Introduction

Our picture of dinosaurs has changed incredibly over the past quarter century. Formerly portrayed as solitary sluggards -- stupid, overgrown reptiles lazing in steamy Mesozoic swamps -- now they leap and dance with endothermic exuberance. Moreover, evidence is accumulating that some dinosaurs were gregarious and had highly developed social behavior.

Evidence for this is of two types: paleontological information and neontological comparisons. Paleontological evidence may come from the relationship between morphology and function, from taphonomic information, or from ichnology. Inferences can also be made by comparing what is known about dinosaurs with behavior seen in their living relatives (crocodiles and birds), or mammals.

The taphonomic picture provides such information as whether or not the fossil animal is found in association with its cohorts or was transported downriver to the burial site; whether an assemblage of bones was formed by catastrophic or attritional mortality, and thus does or does not reflect the true age structure of the population; and whether diagenesis has taken a large or a small toll. It is important to understand what about the assemblage is characteristic of the population if one is attempting to understand the group dynamics of the animals represented. Ichnology is a branch of paleontology that has only recently begun to be tapped for its tremendous information content. Trace fossils are "snapshots of behavior". Such windows into the past are invaluable in attempts at reconstructing dinosaurian lifestyles.

Evidence for herding behavior

Many animals occur in groups. The distinction must be made between those which are truly gregarious and those which congregate through circumstance. Gregarious behavior is exhibited by animals which move together, as in flocks or herds. This differs from the association of frogs which live near each other in a pond because of habitat selection. Gregarious behavior connotes some type of social structure within the group of animals being considered.

Many footprint localities provide evidence of herding behavior in dinosaurs. A minimum of six iguanodontids travelling together is shown by the trackways on Dinosaur Ridge, near Morrison, Colorado. The Triassic rocks on Mt. Tom, near Holyoke, Massachusetts preserve 28 parallel trackways made by tridactyl bipeds. The likelihood of this many parallel trails occurring randomly is minuscule, and the rare trails which do not follow the trend preclude the possibility of a restricted corridor. Footprints of tridactyl bipeds are also preserved in Triassic beds near Culpeper, Virginia, and the configuration of the trackways has led to the suggestion that the herd travelled with adults on the flanks and the juveniles in the center for protection. It paints an intriguing picture: modern musk oxen are known to form a ring, with adults facing outward and young at the center, when they are threatened by predators. The Cretaceous Glen Rose limestone of central Texas contains 23 sub-parallel sauropod trails in the Davenport Ranch area. More than a hundred sauropod trackways are preserved at the Purgatory River tracksite in southeastern Colorado. These tell the tale of sauropod herds that passed on the mudflats bordering inland Jurassic waterways.

Monospecific bone associations also point to dinosaurs having congregated in herds. A coal mine near Bernissart, Belgium yielded over three dozen skeletons of Iguanodon. The Ghost Ranch quarry in northern New Mexico is famous for the large number of Coelophysis specimens found there. While these associations may be explained in other ways, such as congregation of individuals around water holes during a drought, some groups of dinosaur bones clearly indicate cooperative behavior. Such is the discovery of four Deinonychus skeletons with that of a single Tenontosaurus ten times their size at the Yale quarry in Montana, which suggests that the intended prey was able to knock out four pack members before succumbing itself. Of 58 occurrences of Tenontosaurus in the Cloverly Formation, 14 of them were associated with shed teeth or other evidence of Deinonychus, indicating that the herbivore was a preferred prey item of the small carnivore.

Displays for Courtship, Combat, and Species Recognition

An incredible variety of species-specific morphologies and behaviors has evolved among modern animals: witness the peacock's tail, the grouse's dance, the clashes of bighorn sheep and bull elk. They all serve the same purpose: to attract and win a mate, and thus assure the contribution of one's own genes to the next generation.

Species recognition is important so that gametes are not wasted on the production of infertile hybrids. Those behaviors and structures that confer and advantage in combat (with other males) or courtship (of the female) will be preferentially passed on to future generations; thus natural selection works to develop and perfect them. Reproductive isolating mechanisms are most needed where the largest number of closely related sympatric (cohabiting in time and space) species occur, and it is under such circumstances that the greatest development of horns, crests, etc. is expected. Such structures need not have any other function and indeed, this perspective can help to explain features which otherwise make only adaptive nonsense.

