Herding Behavior

Herding behavior (also termed gregarious behavior in the paleontological literature) describes a form of sociality in which organisms maintain sustained spatial proximity with conspecifics during daily activities. In behavioral ecology, herding is understood through cost–benefit analysis: individuals gain anti-predator protection, foraging efficiency, and reproductive advantages at the cost of increased competition for food and heightened parasite transmission. For extinct organisms, herding must be inferred indirectly from taphonomic, ichnological, and sedimentological evidence. The systematic study of dinosaur herding accelerated in the late twentieth century, driven by Jack Horner's pioneering work on Maiasaura nesting sites in Montana during the late 1970s and 1980s, and by Martin Lockley's comprehensive analyses of dinosaur trackways worldwide (Lockley, 1991, Tracking Dinosaurs: A New Look at an Ancient World).

Bone Beds (Bonebeds): Monospecific bone beds—fossil concentrations dominated by a single taxon—constitute the most cited evidence for dinosaur herding. In Dinosaur Provincial Park, Alberta, Canada, at least 20 ceratopsian bone beds have been documented, including those of Centrosaurus apertus and Styracosaurus albertensis (Eberth et al., 2007). The Hilda mega-bonebed, a Centrosaurus accumulation stretching over 2.3 km², is estimated to contain the remains of thousands of individuals and is interpreted as the catastrophic mass death of a large herd, likely caused by a tropical storm-induced flood crossing event (Eberth & Getty, 2005). In Montana, the Westgate Maiasaura bone bed preserves an estimated 10,000 or more individuals across multiple ontogenetic stages, representing one of the densest dinosaur accumulations ever recorded. However, taphonomic scrutiny is essential: post-mortem hydraulic transport, time-averaging, and attritional accumulation can produce monospecific concentrations that mimic herd mortality events without reflecting social behavior in life. Rigorous taphonomic protocols—including bone orientation analysis, articulation indices, weathering stage assessment, and sedimentological context—are required to distinguish genuine mass-mortality assemblages from secondary accumulations.

Trackways: Parallel trackways—multiple trails of the same morphotype preserved on the same bedding plane, oriented in the same direction with comparable stride lengths—provide the most direct evidence of contemporaneous group movement. The Davenport Ranch tracksite in Texas, discovered by R. T. Bird of the American Museum of Natural History in 1940, preserves the trails of 23 sauropods moving together at approximately 2 m/s, with larger individuals leading and smaller individuals trailing behind. This structured arrangement has been interpreted as evidence of a hierarchically organized herd. In Portugal, Lockley et al. (1994) documented a Late Jurassic tracksite containing parallel sauropod trails made exclusively by juveniles, suggesting age-segregated grouping. Early Jurassic trackway evidence from Japan (Azuma & Takeyama, 1991), the western United States, and southern Africa further demonstrates that gregarious movement was widespread among bipedal ornithischians and theropods. A 2024 study published in Scientific Reports reported the first trackway evidence of gregarious behavior in stegosaurs, a group previously lacking such documentation.

Colonial Nesting Sites: Spatially organized clusters of nests within a restricted area indicate communal breeding behavior. The Egg Mountain locality in the Two Medicine Formation of Montana preserves multiple Maiasaura nests arranged at roughly one adult body-length apart, suggesting that breeding females gathered at traditional nesting grounds. In Patagonia, the Laguna Colorada Formation site of Mussaurus patagonicus contains over 100 eggs arranged in clutches of 8–30 within excavated trenches, along with 80 skeletal individuals ranging from embryos to fully grown adults (Pol et al., 2021). The Early Jurassic Massospondylus nesting site in South Africa's Golden Gate Highlands National Park (Reisz et al., 2012) provides evidence of site fidelity—repeated use of the same nesting locality over multiple breeding seasons—paralleling the pattern observed in Mussaurus.

