Australia: The Land Where Time Began

A biography of the Australian continent 

Dinosaur Behaviour - Reproduction

There is a suggestion that among dinosaurs there are morphs, gracile and robust, representing the 2 sexes. As it is difficult to know if the different morphs are actually separate species it has been difficult to prove or disprove this proposal. Males are not always more robust than females, though they often are. Exceptions from this are raptors where the females are larger then the males, as are the females of some whales. Attempts have been made to discriminate male dinosaurs from the females by the use of the depth of the chevron bones beneath the base of the tail failed  because the 2 factors are not consistent in modern reptiles. Among heterodontosaurs some have small tusks and others don't, the tusked specimens have been suggested to be males. The author1 suggests Oviraptorosaurs with head crests and pachycephalosaurs with domed heads may be males if they are not the mature animals of both sexes. Among the T. rex the robust forms are believed, tentatively, to be females because of the inner bone tissue that in birds is associated with egg production.

Sexual maturity is achieved before adult size is reached by humans, some other mammals and reptiles, though this is not the case in most mammals and living birds. On the inner surface of the hollow bones female birds deposit special tissues high in calcium when they are producing eggs. The presence of this tissue has been used to show that in some dinosaurs reproduction began before before the animal reached full adult size. This has been confirmed by the presence of dinosaurs that were still growing brooding nests. The author1 suggests that most dinosaurs probably became reproductive before maturing. He suggests that ceratopsians and hadrosaurs could possibly have been exceptions because their display organs became completely developed only when they approached adult proportions.

According to the author1 the many display features that included crests on the head and body, frills, hornlets, spikes, spines, tail clubs, bristles and feathers that were evolved by various dinosaurs show that there was strong selective pressure to develop display organs that were distinctive, to identify their species to other members of their species and to achieve success in sexual competition, that often took the form of battle between males competing for females. Any display organs that were formed of soft tissues remain unknown as they don't fossilise. He suggests the use of these organs would have varied widely among dinosaurs. The signals would have been used by females to communicate to the males of their species that they were suitable, fertile mates . The displays would have been used by the males to intimidate other competing males as well as to attract and inseminate the females.

Better quality display organs are usually developed by animals that are healthy and in the prime of their reproductive lives as they are able to devote more resources to growing these display organs. The author1 suggests the extreme in these displays, that were essentially peaceful, is best seen in the elaborate head crests of the hadrosaurs. He also suggests that many dinosaurs also used intricate display movements and vocalisations during the courtship rituals that are completely unknown. The dinosaurs would have had to turn their bodies to show their display surfaces as they were oriented on sides for best effect. A major exception would have been the ceratopsians as their head frills are most prominent in front view. On the heads of pachycephalosaurs the domes were prominent from the front and the side, the author1 suggesting they may have been tilted forward to intimidate opponents. The transverse head crest of Crylophosaurus, a carnivorous theropod, and on some abelisaurs, the horns and domes provided frontal displays that were unusual. The same applied to brachylophosaur hadrosaurs that had small heads regarding the crest on their heads.

Among animals with weapons, horns, antlers, large canines, etc., intraspecific competition, whether for mates or territory, is often violent, in some cases even fatal. The author1 suggests that sauropods may have reared up and used their thumb claws in battle. The thumb spikes of iguanodonts would have been potentially very dangerous weapons if they were used in such battles between same species rivals. He also suggests the pachycephalosaurs may have used their domed heads in these combats, and the ankylosaurs may have used their tail clubs against each other, and other ankylosaur species may have locked their shoulder spines and used their strength to determine their superiority as do deer of the present with their antlers. Ceratopsid species with multiple horns may have used these also in the manner of deer. Evidence has been found in the form of healed wounds that they used their horns to injure one another, as male heterodontosaurs with tusks probably also did. It has been suggested that males of many species have evolved the means of avoiding fatal injuries. This is true in animals such as elephant seals with the extra thick skin on the necks, but it is not always the case. Among hippos and lions there is a high mortality rate in these intraspecific battles. He suggests this may also have been the case among male theropods, ceratopsians and iguanodonts.

The penis or penes (when either are present) of reptiles and birds are internal, and both have internal testes. It is suggested that the same condition was present in dinosaurs. In most birds these is no penis, but it is not possible to know if this also applied to many or all dinosaurs. It is assumed copulation was a rapid process among dinosaurs, probably with the female making mating possible by lowering her shoulders and moving her tail out of the way, the male rearing behind her and standing on 2, or possibly 1  leg, and steadying himself by placing his hands on her back. The author1 suggests that as sauropods would need to copulate is further support for the belief that they had they had the ability to stand on their hindlegs. Among stegosaurs he suggests the male most likely held his hands on one side of the female's pelvis, because of the plates on her back.

It is believed that the eggs of dinosaurs were hard-shelled, as in birds, and not leathery as in reptiles and crocodilians. Also like birds there would have been no species in which there were live births, as occurs in some reptiles, if their eggs had hard shells. Dinosaur eggs from the entire Mesozoic are still rarely found. An example of this rarity of eggs is that no eggs, or even egg shells, that can be attributed to sauropods, have been found in the large Morrison Formation. The only known eggs from this formation are some small ornithopod eggs. The situation is different in the Cretaceous deposits, especially the latter part of the period, eggs and nests being known from a wide variety of dinosaurs. The identification of the dinosaur species that produced a particular egg is not always easy, requiring the finding of eggs within the trunk skeleton of a fossil, an embryo skeleton of an identifiable dinosaur or a dinosaur on top of the eggs in brooding posture. Further identification of their origin can use the different shapes and types of eggshell, as the various dinosaurs produced eggs of different types and shapes, though the origin of many known eggs are still unknown. There was a wide range of shapes among dinosaur eggs, from spherical to elongated  and a variety of shapes between the extremes, some being strongly tapering. As occurs in reptiles dinosaur eggs are arranged within the body and in the nests in pairs, whereas in birds they are produced singly. The surface texture of dinosaur eggs also varied, some being crenulated and others bumpy.

