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Tiger sharks that interact with tourists exhibit physical differences

The Tiger Beach in Bahamas is famous not only for its pristine beauty, but also for being frequented by an animal that might scare some people, but is in fact a major tourist attraction: the Tiger shark (Galeocerdo cuvier). Since the sea is crystal clear and only five meters deep, the sharks – which often surpass three meters in length – can be seen easily, and are often drawn to the site by local tour operators who throw fish and other food in the water.

Now, a team of researchers from Brazil and the United States has discovered that females of the species which frequently visit the area are larger and have higher hormone levels than conspecifics that spend less time there. These findings point to possible effects of mass tourism on this species.

“The area was dominated by large females, some of them pregnant. Generally speaking, hormone levels were higher in female sharks that frequented the area, where they were fed, than in others that didn’t interact much with divers. Moreover, the former’s nutritional state was better, and they had more omega-3 in their blood,” said study lead author Bianca Rigel, an expert in Biosciences at the University of São Paulo.

“We can’t say whether tourism is or isn’t harming these animals, as we were unable to collect material for testing before and after interaction with divers, which would have been ideal. However, we now have a body of evidence that will be helpful for future evaluations,” added study co-author, a professor of Biology at the same university.

By collecting blood samples from 33 sharks captured in this area, the researchers discovered that females that spent more time there had higher levels of omega-3 and other acids, as well as nitrogen isotopes – most probably due to the foods they were given by tour operators. Moreover, their hormone levels were also strikingly higher compared to those of sharks that were not frequenting the area – by a factor of three in the case of testosterone, four in that of estradiol, and 16.4 in that of corticosteroids. 

“We don’t know exactly why this was. Levels of these hormones might have been higher as part of their more aggressive social dominance behavior while they were swimming with many other sharks,” Moreira explained. “Another hypothesis is that they were at a stage in their life cycle when they were ready to reproduce. Juveniles haven’t reached reproductive age and naturally have less of these hormones.” 

Although the reasons for these physical alterations are not yet clear, these findings stress the importance of considering life cycle stages, hormone levels, and nutritional states in assessing the impact of tourism on sharks.

The study is published in the journal Animal Behavior

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By Andrei Ionescu, Staff Writer

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