Demon Fish (23 page)

“Biodiversity is based on genetic diversity,” Chapman explains. “Genetic diversity is like lottery tickets: you don’t know which one will win out in the future. The more diversity they have, the more chances they have to win, and the chance to survive for the next thousands and millions of years.”

After Chapman and Shivji’s paper came out, a rush of aquariums started reporting suspected virgin births in a wide range of shark species. The Belle Isle Aquarium in Detroit examined the babies that a white spotted bamboo shark appeared to produce on its own, and determined the two daughters were born to a mother that had never engaged in intercourse. European aquariums have reported that a zebra shark and a whitetip reef shark may have produced offspring single-handedly. In one case, a shark held in a tank within a Texas classroom seems to have engaged in parthenogenesis.

Chapman and other researchers have been able to confirm that this phenomenon has occurred in at least one other instance besides in Omaha and Detroit: a nine-year-old female blacktip shark that had lived in Virginia Aquarium’s Norfolk Canyon Aquarium died during a physical exam while under sedation, and it turned out the shark had been carrying an embryo despite having been separated from male blacktip sharks for at least eight years. Chapman, Shivji, and the Virginia Aquarium and Marine Science Center scientist Beth Firchau found that the shark, named Tidbit, had just reached sexual maturity and was carrying an embryo with no paternal DNA at the time of its death.
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This finding also challenged the scientific assumption that smaller sharks might be more inclined to engage in parthenogenesis because these species live in more isolated habitats and therefore may have trouble encountering males: the blacktip shark is a large species that migrates across the ocean.

At this point, Chapman thinks researchers should start out with the assumption that anytime a female shark produces offspring in the absence of a male, it’s likely to be parthenogenesis: “This form of reproduction is more common and widespread than anybody realized.” And it may not be as disturbing a discovery as Chapman first feared: the two Belle Island Aquarium virgin births were alive and thriving more than five years later, showing no signs that they were any less healthy than sharks produced by two parents.

The revelation also underscores another, broader point about sharks: nearly everything about their reproductive and parenting life is weird.

Male and female sharks don’t intermingle frequently, according to scientific surveys. And researchers are beginning to learn that the nitty-gritty details surrounding when they do spend time with each other—to have sex—are harsh. These revelations highlight a central fact about sharks: they cannot be anthropomorphized the way some other creatures have been. They are vastly different from humans in how they behave, and won’t ever warm the hearts of the public the way penguins can.

For centuries humans have recounted only the most fleeting observations of interactions between male and female sharks. While Aristotle might have composed the first written record of shark sex in the Western world, a fur seal observer with the New Zealand Department of Conservation evoked a similar theme thousands of years later. After witnessing an incident in 1991, A. Strachan wrote, “I have unwittingly been fortunate to witness a mating [between two white sharks]. I had thought at the beginning they were fighting as one animal appeared to be attempting to grasp the other with its great mouth, making great gouges in its side.”
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Many scientists don’t like to talk about shark sex, because they worry it will only reinforce the popular perception that these creatures are brutish and unrelenting. But one day I coax Chapman to give me a lecture on the subject, despite his reluctance. We are sitting in an idyllic setting—out on a dock in Belize looking at the Caribbean—and there are dozens of other things he’d obviously rather discuss. But I’m after the facts, and he obliges me.

Shark sex is, as Chapman puts it politely, “very rough.” Some of this reflects simple mechanics: male sharks have a pair of reproductive organs called claspers, which they insert into a female shark’s reproductive opening, or cloaca. (No matter how sharks gestate their young, they need to engage in internal fertilization in order to produce their offspring.) These claspers, which harden as a male becomes sexually mature, have tiny hooks inside them, which allow them to hold the female alongside as they’re mating. On top of that, during courting among larger sharks the male is usually biting the female to keep her around. This stems from the fact that, with a few exceptions, the female is almost always resisting the male’s advances. Marine biologists have an easy time determining if a female has been mating in the recent past because her skin will be raw and possibly bleeding. Female sharks build up defenses, to the extent they can, to cope with such a brutal coupling. The skin of most mature female sharks is measurably thicker than that of their male counterparts, and the fact that females tend to be larger also helps them withstand the beating they take during sex. Smaller shark species often mate by intertwining their bodies rather than the male dominating the coupling, a slightly less violent form of courtship.

