A crocodile’s salt water surprise

It was a bit unsettling to see a crocodile occupy the same several square meters of ocean where I’d swam only a few days before. The croc must have been over ten feet in length and appeared suddenly and silently in the surf near the beach. He was probably chasing after a morning meal of fish. Unfortunately (or fortunately, depending on how you look at it) this beach is where I happened to be eating breakfast with my family. We had planned to spend the day swimming in the same stretch of water now occupied by the croc.  It was Thursday and earlier in the week this is where we’d been swimming, body surfing and even taking my 16 month-old niece Lucia for a dip.

While we recoiled from the shock, my younger sister Megan and I frantically searched for our cameras. Lucky for us our croc wasn’t going anywhere. He surfaced at least a dozen more times as we snapped pictures and other anxious tourists peered out over the water searching for his outline. No doubt their plans for the day–much like ours–had suddenly changed.

It wasn’t completely unexpected to see a croc that day. After all, we were on the beach in the small Mexican town of La Manzanilla, about three hours south of Puerto Vallarta on Mexico’s Western Coast. La Manzanilla is renowned for the many crocodiles that inhabit a river near the edge of town. We’d seen the crocs before, but we’d never seen them anywhere except in the river, which the locals have partially barricaded to keep the crocs from getting out onto the beach. According to the locals we asked, a hurricane that had arrived about a month before had flooded the river and must have swept some of the crocodiles out toward the ocean.

Our ocean-going friend and his river-dwelling siblings are known as the American Crocodile (crocodylus acutus). The local Mexican population just call them cocodrilos (which, by the way, I’ve never understood. How did the Spanish manage to relocate the “r” from the second position of the word to the sixth?). The most prolific of the four living crocodile species in the Americas, their range extends from the tip of Florida all the way south to Ecuador. You won’t typically find them any farther north or south however, because unlike their reptilian cousin the American alligator (alligator mississippiensis), they don’t fare well in anything less than tropical temperatures. Despite its cold-blooded reputation, Crocodylus Acutus prefers a body temperature of 30-35 degrees Celsius (86-95 degrees Farenheit) and relies on ambient temperatures to maintain this.

But as we learned on the beach that day in La Manzilla, crocs do have something over alligators–the ability to survive in salt water. In fact, they’re so well-suited to life at sea that when biologists immersed crocodiles in a tank of salt water more than twice as salty as seawater, they seemed to do just fine. This adaptation is responsible for their proliferation throughout the Caribbean; islands that the American alligator could never reach.

Crocs can go where alligators can’t because they are able to counter the dehydration that results from moving from a fresh water environment like a river out into the salty ocean. Anyone adrift at sea whose tried to drink seawater to quench their thirst has quickly learned this is a bad idea–and the price for the lesson is usually death by dehydration. That’s because sea water is much saltier than body fluids and in order to compensate, your kidneys will try to flush the excess salt out of your system. The catch is that kidneys are only able to excrete urine that is less salty than seawater, so more water must go out with the salt than came in with it. Crocs have the same issue because they spend the bulk of their time living in fresh water rivers. So how can they switch between the two environments?

For one, crocodiles have the ability to change their urine from a liquid into a creamy-white solid. This saves on water that would have been lost to the flushing process. In addition, like other marine reptiles, crocs can excrete excess salt through specialized glands. The marine iguanas of the Galapagos, for example, use glands in their snout to sneeze out salt. Sea turtles do it through a gland near their eye.

So where are the crocs  salt glands? Well, we’ve all heard the  expression “crocodile tears” which is about superficial sympathy, but it actually came from the observation that crocodiles out of water for a long period of time well up with tears. Naturally you might think that’s where the salt glands are located. But it turns out crocodiles tear up just to keep their eyeballs from drying out. After a lot of study by Australian researchers in the 1980s, it was finally determined that a crocodile’s salt glands are on its tongue. An odd place, to be sure, but when they looked for similar glands on the tongues of alligators none were found.

Of course, none of these seafaring adaptations were known to me or my family on that fateful day in La Manzanilla. Having only seen Mexican crocs in the river and some alligators in the freshwater swamps of Lousiana, we had assumed the only crocs venturing into salt water were the so-called ‘saltwater’ crocs of Australia (crocodylus porosus). Needless to say, after we’d finished our lovely beach-side breakfast, we packed up our gear and without much discussion headed about a half an hour up the road to Cuastecomate, a lovely beach that isn’t anywhere near the mouth of a river and therefore free of cocodrilos–at least we think.

All photos by Eric R. Olson unless otherwise credited

Further Reading:

Grigg, Gordon, and Carl Gans. “Morphology and Physiology of the Crocodylia.” Fauna of Australia. Canberra: Australian Government Publ. Service, 1993.  http://espace.library.uq.edu.au/view/UQ:9776

Grigg, Gordon. “Twenty Years of Wondering and Worrying about How Crocodiles Live in Salwater.” Herpetology in Australia: A Diverse Discipline. Mosman, NSW, Australia: Royal Zoological Society of New South Wales, 1993. http://espace.library.uq.edu.au/view/UQ:9724

Mazzotti, Frank J. “Field and Laboratory Observations on the Effects of High Temperature on Hatchling Crocodylus Acutus.” Herpetologica 42.2 (1986): 191-96. http://ufdc.ufl.edu/UF00066434/00001/1j