Although leeches may have an unsavoury taste for blood, like the rest of us, they still have to eat; they are able to sense unsuspecting prey that come across their watery lairs for a drink or bath. However, as Cynthia Harley from the California Institute of Technology, USA, points out, ‘If you eat living things, those living things may move. How do you determine the location of your prey? Do you, like a ballistic missile, sense them, and move quickly and blindly to their location hoping they will still be there, or do you continually update your picture of where they are?’ During ballistic tracking predators periodically stop moving to re-sense the location of their victims, whereas during continuous tracking, stopping is unnecessary; predators continuously update their sensory picture of their target's whereabouts and tweak their trajectories accordingly. However, continuous tracking requires a great deal of brainpower, as predators have to simultaneously distinguish between their own movement and location and that of their prey. As leeches only have 10,000 neurons and are often stationary between bouts of either swimming or crawling, Harley and her post-doctoral advisor, Daniel Wagenaar, suspected that the leech might use the simpler of the two options. To conclusively test this, Harley recruited the help of two enthusiastic high-school students, Matthew Rossi and Javier Cienfuegos (p. 1890).
Together, the trio set up a small pond in the laboratory and mimicked prey movements in the water by attaching a piston to a moving speaker. ‘First we asked what happens if, after the animal starts moving, we turn the speaker off. If they were ballistically tracking [their target], it wouldn't matter because they've already sensed where the thing is and they've already determined where they're going to go’, explains Harley. However, to their surprise, they found the opposite: ‘Well, they got lost, whereas if you kept the speaker going they were fine’, recalls Harley. So, the first results suggested that maybe the leeches were not ballistically tracking their prey after all.
However, given that motionless bouts, which are key signatures of ballistic tracking, are so ingrained in the leeches' behaviour, the team decided to test the leeches again in a different experiment. They reasoned that perhaps because the prey-like stimulus was removed, the leech assumed that their intended victim had left the vicinity – why waste energy tracking something that is not there? ‘We decided to use two speakers, and once the leech had determined the direction of movement [towards the first speaker], we switched to the other. We wanted see if it would go towards the first one or towards the second one’, says Harley. They never approached the first speaker and instead behaved as if the second speaker was the only one – strong evidence that the leeches were continuously updating the position of their target.
If they were not tracking ballistically, then maybe the leeches were not using their stationary periods for tracking either. The team decided to test whether the leeches tracked whilst swimming or crawling. To begin with, Rossi and Cienfuegos patiently sat and watched the leeches switching on the speaker only when they crawled. They repeated the experiment only stimulating the leeches when they were swimming. ‘We found that stimulating them during crawling didn't decrease their success rate at finding the speaker, whereas when we stimulated during swimming, they never found the stimulus – they were terrible at it’, says Harley. Despite swimming being the faster of the two movements, it seems that when it comes to tracking prey, crawling is essential, and when stimulated, the leeches were more likely to crawl. So, slow and steady wins the prey!