The Erotic Endurance of Whale Hips

Buried deep within the body of a whale, underneath the heaps of muscles and tendons, lie some little, lonely bones. They are whale hips–and they are one of the stranger examples of evolution’s transforming power. Perhaps kinkier is a better word.

Some 54 million years ago, the ancestors of whales and dolphins were four-legged mammals. Their anatomy was well-adapted for moving around on land, including their hips. Here, for example, is an ancient member of the whale lineage, called Indohyus. The hip bones of these early whale relatives had scoops where the balls of their femurs could be tucked away. They had shelves where leg muscles could anchor. While the hips themselves were made up of a cluster of bones, they were fused together, and they were also joined tightly to the spine. Those firm connections allowed the animal to hold its body up against gravity, and use the forces generated by its legs to propel its body forward.

Indohyus reconstruction. Thewissen et al. Nature 2007. 450, 1190-1194
Indohyus reconstruction. Thewissen et al. Nature 2007. 450, 1190-1194

Over the course of about ten million years, the ancestors of today’s whales moved into the water. They evolved seal-like bodies with stout limbs; later, their forelegs became flippers and their hind legs dwindled away. They lost their fur and their nostrils migrated from the tip of their head to above their eyes, where it became a blow hole. (I wrote about this transition in my book At the Water’s Edge.)

By 40 million years ago, the walking whales were long gone. In their place were species like Dorudon atrox. As you can see from this diagram, its body looks a lot like a living whale. And like a living whale, its hips have shrunk and become separate from its spine. Unlike today’s whales, however, Dorudon still had well-developed hind legs–albeit tiny ones.

Dorudon. From M. Uhen, Annu. Rev. Earth Planet. Sci. 2010. 38:189–219
Dorudon. From M. Uhen, Annu. Rev. Earth Planet. Sci. 2010. 38:189–219

Within a few million years, those leg bones were pretty much gone, too. The diagram below shows a series of hip bones from the whale lineage, going from terrestrial species (Indohyus and Pakicetus) to more aquatic ones (Ambulocetus) to totally aquatic (Basilosaurus, which looked a lot like Dorudon) to living whales.

Whale pelvic bones. From Thewissen et al 2009, Bioscience. dx.doi.org/10.1007/s12052-009-0135-2
Whale pelvic bones. From Thewissen et al 2009, Bioscience. dx.doi.org/10.1007/s12052-009-0135-2

All that remained were a tiny set of hip bones. They lay far away from the rest of the skeleton. They no longer fused to each other, and they no longer had the distinctive scoops and shelves that used to be essential for walking.
When we see whale hips at the end of this long evolutionary history, they make more sense. They are vestiges of the terrestrial history of whales. But the fact that they still linger is also puzzling. If hips were adaptations for a vanished life, then why haven’t they vanished altogether?

A new study in the journal Evolution helps to answer that question. Far from being abandoned by evolution, whale hips are still evolving today. While they may not be essential for walking, they still matter a lot to whales. (A quick editorial note: this post gets NSFW from here on out.)

To see why, we have to go back to those hips of land mammals. They are important for walking on land, but they serve other purposes, too. Among other things, they anchor muscles that control the sex organs. If these muscles are anesthetized in men, for example, they have a hard time gaining an erection.

As whale hips stopped mattering to walking, they didn’t stop mattering to having sex. In male whales, the pelvis controls the penis with an especially elaborate set of muscles. In some whale and dolphin species, these muscles make the penis  downright prehensile.

Dines et al, Evolution in press
Dines et al, Evolution in press

Jim Dines of the Natural History Museum of Los Angeles and his colleagues have recently been studying how the sex life of whales drives the evolution of their hips. If a male animal can fertilize more eggs than other males, his genes may become more common over the generations. This process–known as sexual selection–can lead to all sorts of baroque adaptations in animals. Dines and his colleagues wondered if whale hips are also experiencing sexual selection.

Sexual selection gets stronger as the competition between males gets more intense. In some species, males fight battle for the opportunity to mate with females, and they often evolve big weapons like horns or oversized claws. In some species, the competition takes place inside the females, because a single female may mate with several males. Any strategy that lets one male’s sperm do better than another’s may become more common.

Females can make sexual selection even more intense. In some species, they have evolved elaborate reproductive organs that let them choose which male’s sperm she will fertilize her eggs with. Those female adaptations may drive the evolution of even more elaborate male organs that can overcome them.

