February 10, 2012
Yesterday I wrote about the possible mating mechanics of immense sauropod dinosaurs such as Brachiosaurus and Argentinosaurus. But there’s more to mating than the act itself. It is not as if two Diplodocus nonchalantly walked up to each other, had a quickie, and plodded off to feed on a nearby patch of ferns. There was probably some kind of behavioral lead-up to copulation—a way for one sex to strut its stuff and the other to be choosy about a mating partner. With this in mind, one paleontologist proposed that sex might hold the secret of why sauropods evolved such long, gorgeous necks.
The idea that mating behavior might have something to do with sauropod anatomy was inspired by giraffes. Scientists have been puzzling over why giraffes have such spectacular necks for over a century and a half. The most popular notion is that the long necks of the mammals are an adaptation for feeding high up in the trees where competing herbivores can’t reach, but in 1996 zoologists Robert Simmons and Lue Scheepers proposed something different.
Male giraffes fight each other in a peculiar form of combat called “necking.” It’s not as nice as it sounds. Male giraffes swing their long necks to batter each other with the stout ossicones on the tops of their heads. These bouts determine hierarchies among males, and dominant males mate more often than subordinate ones do. Since males with bigger, stronger necks would seem most likely to win the contests, Simmons and Scheepers argued, those males are more likely to pass on their traits to the next generation, and therefore necking might have been the reason why giraffes evolved longer necks. Female giraffes just happened to get evolutionarily carried along even though they don’t engage in the same behavior.
The “necks for sex” hypothesis has been controversial from the start. At present, the weight of the data supports the idea that giraffe necks primarily evolved as a way to sample a wide range of food, not as a weapon involved in battles for mating rights. Studies since 1996 have indicated that long necks really do help giraffes avoid competition with other species for the most nutritious food by going higher up, especially when food may be scarce, and studies of fossil giraffes hint that long necks may have began to evolve in response to changes involved with the spread of grasslands around 14 million years ago. Still, the idea proposed by Simmons and Scheepers has remained a sexy hypothesis, and in 2006 paleontologist Phil Senter applied the idea to Apatosaurus and kin in a paper called “Necks for sex: sexual selection as an explanation for sauropod dinosaur neck elongation.”
Without living sauropods to study, Senter proposed six predictions for what a sexually-selected sauropod feature would look like. For example, on the basis of previous theoretical work, Senter suggested that a feature that was primarily used for display or mate competition would provide no benefit to the survival of the animal and might, in fact, be a risk. In the case of sauropods, Senter argued that the long necks of sauropods would not have provided the dinosaurs with any major advantage over other herbivores in terms of accessing food. Just as Simmons and Scheepers proposed that the long necks of giraffes did not provide a feeding benefit, Senter suggested the same for dinosaurs like Camarasaurus. As a corollary to that, Senter also pointed out that predatory dinosaurs must have targeted the long necks of sauropods to quickly bring down the giants. “The evolution of more neck, and hence more vulnerability to a fatal bite, therefore incurred a survival cost for all but the longest-limbed sauropods [which would have carried their necks higher off the ground],” Senter wrote.
Senter only briefly entertained how fancy, flashy sauropod necks might have been involved in dinosaur mating behavior. Maybe males smacked necks when fighting for territory, or perhaps competitors simply eyed each other to see whose neck was bigger. There was no way to tell. Overall, though, Senter believed that the necks of sauropods were more consistent with what would be expected for a sexually-selected feature than an adaptation for feeding.
However, in a paper published last year, paleontologists Mike Taylor, Dave Hone, Matt Wedel and Darren Naish refuted Senter’s arguments. The long necks of sauropod dinosaurs certainly could have provided survival benefits, particularly in terms of accessing high-quality foods that were beyond the reach of smaller herbivores. Senter had assumed that sauropods held their heads low to the ground and therefore would not have been capable of much vertical reach, but there is osteological evidence to the contrary. Sauropods were physically capable of holding their heads high, and may have favored an elevated neck posture most of the time. More than that, the exceptionally long necks of many sauropods would have provided an energetic benefit by allowing the animals to stand in one place while sampling food over a wide range.
Senter also overstated the vulnerability of dinosaurs like Barosaurus to attack. As Taylor and co-authors pointed out:
The [sauropod] neck was not simply a mass of external blood vessels and nerves, but was constructed from tough elements including the often robust cervical ribs, bony laminae, ligaments and tendons. A theropod could hardly dispatch a moving apatosaur with one swift bite, and a raised neck would further reduce vulnerability.
That is assuming that predatory dinosaurs hunted adult animals at all. Like many modern predators, Mesozoic hunters like Allosaurus and Torvosaurus probably targeted young, relatively small sauropods more frequently.
Underlying all of this, though, was a conceptual flaw in taking a hypothesis proposed for one species—the evolution of necks for sex in giraffes—and applying it to a disparate, wide-ranging, and long-lived clade of vertebrates. If the long necks of sauropods were so costly to evolve and provided no significant survival benefit, then why did so many dinosaurs retain this feature for so long? Taylor and co-authors summarized the rhetorical flaw this way:
If the long necks of sauropods had negative survival value, their retention across the whole clade is analogous to a hypothetical situation where the maladaptively long tails of birds-of-paradise are found throughout Passeriformes [perching birds, about half of known bird species], or where the enormous antlers of the Irish Elk Megaloceros are ubiquitous in Artiodactyla [even-toed, hoofed mammals and descendant forms, including whales].
The proportionally long necks of sauropods must have had some adaptive advantage for the trait to be so widespread and persistent. This doesn’t mean that sauropod necks were only used for feeding, though. As Taylor and co-authors pointed out, traits used in mate competition may also provide survival benefits. As the researchers stated, “It remains possible that the sauropod neck originally arose either as a sexually selected feature or to help gather food, but it cannot be demonstrated that the necks remained monofunctional throughout their evolution, or that they could not be co-opted for a secondary function.” The neck of the giraffe is a perfect example. Male giraffes swing their necks in competition, but their long necks have also been shown to provide them with a competitive edge when it comes to reaching food resources other herbivores just can’t exploit. The question is which impetus was more important in the trait’s evolution.
For sauropod dinosaurs, feeding ecology was more important than sexual selection in the evolution of long necks. But once long necks had evolved, who knows how they might have been used for communication and display? Such prominent necks would have been elongated, fleshy billboards which could very well have been used to establish dominance, attract mates, or otherwise advertise an individual’s prominence. If adult sauropods were too big to be harried by predators, and therefore did not require camouflage, would sauropods have developed bright, striking color patterns along their necks to gain the attention of potential mates and show off that they were the healthiest, sexiest dinosaurs around? Those are the kinds of questions that can keep a paleontologist up at night.
This post is the second in a short series of articles on dinosaur reproduction that will run through Valentine’s Day. Because nothing spells romance like dinosaur sex.
Senter, P. (2006). Necks for sex: sexual selection as an explanation for sauropod dinosaur neck elongation Journal of Zoology, 271 (1), 45-53 DOI: 10.1111/j.1469-7998.2006.00197.x
Taylor, M., Hone, D., Wedel, M., & Naish, D. (2011). The long necks of sauropods did not evolve primarily through sexual selection Journal of Zoology, 285 (2), 150-161 DOI: 10.1111/j.1469-7998.2011.00824.x
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