Leeches model reproductive behavior

No, not that kind of modeling.

Leeches have a bad reputation because they dine on blood. Even forgetting for the moment such human-designed culinary delicacies as blood pudding or blood sausage, let’s just say that sucking blood does not necessarily an incubus make.

Not just blood-sucking boneless terrors

In fact, leeches have recently made a comeback in the shape–the slimy, creepy shape–of their use as medical therapy. Their former role was to suck bad humors from the body. Today, with our improved understanding of molecular biology and relegation of humor to Jon Stewart, leeches serve a different purpose. Pracititioners encountering venous insufficiency and premature clotting during certain surgeries can apply leeches–and their salivary anti-clotting factors–locally to address the problem. By the way, the medicinal use of leeches–which has a history stretching back for milliennia–is called hirudotherapy.

Model leeches

And leeches also make an oxytocin-related hormone called hirudotocin that plays a role in their reproductive behavior. A reproductively aroused leech, it seems, undergoes a maneuver that involves a sloooow, five-minute rotation of its body. The rotation results in alignment of reproductive pores with complementary pores on a presumably adjacent partner.

Animal behavior results, at its core, from an interaction of hormones and the nervous system. But linking the two directly and assessing the influence of hormones on nerves has proved elusive in more complex animals. Leeches, though, have a nervous system more basic than a mosquito’s. And an injection of hirudotocin yields leech reproductive rotation within minutes, accompanied by a leechy mouthing of the potential reproductive partner. In the world of animal behavior research, this is exciting stuff.

Sliced leech anyone?

To track the effects of this hormone through the animal’s nervous system, researchers at Caltech and UCSD examined nervous response to hirudotocin in slices of leech. Then, they did the ultimate direct assessment, removing all of the leech except the nervous system. This approach allowed them to trace directly the activation of the nervous sytem that led to the corkscrewing muscle movements of leech reproductive behavior.

Their next step will be to use voltage-sensitive dyes to detect electrical nerve signals along these paths to see which ones are involved in maintaining the behavior. They may not be drawing out bad humors any more, but leeches are certainly doing their part in helping us tease out the links between hormones and behavior.

For your consideration

Why is it so difficult to link a hormone and a behavior, especially in vertebrates?

This article says that animal behavior is a manifestation of the interaction of hormones and the nervous system. Can you think of some other examples of this interaction?

Animals are not the only organisms that use hormones. Plants do, too, but they lack a nervous system. Identify some plant hormones and determine what plant systems they influence.

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