HOW DO DOLPHINS ACQUIRE MARKS?

Photos © Tilen Genov / Morigenos

Dolphins are social animals that normally live in groups. Within these groups, interactions among individuals are constantly occurring. Dolphins can be gentle or rough, friendly or aggressive towards one another. Play and fights are a normal thing in the lives of dolphins. Fights can occur for establishing or maintaining hierarchy within the group, access to females or other disagreements.

Bottlenose dolphins can sometimes be very aggressive, not only to their own kind, but also to other (often smaller) species of dolphins and porpoises!

 

                                                                          Bottlenose dolphins fighting

                                                                          Bottlenose dolphins playing

During both play and serious fights, dolphins tend to bite each other. They have a numer of conical teeth on the left and right sides of the upper and lower jaw, which they use to catch prey. But teeth can also be used as a weapon. Because dolphins often bite each other, their bodies are full of scratches and scars. Teeth marks of other dolphins are easily visible on the skin. Such marks are usually relatively short-lasting and are not normally used for long-term identification of dolphins. However, we can use them to separate different animals from the same group, in the same day.

   

                                                   Scratches and tooth rakes (paralel lines) on dolphin skin

The dorsal fin of a dolphin is the part of the body that is most exposed. It does not contain any bone or cartilage. It only contains some connective tissue, some fat tissue and a dense web of bood vessels, which help to maintain body temperature (this is called thermoregulation). As the posterior side of the fin is much thinner than the anterior side, it is easily torn and serrated. The resulting missing tissue never grows back. Therefore, smaller or bigger nicks and notches are formed on the edge of the fin. Such marks are long-lasting and can therefore be used to identify individual dolphins.

AN EXAMPLE

Ric is one of the dolphins that we encountered off the coast of Slovenia. We met Ric in 2009. Apparently, Ric was very active in spring 2010, because we found him (we assume it is a “he”) covered in new scratches, scars and even fresh wounds. He probably had a fight with other males. At first he looked beaten up pretty badly, but his wounds healed quickly. Fast wound healing is characteristic of dolphins.

Ric in July 2009. The fin top does not have any obvious notch, while the rest of the fin does, which enables an easy recognition.

Ric in April 2010. The nicks and notches in the fin clearly show it is the same dolphin. But now there are other scratches and nicks present, even real wounds. In some places the skin has been torn off, revealing subcutaneous tissue (light pinkish colour). Skin at the top of the fin is ripped.

Ric in beginning of May 2010. The wounds are healing quickly. The top of the fin, where the skin was ripped, already shows a small nick. The pinkish tissue is still visible in some parts, but the healing process is very fast.

Ric in the end of May 2010. The pinkish colour is gone, the wounds are healed. All that remains are light scratches and tooth rakes. A relatively deep notch has formed on the top of the fin. The rest of the fin is nearly identical to how it was in 2009.

Natural marked animals

That is how our dolphins acquire marks by which we can recognise them. Dolphins acquire new marks throughout their lives. Calves are born without marks, but they start to acquire them while they are young. Older animals therefore tend to have more marks. Males tend to be more marked than females, but this is not always the case.

Some dolphins change very quickly, from year to year. Other stay practically the same for ten years or more.

Because the mark pattern can change (dolphins acquires new nicks in new places or they even “cover” the old ones) it is possible to accidentally confuse one dolphin for another. To avoid that, we use only high-quality photos, we carefully examine all marks and we observe dolphins every year (so that we can track any smaller changes before they become big changes).

Sometimes the marks on two different dolphins can look very similar. Again, it is therefore extremely important that we perform the analysis very carefully and use only high-quality photos on which all the details are visible.

If you would like to see how the fins of different dolphins from our study area look like, please visit the Photo-ID catalogue.