Blood is Not Thicker than Water
Updated: Apr 27, 2021
It is a universal trait amongst all sharks that there is no "parental care"; mothers and fathers do not rear their offspring. Instead, they either lay eggs or give birth (depending on the species) and the newly born sharks (known as "neonates") must fend for themselves. The mothers are not completely thoughtless though; they take great care to leave their pups in an area with lots of food and few predators to give them the best chance of survival. These sites are known as "nursery habitats". During this phase of their lives, the juveniles live in aggregations with other members of their litter and other young sharks, but when they are big enough they head out into the big, wide world, to join the adult population. So do the sharks stay in touch with their litter-mates and other family members? Do they even know who they are? A research team recently sought to find out...
The team, based in French Polynesia, attempted to understand the group structure of blacktip reef sharks (Carcharhinus melanopterus). Blacktip reef sharks live in relatively large groups, generally in shallow waters around coral reefs. Their nursery areas are closer to shore, in shallower water still. These sharks were especially good for this kind of study, because they are non-migratory and spend their whole lives in one area around Moorea Island (Mourier & Planes, 2020)
The researchers watched the sharks and noted which individuals were often close to each other (less than 10 m apart). They were able to identify individual sharks based upon the unique pigmentation patterns of their fins (much in the same way that every human being has unique colours in their eyes).
They also took DNA samples from the sharks in order to determine which individuals were kin. They then used snazzy statistical analyses to determine whether these sharks hang-out with the same individuals a lot and whether they regularly interact with their genetic relations (Mourier & Planes, 2020).
Based on 105 individual sharks they observed, the researchers found that there was an extremely fine-scaled social structure in these sharks, but that these relationships were not with their kin. Sharks were not more likely to be associated with individuals they were related to compared to others. In fact, the genetic analysis suggested that there was a low probability that close-kin would interact (Mourier & Planes, 2020).
What was especially interesting though, was that this social network was dynamic... During the non-mating season, the community had five clearly defined groups (shown as different colours in the image) of different sharks (each individual shown as a spot on the image), which associated with each other regularly (shown as lines on the image). However, during the mating season, the five groups merged into three larger communities and each shark interacted with a higher number of individuals (aka became more "gregarious") (Mourier & Planes, 2020).
These different gangs spent time in different areas within the study site, with some overlap at the edges of their ranges (Mourier & Planes, 2020).
The scientists hypothesised that this spatial and social structure was likely caused by the lack of parental care. They stated it is possible that the juveniles might have had some affinity for their litter mates during their time in the nursery habitat, but as they grew and expanded their home range, they formed associations with individuals which are not genetically related. This suggests that these sharks do not have the ability to recognise their kin by sight or smell (Mourier & Planes, 2020).
The researchers suggested that having these stable social affinities during adulthood might have several benefits for blacktip reef sharks:
Hunting in groups might allow increased foraging success, as they can flush out more prey and block-in any fish which are trying to escape, within the school.
Grouping probably allows some protection from predators, as it can be more challenging to pick off one individual from the group (this is known as "the predator confusion effect"),
Group living might lead to reduced aggression, as the individuals learn to tolerate each other (Mourier & Planes, 2020).
They suggested that the group structure changed during the mating season, with increased gregariousness, as the sharks hunted for a suitable mate to reproduce with. Getting to know more individual sharks, would have increased their likelihood of successfully finding a partner whilst also reducing the likelihood that they accidentally reproduced with a genetic relative (known as "inbreeding"). Therefore, this behaviour is important for keeping the population healthy and thriving (Mourier & Planes, 2020).
Mourier J & Planes S (2020). Kinship does not predict the structure of a shark social network. bioRxiv. Access online.