Traditionally, dinosaurs were seen as creatures primarily active during daylight, dominating ancient landscapes under the sun. This view, often reinforced in popular culture, suggested early mammals occupied nocturnal niches, scurrying in shadows to avoid larger, daytime predators. Recent scientific investigations, however, are revealing a more complex picture of Mesozoic life. New discoveries indicate some dinosaurs were active at night, possessing adaptations for thriving in low-light conditions and challenging long-held assumptions about their daily routines.
Uncovering the Evidence for Nocturnality
Scientists can infer the activity patterns of extinct animals by analyzing specific anatomical features, particularly those related to vision. A primary method involves studying scleral rings, bony rings found within the eye socket of many vertebrates like dinosaurs, birds, and lizards. Though absent in mammals and crocodiles, these rings provide structural support and their dimensions reveal insights into an animal’s visual capabilities.
Nocturnality is understood by the ratio of the scleral ring’s outer diameter to its central opening’s inner diameter, which indicates pupil size. Animals adapted for low-light, such as nocturnal species, typically have a larger pupil opening relative to their eye size to maximize light gathering. Conversely, diurnal (day-active) animals tend to have smaller pupil openings. Cathemeral animals, active intermittently day or night, exhibit ratios in between.
By comparing these measurements in dinosaur fossils to those of 164 living species with known activity patterns, researchers can reliably predict a dinosaur’s activity. The overall size and shape of the eye orbit also contribute to this analysis.
Notable Nocturnal Dinosaurs and Their Adaptations
Evidence suggests several dinosaur species were active at night. Megapnosaurus, a slender, bipedal carnivore from the Early Jurassic, is one such example. Its scleral rings indicate a probable nocturnal lifestyle, allowing it to hunt smaller prey. This dinosaur, measuring up to 2.2 meters (7.2 feet) long and weighing around 13 kg (29 lb), likely preyed on creatures like early lizards.
Shuvuuia deserti, a small, desert-dwelling theropod from Late Cretaceous Mongolia, was pheasant-sized and weighed like a small house cat. It possessed keen vision and owl-like hearing. Its eyes had some of the proportionally largest pupils measured in birds or dinosaurs, indicating excellent low-light vision. The lagena, the part of the inner ear responsible for processing sound, was large in Shuvuuia, nearly identical in relative size to a modern barn owl. This suggests Shuvuuia could hunt in complete darkness, relying on its acute hearing to detect prey like small mammals and insects.
Certain dromaeosaurids, including Velociraptor and Microraptor, are also thought to have been nocturnal hunters. Velociraptor, a bipedal, feathered carnivore roughly 1.5 to 2.07 meters (4.9–6.8 ft) long, likely hunted under moonlight. A fossil of a Velociraptor in combat with a Protoceratops, which was likely cathemeral, suggests this encounter occurred during twilight or low-light conditions. These findings indicate agile predatory dinosaurs actively hunted during the Mesozoic night.
The Advantages of Nocturnal Life
A nocturnal lifestyle offered several evolutionary advantages to some dinosaurs, shaping their survival and ecological roles. One benefit was the ability to avoid larger, dominant diurnal predators. By being active at night, smaller dinosaurs could escape the hunting hours of daytime hunters, reducing their risk of predation. This temporal partitioning of habitats allowed different species to coexist by utilizing resources at different times.
Nocturnality also provided access to different food sources. Many prey species, particularly small mammals and insects, are primarily active at night. Dinosaurs like Shuvuuia, with their specialized night vision and hearing, could exploit these nocturnal prey populations, expanding their dietary options. For smaller dinosaurs, especially those inhabiting hot, arid environments, being active during the cooler nighttime hours offered a way to regulate body temperature and conserve water. This adaptation helped them avoid metabolically demanding daytime heat.
Night activity also reduced competition for resources like food and territory with diurnal species. While large herbivores, such as sauropods, likely grazed both day and night due to their immense metabolic needs, smaller carnivores gained an advantage by occupying the nocturnal niche. This temporal separation allowed a greater diversity of life to thrive by minimizing direct competition for shared resources.