One of the best ways to see sexual selection in action is to compare different species. As I wrote on the Loom a few years back, different species of ducks and other water fowl have huge penises and equally huge reproductive tracts. The longer the penis, the more maze-like the reproductive tract. This pattern suggests the birds are trapped in a sexual arms race.

Another way for males to increase their success is to produce more sperm, so as to overwhelm the competition. Scientists have tested this possibility by comparing primate species where males compete a lot with each other to species where there is very little competition–in other words, where the primates are monogamous. They’ve found that in the more promiscuous species, males have bigger testicles. (We humans have moderately large testicles, suggesting we’ve experienced moderate sperm competition.)

Dines and his colleagues decided to take a similar approach to whales and dolphins. They studied pelvic bones from 29 different species, and compared their dimensions to their mating systems. Some of the species the scientists looked at, like the franciscana dolphin, are monogamous. Other species are more promiscuous. Marine biologists once observed two male Northern Right whales mating with a female at the same time, for example.

A pattern emerged from their analysis–the kind of pattern you’d expect from sexual selection’s fingerprints. The more promiscuous a species was, the bigger its pelvis bones tended to be. The scientists also found that as whales evolved to become more promiscuous, their pelvic bones changed shape. These changes weren’t part of some general change to their skeleton, however. The ribs near the hips didn’t show the same patterns of size and shape change.

Dines and his colleagues can’t say what the change in the shape and size of pelvic bone does to a whale. That would require a level of intimate observation of whale sex that is simply impossible. But they have been able to get some clues by looking at other parts of male whale sexual anatomy. The whales with big hip bones also tended to have big testicles and big penises. This pattern may mean that hip bones are evolving as part of a bigger system. Whales with more competition may be using bigger hip bones to control a longer penis to deliver more sperm to females. (The new study only considers how whale hips may be selected in males. That doesn’t necessarily mean they have no function in female whales, which have hips too. Or perhaps they are the female equivalent of male nipples, carried along for the ride. For now, the subject is too mysterious for scientists to say anything firm about it.)

“Far from being mere relics of a terrestrial past,” Dines and his colleagues conclude, “cetacean pelvic bones are targets of sexual selection.” In fact, the only reason that we can still see these strange vestiges may be that they still matter to evolution, and in the most intimate way imaginable.

[Update: Added sentences about hips in females.]

20 thoughts on “The Erotic Endurance of Whale Hips

  1. That’s a nicely written piece Carl. I wish that evolutionary scientists would spend more time on the somewhat murky topic of vestigial evolution – that attrition through neglect that seems so pervasive. One undergraduate wrote that humans became hairless because clothing made hair irrelevant. Probably not – but our appendix has probably shrunk to a small wormlet because cooking made digestion easy, and made this digestive sac irrelevant. But why do horses still pop out a vestigial toenail on what for us would be the inside of our knees and elbows? Why, when things are 98% gone, do they hang on and fail quite to disappear? In the case of whale pelvii (?;-) it is clear that they’re only vestigial in the ambulatory sense. Anything controlling the anchoring and leverage of the penis – that’s a structure where failure is not an option.

    [CZ: There are certainly a lot of ways that things can evolve into vestiges. A pseudogene that produces no protein or functional RNA molecule doesn’t stick around because it still has a function, but because it hasn’t been deleted yet. And speaking of whales, they still develop hind legs as embryos, but those legs disappear quickly. So the developmental program is still there, but only as a vestige.]

  2. “If a male animal can fertilize more eggs than other males, his genes may become more common over the generations. This process–known as sexual selection–can lead to all sorts of baroque adaptations in animals. ”

    Sexual selection is about being better at securing mates. Being able to fertilize more eggs does not need to be sexual selection. It can be because the male is more fecund (producing more sperm), for example.

    Sexual selection is mostly divided into two components: “female choice”, in which males make themselves more attractive to the females than other males, and “male-male competition”, where males (or the less limited sex) compete for the chance to mate.

    http://en.wikipedia.org/wiki/Sexual_selection

    So in the quote

    “Far from being mere relics of a terrestrial past,” Dines and his colleagues conclude, “cetacean pelvic bones are targets of sexual selection.”

    it really should just be called “selection”.

