1. Why Not To Play “Marco Polo” with Harbor Seals Dehnhardt G, Mauck B, Hanke W, Bleckman H. 2001. Hydrodynamic trail-following in harbor seals (Phoca vitulina). Science 293:102-104. April 21, 2004 (from BBC News July 6, 2001)

2. Introduction Marine animals can face conditions where there is poor visibility Dolphins use echolocation, but not all marine animals, like seals, use this Others use hydrodynamic sensory systems Why use such systems? Hunting The wake behind a swimming fish has particle velocities above threshold for hydrodynamic receptors Lasts for several minutes after the fish passes by So swimming fish leave a hydrodynamic trail that predators can use for prey detection over long distances Can seals use this mechanism of hydrodynamic trail- following to track objects? Fish leave disturbances in the water when they swim (from BBC News July 6, 2001)

3. Methods – Linear Trails Used a propeller-driven miniature submarine (about the size of a large trout) Generated trails using the submarine to determine if it was appropriate for use in the study Determined that it was sufficient in mimicking fish-generated hydrodynamic trails The male harbor seal “Henry” used in the study (from National Geographic News) © Guido Dehnhardt, University of Bochum. First trained to locate the submarine without any sensory restrictions Then blindfolded and blocked hearing with headphones Then released submarine Linear hydrodynamic trails generated Removal of headphones signaled the seal to start searching Searching for the trail was characterized by the seal extending its whiskers to the most forward position and performing slight lateral head movements

4. Results – Linear Trails Analysis of video recordings showed that the seal followed trail exactly Seal located the submarine in 256 of 326 trials (78.5%) In 59 of 70 trials in which the seal failed to find the sub, the seal seemed to have missed the beginning of the trail Indicated by the seal stopping the search in the starting area after a few seconds In the remaining 11 trials (3.4%), failure to find the sub was due to the seal deviating from a previously correctly tracked trail Increased delays between start of the submarine and start of the seal’s search did not affect accuracy of finding the trail A.The seal’s probability of finding the sub by mere chance was only 4%. B.Frames from video recordings showing a trial with a linear hydrodynamic trail. (from Dehnhardt et. al. 2001)

5. But Fish Do Not Only Swim Linearly… Introduced an unpredictable change of course by using two lateral steering propellers Seal spontaneously turns onto new course when meeting course change and found submarine in 26 out of 30 trials Clearly shows true trail-following Use of passive listening or use of vision expected to result in straight-line approach Sequence of frames from video recordings showing a trial with a hydrodynamic trail with a sharp change in course (from Dehnhardt et. al. 2001)

6. …And The Trail May Not Be Encountered At The Beginning Tested to see if the seal could correctly identify the direction of sub movement Running trials from both the left and the right, the submarine ran parallel to the long side of the pool, passing the starting point of the seal at a distance of about 1 meter Out of 50 trials, the seal swam in the correct direction 46 times.

7. Look, Mom! No Whiskers! Whisker movements impeded by a stocking mask In all 30 trials, the seal failed to detect the hydrodynamic trail Shows that trail-following is only possible with unimpaired whiskers and is based on detection and analysis of hydrodynamic information

8. Next Up…Nick Performed same experiments on Nick, another male harbor seal Clear fresh water pool Henry was in turbid waters Found similar results as experiments with Henry Also tested effect of acoustic cues If seals uses acoustic cues, we expect to see a straight approach to the sub Ten trials randomly interspersed throughout the different experiments Seal was allowed to start search right before the engine of the sub stopped running (short acoustic cue from sub’s final position) In these trials, the seal directly approached the sub in an unusually fast reaction Sequence of frames showing a trial with Nick (from Dehnhardt et. al. 2001)

9. Discussion What might be the detection range of a trail-following seal for prey fish? Need to learn more about background noise in the wild Need to learn more about the aging of fish-generated trails under natural conditions Swimming seals produce a lot of water movement itself Preliminary data suggests that seals overcome this problem by having the whiskers vibrate with characteristic frequencies depending on swim speed and biomechanical properties Seals can sense hydrodynamic trails due to changes in vibration Results suggest that hydrodynamic trail-following is used for spatial orientation in the aquatic environment which is useful to pinnipeds for long-distance prey location.