When mating season rolls around, female sharks—at least those that have been observed mating, a rare event in itself—tend to stay in shallow water. This is one of the few ways they can exercise any form of mate choice, since female sharks can resist being pinned down in shallow water. “If they stay in deep water, what’s waiting for them is a roaming band of males,” Chapman explains. “If she’s in shallow water, it’s difficult to roll her over; she will press her cloaca against the bottom. The key is to get her into deep water.”

Once a female is cornered by a group of males in deeper water, they will take turns inserting their claspers in her. Often, a male shark will bite a female in order to grip her during mating. The end result? When a nurse shark gives birth to a litter of fifty pups, Chapman says, “what you’ll see is there’s anywhere from two to seven fathers.” Lemon sharks exhibit the same phenomenon: a litter of twenty pups often boasts several male parents.

Of course, Chapman knows this, for the most part, from DNA analysis rather than from firsthand observation. Researchers rarely get to witness this mating—though nurse sharks are better observed than other species when it comes to intercourse—but advances in genetic testing have expanded our understanding of this ancient ritual. As Chapman notes, “You can actually use genetics to know what mating patterns they have.”

One scientist has, most likely, viewed more shark sex than any other researcher in the world: Jeffrey Carrier, a marine biologist at Albion College in Michigan who conducts his research in the Dry Tortugas, a protected marine reserve that lies seventy-eight miles away from Key West. The animals his team has observed over the past two decades have demonstrated an amazing degree of fidelity to this site: the females return every two years, while the males come each year, during June. “We pretty much know who’s dating whom,” he explains. He’s acquired this level of detail by watching more than a thousand shark mating attempts, a formidable record.

In 2000, Carrier co-authored a paper with the Mote Marine Laboratory scientist Harold L. “Wes” Pratt Jr. in the journal
Environmental Biology of Fishes
with the deceptively bland title “A Review of Elasmobranch Reproductive Behavior with a Case Study on the Nurse Shark,
Ginglymostoma cirratum.
” It is the most definitive account of shark sex ever published, and it makes for fascinating reading.

Here are some of the facts Pratt and Carrier have to offer: among male sharks (as well as skates and rays) “biting or holding by some means appears to be universal.” Female blue sharks—which produce young at a much higher rate than other sharks—seem to get the worst of it, since they “receive so much precopulatory biting that they often appear to be severely wounded while on the seasonal mating grounds.”

One of the best sections in Pratt and Carrier’s article is the chart they’ve constructed detailing the fish’s “courtship and mating behaviors.” When a nurse shark wants to demonstrate “acceptance,” for example, “female arches body towards male, ‘cups’ pelvic fins.” If it’s “avoidance” she’s after, then the shark opts for “ ‘lying on back’ the female rests motionless and rigid.” The sand tiger shark even engages in “stalking,” according to the scientists, but at least it’s not aimed at females. In this case, the sharks target other species within a captive environment, just to ensure that no other animal interferes with their chances of hooking up.

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Carrier and his colleagues have studied their subjects’ intimate dating patterns through a device called Crittercam, whose name sums up its function quite nicely: you attach the camera to the critter, and when you retrieve the footage, you can see what it’s been doing. (Greg Marshall, the National Geographic scientist who invented Crittercam, actually did so after being inspired by watching a shark glide through the water in Belize with a small remora fish attached to its back. Remoras, which are sometimes nicknamed shark suckers, hitch rides on the large predators, and in exchange for getting some of a shark’s leftover food, they pick off some of the parasites that attach themselves to their host. Sharks show no signs that they’re even aware of the remoras’ presence, which is why scientists have modeled their video observation system on the tiny fish.) The Crittercam footage has captured the gang-bang phenomenon nurse sharks engage in, a process Carrier describes as “a cooperative venture, where one male is successful and the other males keep the female from leaving.” One of the most interesting aspects of this act is the fact that an implicit social order seems to determine who gets lucky.

