetime, an estuarine crocodile may grow from a 12-in, 2.8-oz (30-cm, 80-g) hatchling to a 20-ft, 2,650-lb (600-cm, 1,200-kg) adult. A 20-fold increase in length and 15,000-fold increase in weight is quite a feat in the animal kingdom. Imagine, then, how this compares with the extinct Sarcosuchus, which reached 35 ft (10.7 m) and over 19,800 lb (9,000 kg)! Growth is most rapid when young, yet scientists are unsure whether adults reach a maximum size or continue to grow slowly until they die. The enormous sizes attained by extinct species such as Sarcosuchus and Deinosuchus may have been possible by maintaining those fast juvenile growth rates throughout a greater percentage of their lives.

Crocodilians are covered in a thick, leathery skin broken into various sizes and shapes of scales in particular areas. Scales on the back are large and rectangular, lying in parallel rows from shoulders to pelvis and continuing onto the tail. These dorsal "scutes" each contain a bony plate called an os-teoderm ("skin bone") just below the surface. A tough covering of beta-keratin helps minimize water loss, although the more flexible alpha-keratin is found between the scales. Os-teoderms not only offer protection, they are infused with blood vessels and function as solar panels, transporting heat from the surface to the body core during basking. Adjacent osteoderms are closely integrated like the beams of a bridge, providing support for the spinal column. Large nuchal plates protect the nape. Scales on the flanks and limbs are generally smaller, rounder, and softer to allow bending. Those on the belly are even, rectangular, and smooth to reduce friction sliding over the ground. Small osteoderms are found in the belly scales of most species. Thick, rectangular scales are present on the tail, with sharp, upward-pointing scutes providing extra surface area as the tail sweeps through water. Scales on the head are small, irregular in shape and thin, housing blood vessels and sensory nerves. Each species has a unique pattern of scales and osteoderms.

Deceptively, a layer of mud and dust often covers dry, basking adults, suggesting a bland coloration. However, most species exhibit distinctive color patterns, which enhance camouflage and aid communication. Dorsal color is typically tanned yellow to dark brown, overlaid with characteristic dark bands, spots, or speckles. Juveniles of all species are more vivid, their bright colors fading in adulthood. Ventral scales are creamy white with varying degrees of black pigmentation, except for the almost black bellies of dwarf caimans and dwarf crocodiles. Color mutations where pigment is usually absent are rare, genetic anomalies. Leucistic and albino crocodiles are as tempting to predators as their "white chocolate" appearance suggests, but they are popular tourist attractions in captivity, where they must be shielded from excess sunlight. Both short- and long-term changes in skin color have been

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