The thickened skull caps of pachycephalosaurs are assumed to be an adaptation for head-bashing competitions similar to those exhibited by bighorn rams. The horns of ceratopsian dinosaurs may be compared with the horns and antlers of modern ungulates, which are used in various types of actual and ritualized combat. Variations in morphology are broadly similar to those in cows, bison, and rhinoceroses. It has been suggested that primitive ceratopsians (such as Protoceratops) used their single small nasal protuberance for butting contests like those observed in mountain goats. From this humble beginning, later ceratopsians evolved more and larger horns, and bigger neck frills. The shield had a dual function: protection and bluffing -- when the neck was bent forward, it gave the illusion of a much larger animal charging head-on. Similar intimidation displays are seen in some modern lizards.

Jackson's chameleon sports a three-homed face strikingly reminiscent of Triceratops. Fighting individuals lock horns, and push and twist in an effort to dislodge opponents from tree branches. Similar behavior among ungulates is exhibited by antelope, elk, and deer. Can't you just see a couple of Triceratops bulls bellowing and battling during the rutting season?

If Triceratops truly did use its horns for intraspecific competition, rather than solely for defense, we should expect to find specimens with evidence of horn-inflicted damage to the neck frill. In fact, such fossils are common. Single-horned ceratopsians like Monoclonius would be in much greater danger of catching a blow to the frill than the triple-horned types, if wrestling contests were staged. The rhinoceros engages instead in a little fencing and a lot of bluffing, in which the animal struts about and displays the profile of his horn to his opponent without actually charging. In this type of behavior, intimidation and style are as important as strength. Monoclonius may have developed similar behavior. Interestingly, no punctured neck frills of Monoclonius are known. (Another possibility, recently suggested, is that Monoclonius is a catch-all genus for juvenile individuals of several ceratopsian types. If this is the case, the lack of battle scars would be expected in sexually immature individuals.)

The bizarre crests of hadrosaurs can also be interpreted as structures important in mating displays. Other explanations have been proposed over the years (that they developed as snorkels for underwater feeding, now largely discounted; that they housed an expanded nasal epithelium for an acute sense of smell), but these do not account for the diversity of crest shapes. Social interaction does: they evolved as genetic isolating mechanisms between related species, and as ornamentation for use in courtship rituals, much as horn and frill morphology in ceratopsians. Crest size, which was greater in males than in females, may have been used to establish dominance rank within the herd.

The hollow crests of Lambeosaurus and Parasaurolophus would have functioned as vocal resonating chambers as well as cranial embellishments. Vocalization may have been part of courtship and ritualized combat, or used for territorial maintenance in the manner of alligators. Everyone is familiar with the thrum of a bullfrog on a hot summer's night; why not the honking of a hadrosaur?

Nesting behavior and parental care

Hard by the lilied Nile I saw
A duskish river-dragon stretched along.
The brown habergeon of his limbs enamelled
With sanguine almandines and rainy pearl:
And on his back there lay a young one sleeping,
No bigger than a mouse; with eyes like beads,
And a small fragment of its speckled egg
Remaining on its harmless, pulpy snout;
A thing to laugh at, as it gaped to catch
The baulking, merry flies.

--Thomas Lovell Beddoes

The living crocodilians are the only surviving non-avian archosaurs, and we frequently turn to them for analogies with dinosaurs. Female American alligators build mound nests by scraping together vegetation and mud until it is two or three feet high, then scraping out the center to form a bowl-shaped nest. Eggs are laid within the depression and covered so that heat from the decaying vegetation maintains a constant incubation temperature. Between the time when the eggs are laid and when they hatch, the female guards the nest, driving away intruders. When they are ready to hatch, the young call out from within the eggs and their mother opens the nest to allow them to escape. She does not provide any post-hatching care. Nile crocodiles, on the other hand, assist the young in breaking free from the eggs, and the parent carries the neonates in its mouth down to the water. Hatchlings form créches like those of ostriches, penguins and some other birds and remain under the watchful eye of the parent for some time.

Similar, bowl-shaped nests of dinosaur eggs have been found in Mongolia, Argentina, and Montana. The eggs of hypsilophodontids are broken only at the apex, suggesting that the young exited the nest upon hatching. They may have formed a créche under the watchful eyes of the adults, as skeletons of juveniles are found only between the nests. The nests of the duck-billed Maiasaura tell a very different story. Only eggshell fragments are found in them, indicating that the young remained there after hatching, trampling the shells. Tooth wear indicates that the babies had been feeding for some time, which likely indicates that the parents brought them food while they remained in the nest. Skeletons of juveniles are almost without exception found within the nests; very probably they had never left. The altricial young of birds will starve in the nest if their parents fail to return. An Oviraptor parent has been identified guarding the nest in Mongolia, and the skeleton of a small dinosaur may have been intended as food for the young.