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.
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