Friday, April 22, 2011

Opposites Attract?

            A reader recently sent along a set of fantastic pictures of two snakes intertwined on a log overlooking a river. Apparently the e-mail has been circulating for a while; somewhere along the line it was suggested that these snakes were two Cottonmouths mating. Although these two snakes are mating, they’re not Cottonmouths, they are Brown Watersnakes, Nerodia taxispilota. The two species are often confused (a discussion of how they can be told apart is here).

Comparing the head of a Cottonmouth versus a non-venomous watersnake is a good way to tell the two snakes apart, but it’s difficult to make out the head of the big snake in the picture. In any case, the brown boxes on the body of the Brown Watersnake are distinctive.

What’s striking in these pictures is the giant size of the female snake, the male is only a fraction as big. Although they are both adult snakes, it’s hard to know it by looking at them. Female Brown Watersnakes can reach much larger sizes than males.  They are what we call sexually dimorphic, meaning that the size or shape of males and females differ. Humans are sexually dimorphic too, but this isn’t the website you’re looking for if you want pictures explaining how.

There are a few reasons why males and females of the same species might reach different sizes or come in different shapes. In the case of the Brown Watersnake we need to consider why there would be a benefit for females to get big, but not males.  One potential reason relates to how much energy is required to produce offspring. It doesn’t really matter how big a male is, his reproductive investment is always the same, but it takes a lot of energy to provide for a litter of snakes developing inside of you. For many reptiles, a bigger female can produce more babies. It’s easy to conceive how a big female will produce more babies that carry her genes for big body size, they grow up and produce more babies themselves, and so on.

We should ask why the males don’t get big too. Even if there is no real benefit, what would it hurt? Well, there is a cost associated with larger body sizes, you need to catch and eat more food to maintain a big body. For the females, this cost is outweighed by the benefit of producing more young. The males don’t have that benefit, so they stay small.

Another reason males and females differ is to limit competition. If all the individuals in a population eat one type of prey, and there is only so much of that prey type to go around, then the population can’t grow much and everyone is going to be fighting over food.  On the other hand, if half of the population (males) eat something different than the other half (females), then there is a lot more food for everybody.

One of the classic examples of this type of sexual dimorphism is clearly demonstrated among the map turtles. You might be hard pressed to recognize that the  turtle on the right is the same species as the one below, but they are both Barbour’s Map Turtles, Graptemys barbouri. This turtle can only be found in a few river drainages in Alabama, Georgia, and Florida, which is a big reason why they’re considered rare. The male turtle is the one with the regular-looking head on the right, they prefer to eat various soft-bodied insects and other invertebrates. The female is the one with the head that appears grotesquely large (although I assure you the male turtles don’t mind). Females have large heads to help them crack down on their preferred prey, clams and mussels. Females also reach larger sizes than the males, probably for the same reason that Brown Watersnakes do.

A third type of sexual dimorphism that we often observe is the exact opposite than what we see among the watersnakes. Sometimes, males get bigger than the females. When this type of variation occurs, it’s usually because males compete with each other for the chance to mate with a female. Snapping turtles do this, as do rattlesnakes. One of the most rare and exciting sights a nature-lover can see is two big male rattlesnakes reaching into the sky while they wrestle and push each other to the ground as their rattles buzz lazily. A recent acquaintance related to me she saw this happen once in the woods of northwestern Florida. She noted this was many years ago when she had a film camera and wouldn’t you know it, there was only one picture left in the roll.

The final kind of sexual dimorphism that we see in the animal kingdom (at least the last one I can think of) happens when males go to great lengths to impress females.  You’ve probably seen the little green lizards called Green Anoles, Anolis carolinensis. Maybe you’ve seen males doing push-ups while they flash their pink dewlaps (a collapsible fanlike structure underneath their chin). Females have dewlaps too, but they’re much smaller. Birds are the most famous participants of this strategy (think peacocks).

Lots of people ask me how to tell the difference between male and female snakes. The truth is, sometimes it’s hard to tell and you need to look for subtle differences in morphology (i.e. body shape). The length of the tail is a good way to tell them apart, but when I offer this hint I often get the same response, 

“I thought they were all tail”.


Thank you to the reader (who prefers to remain anonymous) who provided the Brown Watersnake pictures which inspired this blog. Barbour's Map Turtle pictures provided courtesy of Sean Sterrett.

Much of what I write is based on my experience in the field, however I also rely on the research of others, citations of some relevant scientific articles are below.

Shine, R. (1978). Sexual size dimorphism and male combat in snakes Oecologia, 33 (3), 269-277 DOI: 10.1007/BF00348113

Gibbons, J., & Lovich, J. (1990). Sexual Dimorphism in Turtles with Emphasis on the Slider Turtle (Trachemys scripta) Herpetological Monographs, 4 DOI: 10.2307/1466966


Trailblazer said...

Great post!

...taxispilota is a cool critter.

Saw them for the first time when I was down in Florida a few years ago (at the Joint Meetings in 2005).

Conservation Jobs said...

This is true with insects too. With many species of spider the female is much bigger than the male.