Sauropodomorpha: The Mussaurus patagonicus assemblage from the Laguna Colorada Formation (Early Jurassic, ~193 Ma) represents the oldest skeletal evidence of structured, age-segregated herding in dinosaurs. U-Pb zircon geochronology dated the fossil-bearing sediments to approximately 192.7 Ma (Sinemurian stage), revising the previously assumed Late Triassic age. The presence of embryos, neonates, juveniles under one year, sub-adults, and adults at the same locality and within the same 3-meter-thick stratigraphic interval indicates life-long social cohesion. Similar colonial nesting has been documented for contemporaneous sauropodomorphs Massospondylus (South Africa) and Lufengosaurus (China), suggesting the behavior may have a Triassic origin. Derived sauropods (Neosauropoda) have abundant Late Jurassic and Cretaceous trackway evidence for herding, including titanosaurid trackways from Bolivia's Cal Orcko site (Lockley et al., 2002) and rebbachisaurid bone-bed evidence from Argentina (Calvo et al., 2007).

Hadrosauridae: Hadrosaurs possess arguably the richest herding record among dinosaurs. The Maiasaura peeblesorum bone bed in Montana, first described by Horner and Makela (1979), contains mixed-age individuals in a catastrophic assemblage. Isotopic studies (Fricke et al., 2009) using carbon and oxygen isotope ratios from hadrosaur enamel investigated whether these animals undertook long-distance seasonal migrations, with results suggesting regional movements but not continent-scale migration. Terrill et al. (2020) applied strontium isotope analysis to a hadrosaur from Dinosaur Provincial Park, finding evidence of relatively limited geographic range, challenging the earlier assumption of large-scale hadrosaur migration.

Ceratopsia: Ceratopsian bone beds are extraordinarily common in the Late Cretaceous of western North America. Centrosaurus and Styracosaurus bone beds in Dinosaur Provincial Park have been studied as textbook examples of herding. In 2009, Mathews et al. described the first Triceratops bone bed (Homer site, southeastern Montana), containing at least three individuals, which was significant because Triceratops had previously been considered a largely solitary animal. The 2025 discovery of mixed ceratopsian-ankylosaurid trackways in Dinosaur Provincial Park (published in PLOS ONE) provided the first direct evidence of multi-species herding in dinosaurs, akin to modern mixed herds of wildebeest and zebra on the African savanna.

Theropoda: Evidence for theropod gregariousness is more contentious. The Ghost Ranch Coelophysis locality in New Mexico preserves hundreds of individuals, though the taphonomic context suggests drought-induced mass mortality at a water source rather than herding per se. The Albertosaurus bone bed at Dry Island, Alberta, originally excavated by Barnum Brown in 1910 and re-examined by Philip Currie, contains at least 12 tyrannosaurid individuals of varying ages (Currie, 1998; Eberth & Currie, 2010). Currie interpreted this as evidence of gregarious behavior and possibly cooperative hunting. A 2021 study (Titus et al., PeerJ) described a Teratophoneus tyrannosaurid bone bed from Utah, providing additional support for social behavior in tyrannosaurids. However, Roach and Brinkman (2007) critically reassessed the pack-hunting hypothesis for Deinonychus antirrhopus, arguing that the fossil evidence is more consistent with non-cooperative mob feeding—analogous to Komodo dragon behavior—rather than mammal-like coordinated pack hunting. The debate remains unresolved, and most paleontologists currently consider large theropods to have been predominantly solitary, with facultative gregariousness in some taxa.

Anti-predator Benefits: The many-eyes hypothesis posits that as group size increases, each individual can devote less time to vigilance and more time to foraging, because the probability that at least one group member detects an approaching predator rises with the number of individuals. The dilution effect further reduces per-capita predation risk: a predator can only capture one individual per attack, so each member of a larger group faces a statistically lower chance of being the target. For ceratopsians with their formidable horns and frills, collective defensive postures—analogous to musk ox defensive rings—have been hypothesized. The 2025 Dinosaur Provincial Park trackway discovery, which found tyrannosaur tracks perpendicular to a ceratopsian-ankylosaurid herd, lends circumstantial support to the predator-defense hypothesis, though the temporal relationship between the tracks cannot be established definitively.

Foraging Efficiency: In modern herbivorous mammals, herding facilitates information transfer about resource locations. For sauropods, whose enormous body sizes demanded vast quantities of vegetation, coordinated long-distance movement may have been essential for locating adequate food and water sources across seasonally variable landscapes.

