It's Shark Week, and the focus as usual has been on the predatory powers of these mighty hunters of the deep. But far more impressive than how sharks down their prey, is how they make the next generation of sharks. With by far one of—if not THE most—diverse range of sexual strategies of any group of animals on the planet, here's a list of six shark sex facts to really leave your jaws dropped:
1. Female sharks have fertilization-on-demand:
Many female sharks don't have to worry about getting knocked up after a one night stand. Instead, they can store up the sperm and get pregnant later—up to nearly four years later—when the mood suits them. Such sperm storage may also allow females who mate with multiple males over time to "pick" which male's sperm they ultimately decide to draw upon—perhaps by flushing out older sperm for a newer, more attractive mate's, through sperm competition, or other means.
2. Doubly-endowed (and proud?) males
Male sharks have two members with which to conduct their business. As phallus-shaped as they come, these "claspers" are more like a rolled up taco than a mammal penis, formed from an elongation and then curling of the inner margin of the pelvic fins. Baby males are born with an obvious pair, which get thicker, longer, and firmer with age.
3. Forget sex toys, sharks have sex teeth
Love bites in sharks pack a punch. Especially for those species where males grow specialized "mating teeth" which they use to bite down and hold onto a female's pec fin. These sharpened denticles occur only in males, and only during mating season. Females, in return, have evolved thicker skin that than their male counterparts. It's an evolutionary arms race between biter and bitten, denticles vs dermal denticles.
4. Hydro-powered ejaculation. Really.
As noted, shark claspers are just a curled up fin. As such, they lack an internal tube, like a urethra in mammals, that serves as a conduit to transport sperm from testes to tip. Without help, shark sperm would just kind of ooze out a small pore located at the base of the anal fin, and trickle down the clasper and into the female—not a very effective (or impressive) means of getting sperm up inside a mate. Good thing the ocean's got hydropower. By using muscular contractions, male sharks can pull seawater in through a small opening that then fills an internal siphon sac along their belly, inflating it like a balloon. Then, the same muscles can squeeze down on this sac just as they ejaculate, sending a stream of water that flushes the sperm down the clasper. The technique turns a would-be drip irrigation system into a firehose. The fantastic photographer Tony Wu has a few great images of white-spotted bamboo sharks mating, the siphon sac of the male swelled up like a giant goiter.
5. Virgin Birth
With all the risk that rough sex brings, its no wonder some female sharks shirk the whole mating scene and simply reproduce on their own—sans male. Known as parthenogenesis, it's a form of reproduction whereby the offspring inherits only DNA from its mother. Different than asexual cloning, parthenogenesis does result in less mixing of genetic material than sex between an egg and sperm would provide. The technique may help endangered shark species survive in the wild, when females have trouble finding mates; but too much solitary conception may lead to reduced genetic diversity in the population and skewed sex ratios—not good for long-term survival of a species.
6. Sibling rivalry as strategic sex strategy?
In one of the freakiest reproductive strategies on the planet, female sand tiger sharks carry litters of multiple pups in their two wombs—which serve as battle grounds for pre-natal fratricide. In each uterus, the largest fetus breaks free of its sac, and swims around killing and then gorging on the remains of its siblings, which you can watch on video here. Gruesome, but a potentially winning strategy not just for the alpha embryo, but also for the female. Fascinating work by Dr. Demian Chapman reveals that often (more often than expected by chance), the two winning pups—one from each uterus—are full siblings, fathered by the same male. The theory is this: less-fortunate, and likely later males who mate with the female after she already had sex with the first male, wind up siring nothing more than food for the first male's offspring. This strategy could allow females to "acquiesce" to sex by multiple males after an initial mating, without having to worry about the quality of the later suitors. She can save energy normally spent on mate avoidance, without having the risk of fertilization and developing inferior offspring.
Shark sex: making predation seem rather boring for millions of years. Am I right?