Saturday, January 26, 2013

Parental Care in Crocodilians: Nothing “Cold-Blooded” About it!



By Michael DeLea

What adjectives come to mind when you hear the word crocodilian? Ancient? Monster? Man-eater? Perhaps you think of one of the 23 species that inhabit the world today, two of which can be found here in the United States. How about attentive parents? Crocodilians exhibit complex social behaviors, none perhaps more interesting than those demonstrated during reproduction. Not only do members of the order Crocodylia outclass the remainder of their reptilian relatives, they rank among nature’s elite when it comes to caring for their young.


Hungry Crocodile? Or attentive parent?

As observed in many reptiles, nest construction can be a complicated and time-consuming endeavor. For example, it has been documented that female American Alligators (Alligator mississippiensis) will make frequent trips to the nest site adding materials and reshaping the nest prior to egg deposition. The design of the nest plays an even more important role when you consider the fact that sex determination is a function of temperature in at least half of the extant species of crocodilians.


Juvenile Alligator mississippiensis I encountered in Southwestern Florida

During the incubation period, crocodilians have been observed demonstrating various degrees of nest attendance and defense. This behavior is in stark contrast to that of other noncrocodilian reptiles that typically lay their eggs and leave the unborn young to fend for themselves. As to be expected, it has been shown that increased attendance of a parent leads to an increased nest success rate. In addition to maintenance, the presence of a watchful parent is a very powerful predator deterrent. In a study of American Alligator nest attendance, the defensive behaviors of the alligators were broken down into a 10-step response sequence that increased in aggressiveness, depending on the perceived threat of the intruder. Protective parents therefore give their offspring a greater chance of survival by fending off predators such as raccoons, bears, and even humans.


Female American Alligator in a typical defensive posture, preparing to defend her nest from intruders.

            After what can amount to months of incubation, the eggs will begin to hatch. Even before the newborns emerge from their shells, they begin calling out to an adult. Upon hearing the calls emanating from the nest, the adult will approach and begin excavating very carefully. The babies will continue to call out to alert the adult to their location until they are unearthed. The task of the parent is not completed once the young are freed from the nest, however. To avoid having the nest flood due to natural conditions that affect water levels, there is usually an expanse of land to be traversed before the newborns can safely reach the nearest body of water. At this point, the parent will pick up their young with their powerful jaws and transport them to the water that will serve as the nursery.


Clutched between jaws that generate the largest bite force ever recorded for living animals (3,700 psi), this crocodile transports her young to safety.

            The extent to which juvenile crocodilians will be cared for varies between species. In some cases the parental care will cease shortly after transportation to the nursery, when they are left to fend for themselves. In other cases, such as the care demonstrated by the spectacled caiman (Caiman crocodilus crocodilus), females occupying nearby habitats have been known to take turns watching over each other’s offspring.




            The ancestors of today’s modern crocodilians have been traced back over 65 million years ago. During that time they have adapted well to life on earth, outlasting the dinosaurs and many other species. When you look at certain aspects of their behavior, paternal care for example, it is obvious that these creatures have evolved to be successful in an ever-changing environment and will continue to do so.


References:

Deitz, D.C. and T.C. Hines. 1980. Alligator nesting in North-Central Florida. Copeia, 2: 348-258.

Hunt, R.H. 1987. Nest excavation and neonate transport in wild Alligator mississippiensis. Journal of Herpetology 21(4): 348-350.

Kushlan, J.A. and M.S. Kushlan. 1980. Function of nest attendance in the American Alligator. Herpetologica 36(1): 27-32.

Somaweera, R. and R. Shine. 2012. Australian Freshwater Crocodiles (Crocodylus johnstoni) transport their hatchlings to the water. Journal of Herpetology 46(3): 407-411.

6 comments:

  1. I'm surprised mothers invest a lot of energy in parental care while nesting, and then pass their babies off to another mother when they enter the water. How might this "community watch" be a benefit to crocodiles?

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  2. That is such an interesting method for sex determination. I wonder if most broods hatch as a single sex, or if the parent makes conscious alterations to the nest to have them come out close to equal. It must make it pretty easy for animal breeders to control the outcome of the offspring.

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  3. Way to paint cold-blooded crocodilians as warm and fuzzy! Do any other reptiles exhibit this degree of assistance with their young? It's interesting how parental assistance can be quite variable as an evolutionary adaptation for fitness.

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