Reproductive Success: Colonial nesting enables shared vigilance over eggs and hatchlings, reducing nest predation rates. The age-segregated structure observed in Mussaurus—with neonates, juveniles, and adults occupying distinct spatial zones within the same breeding ground—suggests a division of roles analogous to nursery herds observed in modern elephants, where juveniles of similar age are supervised collectively while adults forage.

Age segregation—the spatial separation of age cohorts within a herd—is well documented in extant large-bodied herbivorous mammals such as African elephants and bighorn sheep (Ruckstuhl, 1999). In Mussaurus, the phenomenon is demonstrated by the clustering of 11 articulated juvenile skeletons (estimated body mass 8–11 kg) approximately 50 meters from a neonate aggregation, with adults found separately. Bone histology confirmed that the juveniles were all less than one year old and exhibited identical growth stages, suggesting they were members of a single cohort. The dramatic ontogenetic body mass change in Mussaurus—from roughly 0.07 kg at hatching to approximately 1,500 kg in adults—and the associated shift from quadrupedal to bipedal locomotion would have created vastly different activity budgets and mobility constraints at different life stages, making age segregation functionally advantageous for maintaining herd cohesion.

Herding behavior is frequently discussed in conjunction with seasonal migration. Fricke et al. (2009) examined stable isotope ratios (δ¹³C and δ¹⁸O) in hadrosaur tooth enamel from multiple Late Cretaceous localities across western North America. They found systematic east-to-west decreases in isotope ratios, with overlap between only some localities, suggesting limited regional movement rather than transcontinental migration. Terrill et al. (2020) applied strontium isotope (⁸⁷Sr/⁸⁶Sr) analysis to sequential enamel samples from a single hadrosaur tooth from Dinosaur Provincial Park, finding relatively homogeneous values that indicated the animal remained within a limited geological area during tooth formation. These results complicate earlier assumptions of wildebeest-like mass migrations and suggest that the scale of dinosaur herding movements varied significantly among taxa and environments.

The 2025 report of a tracksite in Dinosaur Provincial Park (published in PLOS ONE by a team including researchers from the University of Reading and the Royal Tyrrell Museum of Palaeontology) documented 13 ceratopsian footprints attributable to at least five individuals, interspersed with ankylosaurid tracks, all oriented in the same direction. This was interpreted as the first evidence of mixed-species herding in dinosaurs. Two large tyrannosaurid trackways were also found perpendicular to the herbivore herd, raising the question of predator-prey interaction. The researchers cautioned, however, that the tracks may have been made over days or weeks rather than simultaneously, meaning the site could represent a palimpsest of multiple behavioral events rather than a single snapshot.

Several methodological challenges complicate the study of dinosaur herding. Bone beds can result from non-behavioral processes such as fluvial concentration, drought-induced mass mortality at water holes, or volcanic events that kill solitary individuals independently. Taphonomic analysis—including assessment of bone articulation, weathering stages, hydraulic equivalence, and sedimentological context—is indispensable for distinguishing true behavioral aggregations from abiogenic accumulations. Trackway evidence, while more directly behavioral, faces the challenge of establishing contemporaneity: tracks on the same bedding plane may have been made hours, days, or weeks apart. The debate over theropod gregariousness exemplifies these challenges, with proponents citing multi-individual bone beds (e.g., Albertosaurus, Mapusaurus, Teratophoneus) and critics noting that aggregation around carcasses or water sources does not necessarily imply social herding.

The discovery that complex herding behavior predates the Jurassic radiation of large-bodied sauropods by tens of millions of years has significant implications for understanding dinosaur evolutionary success. Pol et al. (2021) proposed that the evolution of sociality in sauropodomorphs coincided with the acquisition of large body size between 227 and 208 Ma and may have been a key factor enabling early sauropodomorphs to survive the end-Triassic extinction event (~201 Ma) and subsequently dominate terrestrial herbivore niches throughout the Early Jurassic. The parallel emergence of colonial nesting in geographically distant lineages—Mussaurus in South America, Massospondylus in Africa, Lufengosaurus in Asia—either indicates convergent evolution of sociality driven by similar ecological pressures or suggests an even older, possibly Triassic, origin of the behavior in the common ancestor of these lineages. Either scenario underscores that herding was not merely a Late Cretaceous phenomenon but a deep-rooted behavioral strategy fundamental to dinosaur ecology across the entire Mesozoic.