The basic crocodilian breeding system is polygynous (one male, multiple females), although short-term monogamous pair bonding has been described in Nile crocodiles and possibly other species. Multiple paternity (several males, same female) has also been recorded in some social situations. Once sexual maturity is reached, dictated by size rather than age, reproductive activity follows an annual cycle. Environmental triggers such as changing temperature, rainfall, humidity, and day length trigger hormonal changes in each sex. In males, testosterone levels rise, testes increase in weight, and sperm production increases. In females, estradiol levels rise, triggering the liver to produce vitellogenin for yolk production in the ovaries, and follicle size increases prior to ovulation. Double clutches have been reported in mugger crocodiles (C. palustris) and Nile crocodiles, influenced by extended environmental conditions favorable to breeding.
As habitat and climate change geographically, the timing and duration of reproductive activities varies between species and even within a species' distribution. Courtship, mating, and nesting may occur over a period of several months (as in Nile and estuarine crocodiles), or may be concentrated into several weeks (as in American alligators). Johnstone's crocodiles, or Australian freshwater crocodiles (C. johnstonii), nest within a two to three week period, a phenomenon known as "pulse nesting."
Crocodilians exhibit two nesting strategies: hole nesting and mound nesting. In the former the female selects a soft substrate, such as sand or mulch, and excavates a chamber several inches deep using her hind legs. After laying her eggs, the chamber is concealed again. In mound-nesting species, the female first scrapes material, such as vegetation, soil, or mud, into a mound using her front and hind legs. She also rips up fresh vegetation with her jaws. The resulting mound can be over 3.3 ft (1 m) high and 6.6-9.8 ft (2-3 m) in diameter. Once the mound is complete, the female behaves like a hole nester: she excavates a chamber into the top of the mound with her hind legs, lays her eggs, conceals them, and compacts the nest using her hind legs and body. Only Crocodyl-idae and Gavialidae excavate hole nests, in those species that nest during the dry season or when little vegetative matter is available. Mound nests are built by Alligatoridae and some Crocodylidae, typically nesting during the wet season or in areas that inundate easily, as the additional height reduces the risk of eggs being drowned by floods. American and Cuban crocodiles have been reported to choose either hole or mound nests, depending on climate and habitat.
Nesting location may be determined by available materials, proximity of water, temperature, and even social factors.
Females of more territorial species choose solitary nesting sites, isolated visually or by distance from those of other females. Favorite sites may be reused each year, although not necessarily by the same female. More gregarious species may use communal sites. Although under the vigilant watch of several females, there are disadvantages to communal sites. Late nesters may inadvertently dig up older eggs, and predators have an easier time finding eggs where nests are concentrated. Frequency of nesting is also under social pressure. In the wild, between 10% and 80% of females of a given species may nest each year; the percentage determined by the amount of nesting habitat available, the territorial nature of the females, and species-specific differences.
As with porcupines, people are curious how crocodilians manage to mate without causing each other grief! In reality, it is a gentle affair (once the competition has been dispensed with, that is). To copulate successfully, males must court females to gain their consent. Males of some species, such as es-tuarine crocodiles, establish territories that contain a number of females, others, such as American alligators, display competitively to attract females. Courtship may be elaborate or subtle, involving mutual signaling on a visual, olfactory, auditory, and tactile level. Alligators combine rumbling bellows with infrasonic vibrations, the water dancing across their backs proving a potent aphrodisiac for females. Gular musk glands are rubbed across the head and neck in mutual appeasement, and several minutes of head- and tail-raising postures are necessary for consent. Once she consents, the female allows the male to press her underwater. To align his vent with hers, the male rolls the female's body in one direction while rotating his tail in the other, using limbs for purchase. Male crocodil-ians have a single penis, unlike the hemipenes of most other reptiles. Copulation can last several minutes, and may be repeated over a period of days, although most males attempt copulation with as many females as possible. In captivity, males have been reported copulating with almost 20 females. Copulation may take place in shallow or deep water.
After fertilization, eggs are retained in the oviduct for two to four weeks, although periods of six months have been reported in some spectacled caimans. Embryos grow to around 20 somites (cells) before laying, with development believed to resume once eggs are exposed to air. Freshly laid eggs are covered in a fraction of an inch (several millimeters) of mucus, which cushions their impact as they slide into position. Mucus may also prevent gas exchange and hence further development until laying is complete and the mucus dissolves. Clutch size varies greatly: larger species lay up to 70 eggs, smaller ones as few as 10. It can take between 20 and 90 minutes to lay the entire clutch, during which time the female becomes curiously docile. Each egg contains the yolk (nutrient storage), albumen (water supply), leathery inner shell membrane, and hard, calcified outer shell membrane (protection, control of water and gas exchange). The embryo, lying atop the yolk, attaches to the inner shell membrane 24 hours after laying. As development continues, an opaque white band spreads around the egg's axis before eventually encompassing the entire egg.
