When we experience something over and over again, we tend to get used to it or even ignore it. This is true of most things, whether that be a wonderous view we pass on our way to work or the discomfort of walking around in shoes that don't fit quite right. Animals seemingly have a very similar reaction to annoyances, like the cows that ignore the flies buzzing all around them. But is this true of all animals and for all annoyances? Jessica Hoynoski, John Dohn, Avery D. Franzen and Brian Burrell of the University of South Dakota, USA, wanted to know whether enough poking would make animals stop responding to and just ignore the rather annoying prods they felt.
In order to do this, the team poked medicinal leeches (Hirudo verbana) 40 times in the sucker on their tail and recorded how the animals reacted. At first, the leeches responded in two ways: either shortening the area of their body where the tail sucker is located or shortening their entire body. But as the poking continued, the leeches did something unexpected – instead of ignoring the pokes, they started to evade the poker. The leeches started hiding their tail sucker underneath their bodies so that the researchers couldn't reach it or crawled around in their dish, which made it too difficult for the researchers to poke them anymore.
Once Hoynoski, Dohn and colleagues realised what was happening, they started recording how many pokes it took each leech to start evading them. The blood-sucking worms fell into three categories depending on how many pokes it took: ones that started evading early (fewer than 12 pokes), ones that started evading late (more than 25 pokes) and ones that started evading the poker somewhere in between (12–23 pokes). Interestingly, some of the late evaders didn't respond to pokes by evading, but continued shortening their tails or entire bodies whenever they were poked. This suggested to the researchers that each leech has a different strategy for how it responds to unpleasant experiences: either sit there and take it or try to get away. This finding also generated a new question. What would the leeches do if the pokes happened to a place that was ‘painful’ to them?
The team tested this on leeches that had an injured tail sucker. First, they poked the sucker the day it was injured and found that all of the leeches started to evade the poker within 12 pokes. Not only that, but the leeches also started moving to evade the poker rather than just hiding their tail sucker. When the team tried poking the injured leeches again 7 days later, the blood-feeding worms still moved around quite a bit more than usual, but not as much as they first did. This suggested that the leeches were recovering from their injury. The leeches were also not crawling around their dishes as much, preferring to hide their tail sucker instead, but still choosing to evade the poker rather than tolerating it.
We don't know much about whether animals like leeches feel pain or not, at least not like humans do. However, Hoynoski, Dohn and colleagues suggest that this study shows that this sensation may be similar in animals as different as leeches and humans. Maybe we have more in common with leeches than we thought.