    Schmemantics, I know…

    [CZ: Um…well, I guess the only thing I’d say is that I was quoting the scientists themselves, who somehow managed to get that phrasing into the premiere scientific journal about evolution.]

  3. “But why do horses still pop out a vestigial toenail on what for us would be the inside of our knees and elbows?”

    No it wouldn’t. Those are not vestiges but atavisms, and they appear where they originally did, which does not correspond to our knees and elbows.

  4. Re horse’s “vestigial toenails” —- I think you’re thinking that the “chestnuts” on the inside of the legs of domestic horses are some sort of left over limb stub. That’s a common misconception in popular books on horses, but it’s not true — they appear to be some sort of scent gland. Whatever, nothing to do with the limb elements.

    However, another question is why do horses retain the “side toes” — i.e., the proximal portions of metapodials 2 and 4? This is probably because they still play a role in the articulation of the wrist and ankle joint with the lower part of the foot. No need to retain the whole toe, but keep the top bits as they’re part of a functional joint.

  5. Ok, I stand corrected on the semantics. From a colleague (Jason Keagy):

    “The main evidence they have is a correlation between pelvic bone size and shape and testes size and mating system which implies that there is some link with the intensity of sperm competition. I didn’t read the entire paper, but I have seen videos of whale sex and the penises look and behave like elephant trunks. It’s possible that the pelvis is necessary to anchor the muscles used to move the penis and push away other male penises. That would qualify as sexual selection. I think anything related to genitalia that also increases access to eggs would qualify as sexual selection.”

  6. It seems to me that the conclusion of the observations of Jim Dines are weak. How cancorrelation between pelvic bones size and promiscuity in mating apply to Harbor porpoises? what I know of their mating habits is that they compete with the quantity of sperm produced and strength of ejaculation. They have gigantic testes(among the biggest relative to size), a huge prostate gland and… very tiny hip bones…

  7. I wonder about an analogy with the transition of jawbones to earbones in other mammals. Might whale hips function as vibration (contact) sensors? After all a whale can’t see the point of penetration!

  8. Great article; I really enoyed it. Thanks for taking the time to put your thoughts and knowledge into print.

    Ray

  9. That muscles for sex organs are anchored on whale pelvic girdle rudiments has been known for a while: I learned about it from, of all places, a Creationist tract! (Evolutionists point to vestigial organs as evidence of evolution, the tract argued, but since the supposed vestige have real, present-day, functions, they aren’t really vestigial, so the evolutionists’ supposed explanation is not required…)

    One of life’s little ironies: turns out that, even if the rudimentary hip bones have a function, evidence of evolutionary processes can be found in examining those functions (correlation of hip-rudiment size with mating system). So: evolutionary theory is a progressive research program in a way that Creation Science isn’t.

  10. Scientists have tested this possibility by comparing primate species where males compete a lot with each other to species where there is very little competition–in other words, where the primates are monogamous.

    A small nitpick. Little sexual competition in breeding males does not imply monogamy. The prototypical primate with low male sexual competition in breeding and small testicles would be the gorilla. It is polygynous rather than monogamous but since only one male will be mating with the females of a given harem at any one time there is no male sexual competition.

  11. My 10th grade Biology students have a question. They wonder why it is impossible to observe whale sex intimately.

    Thanks!

    [Dear Joe: Just think about blue whales wandering the length of an ocean, spending most of their time underwater. They spend only a tiny fraction of that time actually mating. Now imagine trying to catch them in the act!]

    1. You could watch whales mate, if you could find them doing that. There are many things in the world we have not found or seen simply because we aren’t there when the animal or the act occurs. Someday someone will accomplish that fete.

      And, there are many things still waiting to be discovered because they are too small to be seen without a microscope. While humans have learned so much in our short time on earth, there is so much we haven’t discovered it is mind boggling.

  12. Dr Emerson DVM
    Was the witty use of fete instead of feat at the end of paragraph one in your comment re male, whale hips your choice or Spellcheck’s?

  13. The right partner(s) indeed; tho, of course, after nearly 30 years of monogamy; I.e., marriage, the issue is hypothetical, or at best imaginary, for me. In any case. I suppose a better question might be, from the viewpoint(s) of the study, what characterized the whales’ fittest partners, as shown by the number & shape of their progeny?

  14. You need to re-read that article at evolutionnews if you came away with the impression that the author there doesn’t understand what “vestigial” means.

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