Several important variables influence embryo development. Nest temperatures may fluctuate between 84.2°F (29°C) and 93.2°F (34°C), but small changes have significant effects. On average, incubation time lasts 70-90 days, yet higher temperatures reduce this and lower ones increase it. Temperature also determines sex. The phenomenon of temperature-dependent sex determination (TSD) is found in crocodilians, marine turtles, and some lizards. Unlike genetic sex determination (GSD), the sex of the embryo is determined not by sex chromosomes at fertilization, but by a critical temperature-sensitive period during incubation (the middle third). In all crocodilians, the greatest percentage of males is produced around 87.8-89.6°F (31-32°C), with more females produced above and below this temperature. Temperatures above 93.2°F (34°C) and below 84.2°F (29°C) produce almost 100% females, although females produced at higher temperatures suffer higher mortality and genetic deformities. Above 95°F (35°C) and below 80.6°F (27°C) embryos rarely survive. The size of hatchlings, their growth rate, and even preferred basking temperatures are influenced by incubation temperature.
All species nest during warm seasonal climates, providing appropriate ambient temperatures for incubation. Solar radiation provides additional heat, although many nests are built in the shade to reduce overheating, and nesting substrate buffers the eggs from extreme temperature fluctuations each day. Mound nests generate heat from the breakdown of plant materials, and even the developing embryos provide some metabolic heat during the later stages of incubation. The smooth-fronted caiman (Paleosuchus trigonatus) typically nests in closed-canopy forests, where ambient temperatures may be insufficient for optimal egg development. To address this, caimans build nests within or adjacent to termite mounds— metabolic heat produced by the termites helps to warm the nest. The female's presence is often required to break open the nest and free the hatchlings. Rainfall helps to cool nests, and some females have been observed urinating on the nest. As a suggested cooling mechanism the volume involved may be insufficient, but it may play a role in chemical marking as hatchlings recognize chemicals in contact with their eggs.
Pores permeating inner and outer shell membranes allow gas exchange, and both high humidity and oxygen are necessary for development. Embryo demands increase further as development continues. Development seems highly susceptible to perturbation, yet crocodilian nests provide an effective environment for incubation. But not always. Exposed nests can overheat, and those built in areas prone to flooding are easily submerged. More than 12 hours underwater, particularly later in development when oxygen demands are higher, can spell disaster. Another threat faces developing eggs—nest predators such as monitor lizards, wild pigs, raccoons, even ants. Females of most species defend the nest, often fasting for over two months to remain vigilant, but predators can still catch her off guard.
Eventually the demands of the embryo exceed the capability of the egg and hatching occurs. Shortly before emergence, the fully developed embryos may vocalize. Calls propagate from one egg to the next, producing a chorus audible to the adult from 165 ft (50 m) away. The female is typically much closer to the nest, often observed resting her throat directly above the nest chamber close to hatching. The female scrapes back sand, mud, or vegetation from above the eggs with her front legs, and the vibration stimulates the eggs to hatch. Some hatchlings head for water, but others remain and vocalize, often with head raised to encourage the female to carry them in her jaws. By lowering the tongue, a gular pouch is created to hold hatchlings. Some species transport hatchlings one at a time, others, such as Nile crocodiles, transport several. The female carries them to water, opens her mouth, and washes the hatchlings out with a sweeping motion of the head before returning to the nest. Hatchlings form small pods or crèches, hiding amongst shoreline vegetation. Not all species perform hatchling transport, and puncture marks caused inadvertently by sharp-toothed females may indicate why not. Infertile or dead eggs are normally eaten by the female, which led to early speculation that females ate their young! In captivity, territorial males of certain species have been observed assisting the female to open nests, hatch eggs, and transport hatchlings. Wild Nile crocodile males occasionally guard juveniles after hatching, but male parental care is atypical.
Female protection of juveniles after hatching may last for days (as in Johnstone's crocodiles), weeks (as in Nile crocodiles), or even up to two years (as in American alligators). Hatchlings remain in close proximity to the female, often using her as a convenient basking platform. Vocalization is well developed in crocodilians, and is an important component of juvenile life. Contact calls maintain group cohesion and alert siblings to the presence of food. Distress calls scatter individuals and bring the adult female aggressively to bear. The level of parental care in crocodilians is fascinating. In spectacled caimans, pods from different females may combine into larger crèches tended by resident adults. Even more remarkable, adult females of three species (broad-snouted caimans, Orinoco, and Siamese crocodiles) have been observed feeding juveniles. The female macerates the carcass and juveniles tear off small pieces. This level of parental care is unprecedented for reptiles, and perhaps reminds us of the closer tax-onomic affinities between crocodilians and birds.
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