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A Theory on Sexual Dimorphism in Madagascar Leaf-nosed Snakes

Living on the island of Madagascar is a most unusual species of serpent: the Madagascar leaf-nosed snake (Langaha madagascariensis). This serpent is one of the most extreme examples of sexual dimorphism. “Sexual dimorphism” is when different sexes of the same species have different visual characteristics.

Read on for a guest article by David D. Crespo! 

What do Madagascar leaf-nosed snakes look like?

The difference in appearance between the two sexes is so striking, you would be forgiven for thinking they’re two separate species! 

Color

Males have an ochre brown back with a bright yellow belly, the females are a uniform mottled grey or light brown color. Males have orange-y irises, while females’ are more brownish.

Nose shape 

While males have noses that are long, narrow, pointed, and sometimes a little curved, females have broad and flat ones. In addition, their noses have a bumpy, almost serrated, edge. The reason for these appendages and disparities between them is still unknown due to limited field study. 

Size

One thing they do have in common is they will both grow to be about one meter (about three feet) long once adulthood is reached. Similar to most tree-dwelling snakes, their bodies are thin and lightweight to prevent branches from breaking underneath them. 

What is the Madagascar leaf-nosed snake's habitat?

In order to understand sexual dimorphism in Madagascar leaf-nosed snakes, one must first consider their habitat. This species inhabits two primary habitats: dry deciduous forest and lowland rainforest. The former is in the north, northwest, and west of the island; the latter is in the east and northeast.  

Since these snakes are highly arboreal, they prefer spending time in the trees. An interesting behaviour is that they enjoy hanging the front half of their bodies upside down from low-lying branches. In fact, this is how the snakes sleep!

Why are Madagascar leaf-nosed snakes sexually dimorphic?

There have been a few theories proposed to try to explain the evolutionary drivers behind the differences in each sex’s nose colour and shape.

One theory suggests they help run rainwater off the snakes’ surface quicker and more effectively. This would help them keep dry during otherwise rainy days. 

Another suggests they help disguise the snakes by making them look like the seed pods of certain plants such as Bignonia (a genus of crossvine). This would also explain the way they hang off of tree branches.

Either of these theories could be correct, but neither of them quite explains why the noses need to look so different from each other. 

My theory:

Both males and female Madgascar leaf-nosed snakes are present in dry deciduous forests and lowland rainforests, even though these habitats are completely unalike. Rainforests are thick with plant life that doesn’t shed leaves and remains green all year round. They are always warm and receive huge amounts of annual rainfall, which means they’re constantly humid like the inside of a steam room.  In contrast, dry deciduous forests (as the name suggests) are relatively dry because they receive less frequent rainfall, have plant life that does shed its leaves, and therefore don’t stay green all year. Instead, they experience two seasons: dry and wet.

In a dry deciduous forest, rainfall normally only happens from late October to April (the wet season) rather than the entire year, like the rainforests. The dry season is when there’s little to no rainfall and is when the majority of the plants shed their leaves. This dry season lasts from May to October and coincidentally includes the snakes’ breeding season, which begins in late May and ends somewhere between June to July. It also includes the egg-laying/nesting season, which, according to captive specimens, takes place from August to September. 

In this habitat, males hide by staying motionless throughout the day. This means that if females want to mate, they have to actively go out are search for them. This may seem unfair, but wandering around in broad daylight through bare, leafless tree branches would (for a male) be the same as holding a sign that says “EAT ME!” 

Females are much better at camouflaging in this environment due to their duller coloration and the shape of their snouts. Their broad, laterally flattened form, along with the ridges on the edge, makes them bear a resemblance to broken branches. In a place and time that largely lacks greenery, this appearance allows them to seek out their hidden partners whilst avoiding any unwanted attention.  

In the rainforests, the roles get reversed, and males are the ones who search for their mates. Their more flamboyant coloration and long, tapering snouts make them resemble a growing leaf, stem, vine, or seed pod. This makes them better at blending in while moving through the lush, vibrant foliage in search of their secret sweethearts.  

If a female tried to do the same, she would be a lot more conspicuous. After all, a bare, broken branch with no organisms using it isn’t something normally found in the rainforest — especially severed ones that are moving around by themselves. 

In other words, both sexes take turns “hiding” or “seeking,” depending on the habitat and time of year, putting their lives and bloodlines at risk until they eventually find each other. 

To lower their chances of getting caught in the act, Madagascar leaf-nosed snakes wait till the cover of night for their romantic rendezvous. Likewise, the females wait until night to lay their eggs to protect their precious young from daytime predators. 

Whether this particular theory of mine is fact or false is yet to be concluded.

About the Author:

david crespoDavid D. Crespo is a second-year college student in Animal Management, independent wildlife writer, and reptile enthusiast. In the past, he’s had his fair share of reptile encounters, from haring a nursery with tortoises, to receiving a bearded dragon named Lola on Christmas Eve, to observing wild alligators in the Florida Everglades.

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