Sunday, June 27, 2021

Do Bull Sharks have Friends? New Paper!

Spoiler: maybe yes maybe not!

In brief, I opined that as their prey is comparatively scarce, evolution would select for large predatory Sharks to be solitary in order not to have to share their kills. 
But whereas that appears to be largely correct for Tigers and Whites, anecdotal evidence and some papers (and here) indicate that Bull Sharks appear to mostly travel in pairs or small groups, and I was eager to see whether our enormous data base could help shed further light on the issue. Yes our Shark feed is obviously an artificial aggregation - but I speculated that if certain individuals would preferentially turn up together, this might be a strong indicator that they might also be traveling together when not attending our dive.

The initial findings were promising.
The data showed some unequivocal, albeit rather weak long-term associations as illustrated e.g. in the following sociogram.

Sociogram depicting the social ties between individual bull sharks observed on the study site on ≥34 dives for the time period from 2011 (C). Only GAI values in the highest 30% were included to highlight the strongest associations between dyads, with thicker edges indicating higher GAIs for both individuals observed throughout the entire sampling period (red nodes), and individuals which were not observed throughout the entire sampling period (blue nodes) - click for detail.
Me too, initially - but then came the crux with the bloody interpretation!
Did those Sharks really turn up together because they like each other? Or was it because their core ranges overlap with the Shark Reef Marine Reserve = they just happen to live in the vicinity? Or because they share some behavioral trait that makes them bolder/more curious/more opportunistic/weaker hunters = more prone to visit our feed? Or are they maybe the remnants of a cohort of siblings who stayed together in the river nursery and have traveled together ever since? Questions questions!
Short answer: who knows - which is kinda disappointing!

But, we now have a starting point.
Those numbers in the sociogram all correspond to known individual Sharks, and somebody could now invest some time into observing how they actually interact during the dive - and assuming that the visual observations confirm the initial evidence, one could then try and equip selected individuals with adequate sensors like cameras and/or say, business card tags to see what happens once the Sharks leave Shark Reef and/or take tissue samples to determine their relatedness, etc.
Yes that's a lot of work - but that's what we do, so keep watching this space!

Long story short, so far so good!
Enjoy the new paper
PS - Juerg here, courtesy of Tom.
IMO the semantics are a lot of splitting of hairs (= do we KNOW that they're not friends?), the more as I believe that some are, and that some (like Blunt and Maite) are very much the contrary - but then again who am I to say!
So thanks for the publicity, much appreciated!

PPS - another really nice piece here - well done!

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