“We think there’s a social overlay, a social hierarchy, in a way that’s startling,” Carrier says. As in the world of humans, lions, and other animal societies, it’s the dominant male nurse sharks that get the most play.

In the case of nurse sharks it’s the big, dark ones that reign supreme, according to Carrier and Pratt, which explains why females may shift from shallow to deeper water depending on the suitor. “The ‘refuging’ female is observed to retreat repeatedly from smaller, lighter colored males which are perhaps younger, weaker or inexperienced. The arrival of one of the larger, darker dominant males often elicits a response in the female to remain in the lagoon’s deeper water, or at least not to retreat when approached.”
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There is a nice inverse of this among hammerhead sharks, which gather in large schools each year to mate. Among that species, the dominant females occupy a prominent place in the center of these schools, pushing more submissive females to the periphery. The UC Davis professor Peter Klimley, who has swum with hammerhead sharks and filmed them while they’ve schooled, describes it as sort of a mix between the old television series
Wild Kingdom
and a traditional 1950s social mixer where young women are waiting for their male suitors to make the first move. On the one hand, the scars on the upper portion of female hammerheads’ bodies testify to the extent to which they fight each other over which sharks get to stay in the school’s inner sphere. “This is what happens a lot in the animal kingdom,” he says. “It takes one or two ritual combats to establish a hierarchy.” On the other hand, the male hammerheads that manage to get near the school push themselves into its center, casting aside the females on the periphery.

Once they make it in, moreover, male hammerheads perform elaborate moves to copulate with one of the dominant females. After pushing their way into the inner sanctum, they rotate their claspers so that they’re bent in half at a right angle and then throw their torso forward in what Klimley calls “a very sexual manner.” Other times, the sharks will conduct a corkscrew somersault. This acrobatic maneuver gives off flashes of light, which help attract attention.

While researchers know what happens when some sharks mate, that’s about where their understanding of shark sex ends. “That part’s not in doubt,” Carrier says of the mating attempts he’s witnessed. “We just don’t know what the hell’s happening afterwards.” After that point Carrier and his colleagues are no longer able to track the nurse sharks for months, until they return the following summer to repeat their courtship pattern once more.

But new technology now holds the promise of allowing scientists to move beyond what they’ve learned through Crittercam. Accelerometers—also known as acceleration data loggers—are the same motor-sensitive computer chips used in smart phones, iPods, and the Nintendo Wii, and they are providing unprecedented detail about how sharks move beneath the water. Accelerometer tags, which are a bit larger than AA batteries and are designed to fall off a shark’s dorsal fin, can detect every flick of the animal’s tail or tilt of its body. Nick Whitney, a postdoctoral scientist at Mote, worked with a team to fit four female nurse sharks with accelerometers in the Dry Tortugas and was able to document a range of mating behaviors, including thrashing, barrel rolls, and headstands. With any luck, Whitney and his colleagues will be able to attach the tags to more elusive species, such as hammerhead, sandbar, and bull sharks, in order to better grasp these animals’ mating patterns.

One thing researchers already understand is that the brutal competition that defines mating among sharks does not stop there. Once the eggs are fertilized, shark fetuses sometimes eat each other in utero, acting out the harshest form of sibling rivalry imaginable. To some extent this is merely an extension of the sort of competition for paternity that usually takes place outside the womb: Chapman has determined through genetic testing that while a sand tiger shark is often fertilized by multiple partners, during gestation the offspring of a single father will consume its half siblings in order to emerge as the sole survivor.

There is another upside to this embryonic cannibalism, or ovophagy, which occurs in both sand tiger and white sharks: it helps prepare young sharks for the difficult conditions they will face once they exit the womb. In the case of sand tiger litters only two babies emerge each time, one from each of the mother’s uteruses, since the surviving offspring has eaten everything from fertilized eggs to embryos inside the womb.