NEW STUDY BY MORIGENOS ON PECULIAR SOCIAL PARTITIONING IN A DOLPHIN SOCIETY

A new study published in the journal Marine Biology investigated the social network of dolphins in the northern Adriatic Sea. It showed that dolphins living in the Gulf of Trieste form distinct social groups and some of these groups don’t seem to like to talk to each other.

It is widely known that dolphins usually occur in groups. In the case of common bottlenose dolphins (Tursiops truncatus), group composition often changes, with animals joining or leaving the groups. But these groups are not random. Individual dolphins prefer to spend time with particular other dolphins, which could sometimes be described as their “best friends”.

We investigated the social network of dolphins living in the Gulf of Trieste (northern Adriatic Sea) over 9 years. We discovered something quite remarkable. It turned out that the resident dolphin society is composed of three distinct social groups: two large social groups with stable membership and long-lasting friendships, and a smaller third social group, nicknamed “freelancers”, with much weaker bonds and no particularly long-lasting friendships. But this isn’t the remarkable part yet. It turned out that the two large social groups seem to avoid each other most of the time. However, instead of than having different “territories”, they actually overlap in space – but not in time. In other words, we found that dolphins share at least some part of their home range, but they use it at different times of day. This pattern was so persistent through the years that we internally started referring to these two social groups as “morning group” and “evening group”. Such temporal partitioning based on time of day has not previously been documented in whales and dolphins, nor in other mammals it seems. The “freelancers” displayed no such pattern.

“We were quite surprised by this” said Tilen Genov from Morigenos, the lead author of the study and a PhD student at the Sea Mammal Research Unit, University of St Andrews, UK. “It is not uncommon for dolphin social groups to segregate in space, but here they segregate in time. It appeared a bit unusual.”

“We still don’t know the entire extent of their ranging patterns, so it is possible that their ranging patterns differ overall. But we do know they overlap in at least part of their range, and they seem to share it by sticking to particular times. We would sometimes even see one group in the morning, and then another group in the same area in the late afternoon of the same day.”

Interestingly, the two social groups also differed in ways they interact with fisheries, as one regularly interacted with trawlers, while the other did not (“trawler” vs. “non-trawler” dolphins). Dolphins therefore employ different strategies when it comes to obtaining food. Previous studies elsewhere have shown that such tactics are learned and passed on from mothers to young. So the next logical question was: Are differences in fishery-related behaviour affecting the segregation patterns? Apparently not. Even when taking fishery-related behaviour into account, this failed to explain the time-of-day segregation.

It remains unknown what the reasons for these differences are. Both social groups contain both males and females, so segregation is not dependent on the sex of the animals. There may be genetic factors (dolphins within social groups may be close relatives) or there may be diet differences, which would partly explain why some dolphins follow fishing boats and others do not. All this is the topic of further investigation, currently ongoing. However, a study published by Morigenos just last week showed that these dolphins are all equally contaminated with PCBs, toxic man-made chemicals, regardless of potential differences in what they eat.

This study demonstrates how different segments of the same animal population may behave very differently and have differing effects on human activities such as fishing. In turn, they may respond differently to human impacts, as temporal partitioning may make animals either more or less vulnerable certain types of disturbance. This study also showed that groups are more stable than is usually the case for this species, and that bottlenose dolphin social structure may be more variable than was previously assumed.

Morigenos has been studying dolphins in the Gulf of Trieste and adjacent waters of the northern Adriatic Sea since 2002, looking at their population size and distribution, behaviour, social and genetic structure, and the effects of human activities on them.

The paper is available open access at the following link: https://doi.org/10.1007/s00227-018-3450-8