Friday, April 30, 2010

Which Came First, the Snake or the Egg?

This fall, a few colleagues of mine were hiking around southwestern Georgia when they noticed a familiar serpentine shape alongside a fallen log, a timber rattlesnake. Closer inspection revealed almost two dozen newly born rattlesnakes huddled together with their large and likely exhausted mother. Just a few months before this auspicious find, a kingsnake captured at the same site was being held for further study in the lab when it surprised everyone by laying over a dozen leathery eggs. The eggs were placed within an incubator and thirteen tiny kingsnakes were safely released into the forest after they hatched.

We call both animals snakes, yet they have vastly different reproductive strategies. Although timber rattlesnakes give birth to live young, kingsnakes lays eggs. Let’s take a moment to think about how unusual this is. What if half of all monkeys laid eggs or if babies popped out of some of the animals we collectively refer to as ducks or geese? Rat snakes, hognosed snakes, coral snakes, and indigo snakes lay eggs, but copperheads, water snakes, and garter snakes have babies. Why the dichotomy? It would be logical to assume all snakes that lay eggs are more closely related to each other than they are to snakes that give birth to live young, but that’s not always the case. And that suggests different groups of snakes independently altered the way they reproduce.

It is generally accepted that the ancestor of all snakes laid eggs, so there must have been some advantage for certain snakes to develop an alternative strategy. To investigate this further, we must look for clues that would indicate why giving birth would increase the survival of offspring. If more offspring survive, then they have a better chance of passing along their parent’s genes, and thereby a better chance of passing along any advantageous trait to further generations. Those that philosophize about these matters observed that species which give birth to babies tended to be from more northern climates (notice I used the word tend because we can almost always find exceptions to patterns observed in nature). Using this clue, we can attempt to imagine why snakes in colder areas found it advantageous to abandon egg-laying.

Snakes are ectothermic (you may be more familiar with the term “cold-blooded”), which means they rely on the external environment to regulate their internal body temperature. Referring to reptiles as “cold-blooded” has fallen out of favor because researchers determined snakes have a remarkable ability to maintain temperatures comparable to ours, just through their behavior. So, in northern regions where temperatures can get cold and unpredictable, you can imagine it’s more beneficial to develop inside of a large snake than an egg exposed to the elements.

Because a mother snake is warmer than an egg, babies develop faster. And a faster developing baby will have more time to find a safe place to hide before the onset of winter, another advantage. That some snakes developed live birth due to these survival advantages is generally referred to as the “cold-climate hypothesis”. Recent research has suggested snakes that develop under constant temperatures are simply faster, healthier, and survive in greater numbers than snakes that develop under more unpredictable scenarios.

There are likely other advantages to giving birth to live young, regardless of the temperature. For example, if you’re a raccoon or a fox and you come across a nest of snake eggs, the chances are pretty good you’re going to take advantage of this easy meal. Now, what if you come across an angry mother rattlesnake full of babies? In this case, you may realize there’s no such thing as a free lunch. It’s not hard to imagine why some snakes have found this strategy to be an advantage.

After this discussion, you may ask yourself if there is any benefit to a reproductive strategy that includes egg-laying. There are in fact some upsides. It requires a lot of energy to produce a fully functioning animal inside you, when a snake lays an egg they’re able to save some of that energy, perhaps better equipping themselves to produce a lot of eggs at one time or breed more frequently. Although eggs may be more likely to be eaten than live young, there can be enough eggs laid to make up for this difference.

You may have noticed that I did not use some scientific terms you’re familiar with, such as viviparity, oviparity, or ovoviparity. As with so many other aspects of biology, it’s sometimes hard to characterize animals with terms we make up, because there are often cases where a species does not cleanly fit into a particular group. That’s the case with snakes, and there is some debate among scientists just how to best characterize all the different reproductive strategies that snakes exhibit.

Pictures here appear courtesy of Fingerprince Prints Photography.

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