Risso's dolphin in wild population seem to obtain scratches with aging. What is causing them and what do they mean?
[ From Jefferson et al. 2015, Marine Mammals of the World, 2nd edition, p 212: "At sea, the best identification characteristic is the coloration and scarring. Adult Risso's dolphins range from dark gray to nearly white, but are typically covered with white scratches, spots, and blotches. Many of these are thought to result from the beacks and suckers of squid, their major prey, but others may be caused by the teeth of other Risso's dolphins. If fact, this species is the most heavily-scarred of all the dolphins." ]
Although little is known about these animals, they seem to obtain these scars from scratching in fights with their prey, giant squid, and from the teeth of other Risso's dolphin.
Most of the linear scars are thought to be caused by intraspecific interactions, e.g. scratches from each others teeth, though some more circular or oval scars are thought to come from squid. Cookie cutter scars are not commonly described for Risso's dolphins though they are common in other whales and dolphins.
Encyclopedia of Marine Mammals. 2002. W.F. Perrin, B. Wursig, J.G.M Thewissin. Academic Press: New York.
As others have said, most of these marks come from intraspecies interactions. However, what makes Risso's different is that they more or less keep their marks for a long period of time.
For example, here is a blog from Morigenos (Slovenian Marine Mammal society) explaining how they use rake marks to help them identify different bottlenose dolphins from year to year. That the marks can fade or change as new marks are added. Noting these scars is a common tactic when using photo-identification to mark individuals when studying a dolphin population. However, because rake marks and other scaring can fade, it is far more common to use the back edge of the dorsal fin to identify different individuals (Clabby, American Scientist).
Risso's dolphins, however, loose pigmentation in their skin while their wounds are healing (MacLeod, 1998), meaning that their scars last for a longer time period. So much so, that individuals within a population can be recognized up to 3 years based on their scars alone (Mariani et al.,2016).
Why is this? Likely due to male on male competition. Younger Risso's have a faster scar recovery rate, indicating that the more scars you have, the beefier you are and the more combats you have survived, making you a better mate. However, the skin pigmentation alteration is present in both females and males.
The Mariani paper really does a great job of summarizing what is known about Risso's dolphin marks, and how long they last. It's freely available online, so it is 100% worth the read.
Monica Mariani, Angelo Miragliuolo, Barbara Mussi, Giovanni F. Russo, Giandomenico Ardizzone, Daniela S. Pace, Analysis of the natural markings of Risso’s dolphins (Grampus griseus) in the central Mediterranean Sea, Journal of Mammalogy, Volume 97, Issue 6, 5 December 2016, Pages 1512–1524, https://doi.org/10.1093/jmammal/gyw109