Hammerhead sharks’ unique traits may doom them

Hammerhead sharks are some of the most easily recognizable predators in the world, and are among the better-liked sharks. They were the most popular choice when researcher Austin Gallagher and colleagues asked citizen scientists to name their favorite shark. But despite their unique appearance and their ability to capture the imaginations of regular people – an impressive feat itself, for an animal so routinely feared – hammerhead populations are in decline throughout their historic range, thanks primarily to overexploitation and to bycatch.

As misunderstood apex predators, a quarter of shark species are in decline, but the big charismatic ones are faring the worst. In part, that’s because they’re the most valued species for commercial fisheries, but for many it is also thanks to their slow life histories. Each female hammerhead gives birth to only a few offspring over the course of her lifetime, and those offspring wait quite a few years before becoming sexually mature. But Gallagher, along with shark scientists Neil Hammerschlag, David Shiffman, and Sean T. Giery, argue that there’s more than life history that makes the hammerhead uniquely vulnerable to endangerment and extinction. The very features of hammerhead behavior, anatomy, and physiology that makes them so unique, and which initially evolved because they were at one time adaptive, may now be making them particularly susceptible to human-driven threats. Are hammerhead sharks “evolved for extinction?”

Among sharks, the three large hammerhead species (great hammerhead, scalloped hammerhead, and smooth hammerhead; there are other, smaller hammer-headed species) are especially encephalized; their brains are larger than expected compared to the size of their bodies. Their large brains may evolved as a result of a unique physiological trait in which the yolk sac of their fertilized eggs merges with the uterus to form a sort of proto-placenta, which nourishes the growing embryo. The more efficient delivery of energy and nutrition from mother to offspring is a necessary precondition for the development of larger brains, but the substantial investment in each individual offspring by the mother also means that she can bear fewer young throughout her lifetime. As apex predators, hammerheads have been able to get away with having fewer offspring for most of the 10-20 million years in which they’ve been swimming the earth’s oceans. But now, even low levels of fishing are capable of decimating the ability of hammerhead populations to sustain themselves.

Another of the hammerheads’ unique, evolved features is their schooling behavior. While many sharks do form aggregations, at least temporarily, the scalloped hammerhead is the only large-body shark species that is known to possess “highly organized and complex” social grouping, of more than 200 individuals. “Presumably this behavior is adaptive,” says Gallagher, “because it allows males to easily locate, court, and copulate with the largest and fittest females in the school.” But because human fisheries are aware of the predictable locations of these schools, scalloped hammerheads are especially vulnerable to targeted exploitation.

And then there’s the hammer itself. The hammer-shaped head allows the sharks to detect electric fields from farther away than other species, but it may have also caused their mouths to become smaller. And that, in turn, may impose a limit on their ability to efficiently pass water over their gills and take in the oxygen they need to breathe. Could that explain, at least in part, the hammerheads’ high mortality rate when immobilized due to bycatch?

Together, Gallagher and his colleagues argue that characteristics of highly specialized apex predators that may have been at one time adaptive are now actually making those species uniquely susceptible to declines thanks to human activities. While it is true that most large shark species are susceptible to exploitation and bycatch, Gallagher and his colleagues argue that the specialized ones, like hammerheads, fare even worse than more generalist species, like tiger sharks. In other words, the most specialized a species is, the more prone it is to population collapse. That’s because those traits have become detrimental, rather than just adaptively neutral. Still, “hammerhead sharks have the reproductive potential to recover from population depletion if mortality levels are decreased,” writes Gallagher.

One obvious solution is to reduce targeted exploitation and commercial fishing – a complicated notion, to say the least. In the meantime, there are at least easy ways to reduce the unintentional capture of hammerheads due to bycatch. Another obvious solution is to enclose known schooling areas as protected marine reserves. The hammerheads’ electrical detection abilities, thanks to their hammers, may also be co-opted to deter them from fishing lines. Recent research has shown that lanthanide metal hooks produce an electric field when in contact with seawater, and the sort of electric field they produce keeps hammerheads away. Scalloped hammerheads are less likely to catch themselves on lanthanide hooks than on standard ones.

Finally, their hammer-shaped heads may actually be their saving grace. It is their striking appearance, after all, that makes hammerheads relatively popular among the general public. Insofar as the public can be more motivated to care for the conservation of familiar, charismatic species, then the hammerheads’ strangely shaped faces may actually be their best hope for survival. – Jason G. Goldman | 25 June 2014

Source: Gallagher A.J., D. S. Shiffman & S. T. Giery (2014). Evolved for Extinction: The Cost and Conservation Implications of Specialization in Hammerhead Sharks, BioScience, DOI: http://dx.doi.org/10.1093/biosci/biu071

Header image: Copyright Tom Burns, used with permission.