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Arnold Arboretum

Snakes and leeches

May 18, 2011 by Cam Webb

A Wragler’s pit viper, just hanging out

“So, is the forest dangerous?” I get asked this a lot. The short answer is, “Probably no more dangerous than life in a U.S. city, and definitely less dangerous than anything involving motor vehicles.” Venomous snakes are often perceived to be a big danger, but the fact is that we almost never see snakes in the forest. Sure, Borneo has some of the most venomous snakes in the world (including king cobras, banded kraits, and a whole host of vipers), but encounters are so rare that we basically forget about the risk. My field attire of choice consists of Crocs, shorts, and a T-shirt—this gear would probably appear foolhardy to colleagues working in American or African tropical forests, where stout lower-leg coverage is highly recommended to protect from snakebites. The other night I asked the local members of our team, who have spent much time in the forest hunting and working on illegal logging crews, if they or anyone they knew had been bitten by a venomous snake. None could remember such an event. They did say, however, that people are sometimes bitten by snakes in the fields near the villages, when clearing scrub and grass in preparation for planting rice. The one kind of snake we do see sometimes is Wragler’s pit viper, a small bright green viper that sits coiled on a branch, usually a meter off the ground. They’ll wait there for days in the same position, unmoving. Bumping into one when bushwhacking is indeed a small, nagging fear I have, but it hasn’t happened yet.

A leech so full it can’t keep its food in its mouth

Leeches, on the other hand, are extremely common though not dangerous. Their abundance in the forest waxes and wanes with the amount of rain; they retire, probably to the leaf litter, in dry weather. After people get over the initial reaction of a squirming, eyeless creature sucking their blood, leeches simply become an annoyance. They carry no disease, and after bleeding extensively as a result of the anti-coagulant (hirudin) the leeches inject, the bite wounds usually heal quickly with some mild itching. The annoyance stems primarily from the mess that the unstaunchable bleeding makes—field clothes are always stained with brown patches, and my hammock is dirtier than I would like! Much nonsense surrounds the question of how to remove a leech: you just pull them off. A much harder question is how to get them off your fingers. Their anterior and posterior suckers are extremely efficient, and trying to roll and flick them off is quite futile and frustrating, except when your hands are very dry. I usually end up rubbing them off onto the rough trunk of a tree.

Despite their downsides, I am always pleased to see that a forest has leeches because their presence indicates a healthy population of ground-dwelling mammals. Having said this, it is quite a mystery how even a large population of deer and pigs could satisfy the very high density of leeches sone observes in natural forest. A leech must be able to survive on very infrequent feedings, perhaps even years apart. As far as I know, these fascinating and intimate creatures are understudied, and much remains to be learned about their natural history.

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