Evolution, sex and dinosaur necks

BBC Nature Wonder Monkey Blog: Evolution, sex and dinosaur necks

They are among the largest and most fascinating creatures ever to have walked the Earth.

I’m talking about sauropods, the group of four legged dinosaurs that are almost instantly recognisable due to their long necks, each of which reaches out to a small head, and long tails.

Among the sauropods is the famous Diplodocus, and less well known, but even more remarkable species such as Argentinosaurus, which holds the record for being both the heaviest land animal ever, and the longest.

But what have these giants got to do with sex?

Well scientists are debating what exactly caused these huge reptiles to evolve their huge necks.

A recent theory proposed is that sex, or more accurately sexual selection, was the main driver.

The idea is that down the generations, male sauropods evolved ever longer necks to dominate rivals for the affections of females.

Dinosaurs are long dead, making it harder to test ideas about why certain traits evolved, and what they were adapted for. But evidence can still be brought to bear to analyse the different hypotheses.

For example, for much of the 20th century, sauropods were imagined to be water-loving beasts, which lived or spent much of their time in water, using their long necks as snorkels.

In the 1970s that idea fell into disrepute as multiple lines of evidence, since validated, showed sauropods to be mainly land-going animals.

That then led palaeontologists to imagine that sauropods used their long necks to reach huge amounts of vegetation – enough to yield the energy needed by their huge bodies.

A long neck, the reasoning goes, enabled Argentinosaurus and its ilk to graze plant material from a large “envelope”, from ground grasses to leaves in trees many metres high.

But then along came the sexual selection hypothesis, first proposed in 2006.

It argues that male sauropods that inherited a longer neck, caused by a chance mutation, would be more attractive to females.

The length of their neck would signal their virility and suitability as a sire.

Neck thumpers (image: Arup Shah / NPL)
A long neck could also have been used to wrestle competitor males, dominating them, just as male giraffes often joust by “necking" and "head clubbing" one another, with males with the longest necks and heaviest heads tending to win. Galapagos tortoises may also use the length of their necks to establish dominance.

Long necked males should therefore sire more offspring, on average, and pass down the long-necked genes, driving the trait through the population.

Just as palaeontologists argued over whether sauropods were terrestrial or aquatic beasts, they are now debating the merits of whether sexual selection or eating vegetation explains the long neck of the Diplodocus and others.

And the sexual selection idea has just been examined in detail, and dismissed.

Dr Mike Taylor of the University of Bristol and colleagues tested the arguments put forward to support the idea, and found them wanting.

Firstly, they say there is no evidence in the fossil record of a sauropod species that has males with relatively longer necks than females, or visa versa, which would be expected if it was a "sexy" trait.

While is impossible to witness whether extinct dinosaurs “necked” as giraffes do, their fossilised bones suggest they did not – they do not become any thicker to resist the blows, which would be expected, or show any signs of trauma associated with such behaviour.

There are a host of other more technical reasons for why a long neck wasn’t a sexy neck, Taylor and colleagues describe in the Journal of Zoology.

Their arguments are pretty convincing.

Such debates occur more often than you might expect, at least when it comes to sexually selected traits.

An eye for the females? (image: Phillipe Clement / NPL)
For example, there is still no firm agreement as to whether female peacocks find the elaborate trains of male peacocks attractive (length and eyespot number play a role, though precisely what is unclear).

There have also been similar debates about why giraffes have such long necks.

Indeed the proposal that the giraffe evolved its long neck as a sexual signal led to the proposal that sauropods do similar.

But in 2009, that hypothesis finally bit the dust after Professor Graham Mitchell of the University of Wyoming, in Laramie, US, and colleagues put the sexual selection hypothesis to the test by examining 17 male and 21 female giraffes.

They found if long necks were a sexually selected trait, they expected to find a number of things:

Long necks should be more exaggerated in males than females.

They should evolve to be bigger in size more than other parts of a giraffe's body.

They should confer no immediate benefit to survival, and may come at a cost.

Their results didn't support any of these propositions.

This refutation is similar to that by Dr Taylor and his colleagues. Sauropod necks aren’t more exaggerated in males than females, and they aren’t particularly costly.

Dr Taylor’s team make one final point.

There is no example, anywhere, of a type of four-legged animal, of which there are many species, that has evolved a single trait to be sexy. Crabs evolve big claws to show off, some flies evolve giant eye stalks, birds of paradise shake their sexy tail feathers.

But dinosaurs? Not likely it seems.

A sexy neck just didn’t get the reptilian juices flowing.