From the research notes of Dr. Khatin von Schorzberg:
It's an observation I have been pondering ever since I found about this project – for even the best-known fossil species, a vital aspect of their lives had eluded our grasp. Behaviour is almost entirely lost in the fossil record – just as the song of a whale, the midden of a bowerbird and the tail of a peacock could never be deduced from inanimate remains, many of the most spectacular sights of the past would never be seen – or even guessed – until now. How Marsh and Cope – even Darwin himself – would have sold their souls for this opportunity to observe the animals they discovered in the flesh.
What has inspired me to make this observation? It seems that mating season for the diplodocids in the Morrison section has arrived – and one of those spectacular sights is finally able to be scientifically observed. Males display vigorously for females – by rearing up on their hind legs and swaying their necks, tails and inflatable throat sacs (presumably an energy demanding stunt to prove strength).
Each female mates with several males – interestingly, proportionate to their position in the pecking order. Dominant females will have between up to five "consorts", whilst lower-ranking females must settle for one or two.
It seems that we have observed something comparatively rare (although not without precedent in archosaurs or even reptiles) – a polyandrous mating system, in which females mate with multiple males. In diplodocids, each herd seems to consist of a long-lived matriarch female, her close female relatives and a small group of loosely associated males.
Perhaps interestingly, diplodocid females appear to have an extended post-reproductive lifespan – a phenomenon rarely observed in animals and something previously unrecorded in archosaurs or even non-mammals. In every diplodocid herd I have observed, the older females seem to have no interest in mating – and the males seem to avoid courting them.
This is more likely to be behavioural, rather than any indication of reproductive cessation (as seen in humans and some cetaceans). Declining body condition would seem to be the most likely factor for this (as is - possibly - seen in elephants), but further study (combined with study in their home time) is needed to ascertain whether this is the case.
An important note – our matriarch females seem to be far older than the others – and all the females are, to varying degrees, genetically related. Some of these matriarchs may be at least a hundred years old. It is entirely possible that some of the younger animals may outlive the people who currently care for them… which is a humbling thought.
This is to go deeper into the spec-bio aspect… and attempt to redress possible criticism that the dinosaurs – especially the large herbivorous dinosaurs – were acting too mammal-like. Which was not at all what I had been intending.
The polyandrous reproductive behaviour is based on a rather unusual source – small, wading birds found in the tropics called jacanas. In these birds, females mate with multiple males, and each male guards a single nest. This was elaborated for the sauropods – basically, what a herd-living animal with this system might be like.
It's important to note that sauropods were a diverse bunch and lasted a long time – and there's evidence that adults and juveniles associated in some diplodocoids. So what is depicted here is at least within the realms of possibility.
The post-reproductive lifespan bit is rather more speculative – and is not seen in many species in the wild (it is fairly common in captive animals simply because they live longer). Despite the well-known examples of humans and orcas, elephants and chimpanzees both seem to display post-reproductive lifespan – although, especially in the case of elephants, this might be more due to declining body condition rather than physiological reproductive cessation.