Dinosaur Herds and Social Behavior

For much of the 20th century, dinosaurs were portrayed as solitary, unintelligent creatures with no more social complexity than a modern Komodo dragon. The fossil evidence accumulated over the past four decades has overturned that view dramatically. We now know that many dinosaur species lived in groups, migrated seasonally, nested in colonies, and provided parental care for their young — behaviors that have profound implications for our understanding of dinosaur biology and intelligence.

Bonebeds — accumulations of bones from multiple individuals of the same species at a single site — are the most compelling evidence for social behavior in dinosaurs. When a single bonebed contains the remains of dozens or hundreds of individuals from the same species, all apparently dying in the same event, the most parsimonious explanation is that they were living together as a group. Some of the most striking bonebeds come from ceratopsian dinosaurs. The Hilda bonebed in Alberta, Canada, contains the remains of an estimated 1,000 to 3,000 Centrosaurus individuals in a deposit interpreted as a mass death during a flood event — a group that large could only exist if Centrosaurus lived in massive herds comparable to the bison herds of historic North America.

Hadrosaur bonebeds have been equally informative. Multiple species of hadrosaur are known from multi-individual assemblages, including the famous Maevarano Formation sites in Madagascar that preserve hundreds of Majungasaurus and their prey. The Egg Mountain locality in Montana, excavated extensively by Jack Horner, produced evidence of communal nesting in Maiasaura — the "good mother lizard" whose scientific name reflects this discovery. Maiasaura nests were constructed as bowl-shaped depressions in the ground, spaced roughly body-length apart (consistent with a colonial nesting strategy), and contained hatchlings whose leg bones were not yet fully ossified, meaning they could not walk well and required parental feeding at the nest. Plant material in the nests suggested that parents brought vegetation to feed the young — the first clear evidence of extended parental care in non-avian dinosaurs.

Dinosaur trackways preserve moments of group behavior with remarkable immediacy. When multiple parallel trackways of the same species travel in the same direction, it suggests coordinated group movement. At the Davenport Ranch site in Texas, a trackway site preserves 23 sauropod trackways all moving in the same direction, with the smaller individuals on the interior and larger individuals on the exterior — a spatial arrangement that suggests the group was protecting juveniles, exactly as some modern herd animals do. While alternative interpretations exist, such sites provide a direct behavioral snapshot impossible to obtain from isolated bones.

The question of migration in dinosaurs is still being resolved. Growth ring analysis of hadrosaur bones from the North Slope of Alaska suggests that some individuals lived at high latitudes year-round, while others may have migrated seasonally. Isotope analysis of strontium in dinosaur teeth can track geographic movement, because different geological substrates leave distinctive strontium isotope signatures in groundwater and hence in the teeth of animals drinking it. Studies of sauropod teeth from the Morrison Formation have found strontium signatures suggesting some individuals moved hundreds of kilometers seasonally — long-distance migration comparable to modern ungulates.

Crest structures in hadrosaurs like Parasaurolophus and Corythosaurus have been the subject of acoustic analysis. These hollow crests connected to the nasal passages and functioned as resonating chambers that would have produced distinctive low-frequency calls. Computed fluid dynamics modeling of Parasaurolophus crests suggests that the different crest shapes in different species produced calls with distinct frequency profiles, potentially allowing species recognition and individual communication within herds. The social complexity implied by elaborate acoustic communication systems is a far cry from the solitary reptilian dinosaur of early 20th century imagination.