This diary is appearing for a number of reasons but the most important one is that I have a cold that doesn't seem to want to go away anytime soon. Also the next semester is going to start with a vengeance tomorrow. So I don't have a lot of energy for a heavy duty science diary for this week's 'A More Ancient World'. Plants and color will get postponed a week.
So let's talk about Alligators and why they are black.
On New Year's Eve it was a balmy 72 degrees and we took the afternoon to visit St. Mark's National Wildlife Refuge just south of Tallahassee. As is typical for a sunny day in the winter, we saw a ton of alligators. Probably several tons as by my estimate we saw about 35 individuals. Of these, only 2 were in the water. The rest were hauled out on shore and the one above had climbed up onto the dike and was lying across the road (we had to turn around and go back as it showed no inclination to move as we approached). At the end of the day it occurred to me that in 2010 I had probably seen more alligators in the wild than in all my life before then. So why not salute this momentous event with a diary?
When kids draw pictures of alligators or crocodiles they typically show them as being distinctly green. As you can see this is definitely not true of alligators. Babies are patterned black and a whitish yellow. As they age the pattern disappears and adults are mostly solid grey to black.
Most crocodilians, such as this American crocodile in the Everglades are tan to greenish gray in color, blending in well with the river banks and logs on which they bask. Why are alligators so dark?
There are two species of alligator in the world, the American Alligator, Alligator mississippiensis and the Chinese Alligator, Alligator sinensis. They are the only two crocodilian species in the world found in temperate climates. All other crocodilians are found in tropical regions.
Crocodilians are ectotherms. The common terminology for this is 'cold-blooded' which scientists avoid using as it is misleading. An ectotherm does not generate its own body heat through metabolism but extracts heat from the environment. It has been known for years that some ectotherms can maintain remarkably constant body temperatures through behavior. Desert lizards, for example can have constant body temperatures throughout the day despite dramatic differences in air temperature between early morning and mid-afternoon. Raising their bodies off the ground vs flattening against a warm rock. Orienting parallel vs perpendicular to the sun's rays and so on.
An crocodilian faces a somewhat different challenge than a desert lizard for two reasons. First they are large. This means a lot more heat is needed to warm them up and they will also cool off a lot more slowly.
Second they are aquatic animals. Behavioral control of temperature is not feasible for animals living in the water. Once you get even a little way beneath the surface of the water most of the solar energy will have been absorbed by the water and cannot be taken up directly. In other words you can't bask underwater. In addition, water is a much better conductor of heat than air. Think about how comfortable you would feel standing around in a bathing suit in 70 degree air vs lounging in a 70 degree swimming pool. The conductivity of water means that it is very difficult to raise your body above the ambient water temperature without and external source of heat.
So temperate aquatic ectotherms are often active at lower body temperatures than terrestrial animals (e.g. fish and amphibians). Others such as turtles leave the water and bask to raise their body temperatures before returning to the water.
Hence the black alligators. A turtle can take advantage of a sunny hour and warm up enough to be briefly active on a spring day in May in Canada even though the water is still bitterly cold. A crocodilian would barely be getting started to get warm enough to feed when the sun would go down. However during sunny winter days in the the temperate south an alligator should be able to have an hour or so at dusk when it is still warm enough to catch some dinner.
In the tropics most of the water is never going to be all the cold. Warming up and cooling down are going to be fairly subtle affairs. Here in north Florida many winter nights are well below freezing. During cold spells the gators will spend days at a time in burrows with underwater entrances. But during warm sunny spells their black color will allow them absorb every ounce of energy they can.
A few other fun alligator facts.
They are one of the great successes of the endangered species act, largely because their primary threat was hunting rather than habitat loss and because they have a high reproductive rate. Today the Florida population of alligators is estimated at one million.
"Shaft" - an Alligator who has survived a harpooning to find safety in a State Park
Crocodilians belong to the same branch of vertebrate evolution as dinosaurs/birds. They are therefore the closest living relatives of birds (and birds are conversely their closest relatives). Although they don't look much like birds they do share some characteristics such as a four chambered heart and care of offspring.
There are only about 25 living crocodilian species compared to some 10,000 living bird species. I've seen 20% of crocodilian species in the wild, something I certainly can't say for birds.
An alligator and one of its relatives.
All crocodilians and most turtles (and some other organisms) have temperature dependent sex determination. They don't have sex chromosomes but instead the sex of each individual is determined by the incubation temperature of the egg. This depends on the ambient temperature but also on the depth at which the eggs are buried. Alligators pile vegetation over their eggs so they have some control over the incubation temperature.
A baby gator - they are usually secretive because so easily eaten.