Among reptiles the eggs are of proportional size to the size of the female's body, but the eggs of birds are proportionately larger. The small dinosaurs laid eggs intermediate between those of reptiles and birds. The author1 points out that there is no known dinosaur that matched the 12 kg (25 pound) eggs of the extinct flightless elephant bird Aepyornis that weighed about 400 kg (800 pounds), much smaller than many dinosaurs. The sauropod eggs that would have weighed about 1 kg (2 pounds). The largest known dinosaur egg so far, that is suspected to have been laid by an oviraptor that would have weighed more than 1 tonne, weighed about 5 kg.

There 2 types of reproductive stratagems, r-strategy and K-strategy. Animals that follow the K-strategy are slow breeders that produce few young and those using the r-strategy produce large numbers of young to allow for high losses that occur among the juveniles. When a species produces many young they have benefits such as being able to expand their population rapidly when times are good, a strategy that is useful when the conditions such as climate are extreme or variable, allowing the species to recover rapidly from conditions that cause large losses such as drought. The animals using the r-strategy have been referred to as "weed species" as they can rapidly colonies new areas as they become available as well as recover their population quickly after crashes. The author 1 suggests that dinosaurs were r-strategy breeders that produced many eggs each breeding season. He also suggests that this could be the reason dinosaurs  produced smaller eggs than birds, as the birds provide care to their eggs and young until the young can fly and continue feeding them until they are mature enough to look after themselves.

Among modern birds one form that use the r-strategy are the large ratites that produce many  eggs. Among the dinosaurs the sauropods are believed to have produced the most eggs in a single nest, up to a few dozen. In this respect the giant dinosaurs differed greatly from the giant mammals that are K-strategists producing few young that are cared for for several years. None of the dinosaurs produced a nutrient such as milk to feed its young. He also suggest that one form of care some dinosaurs may have provided for their young is the regurgitation of a substance that is milk-like, from their digestive tract as pigeons of the present do, though no direct evidence of this is known.

For many years it was assumed that dinosaurs, being similar to reptiles, didn't provide any care to their young, abandoning their eggs as soon as they were produced and possibly buried, as most reptiles do. Among reptiles there a few that depart from this reproductive behaviour, some lizards stay with their eggs in the nest and pythons incubate their eggs with heat produced by their muscles. It is common for crocodilians to guard their eggs and hatchlings. All birds care for their eggs, nearly all incubating the eggs with body heat, except for the megapodes that bury their eggs in mounds of rotting vegetation that generates heat by fermentation to incubate the eggs. The male parent regulates the temperature of the mound by adding or removing vegetation, detecting the temperature with the bare skin around his head. The chicks emerging from the mound are well developed by the time they hatch they can be independent enough to leave the nest immediately. Ratites care for their chicks after they have and guide them to food sources and protect them. The chicks of most birds are altricial, being so poorly developed when they hatch they need to be fed and kept warm by their parents for some time, and their feathers only grow after they hatch.

According to the author a number of discoveries have recently revealed then manner the eggs of dinosaurs were deposited and the degree of attention they and the hatchlings received varied widely in a number of regards was similar and distinctive from extant tetrapods.

Some eggs of unidentified dinosaur species have been found that imply by their structure that they were buried and abandoned. The author1 suggest that this was probably how the sauropods would have deposited their eggs. He suggests that the nests that can be attributed to the sauropods were large and vegetation covered and their rather irregular construction differs from the more organised nests that have been attributed to dinosaurs that cared for the nest and young. He also suggests that if adult giant sauropods stayed with their nests there would be some consequences such as the local vegetation being denuded and the risk of trampling on their eggs, and even the hatchlings, as large numbers of nests were constructed in the same area.

As so many nests were filled with so many eggs in the same area it increased the chances that at least some of the young would survive this very vulnerable stage of their lives. A fossil has been found of a large snake that was feeding on a newly hatched sauropod when it died. It is indicated by evidence from trackways that the small juveniles of sauropods formed pods that didn't travel with the adult herds for a few years until they reached a body mass of about 1 tonne and were no longer in danger of being trampled accidently by the adults. It has been suggested that it is unlikely the adult sauropods paid any special attention to their young when they finally joined the herd, or even were aware of their relationship. In this scenario he suggests the young simply joined the adult herd to be more secure from attack by large predators as the adults would have been formidable opponents for the biggest of the predators. No trace of sauropod eggs have been found of vast Morrison Formation in which there are the remains of a wide variety of sauropods, though the egg shells of smaller dinosaurs have been found.

Juvenile ankylosaurs are also believed to have formed pods. A find of more than a dozen articulated skeletons of large juvenile Pinacosaurus that were grouped together, and apparently died at the same time by a dune slide, indicated that they were travelling in a group, as no adult appears to have been present with the pod.

The author1 suggests that the structural organisation of the nests of hadrosaurs indicate they were probably monitored by the parents. He also suggests that it is possible the mound nests of hadrosaurs may have been monitored in the manner of megapodes. In at least some cases the nests of hadrosaurs appear to have been grouped in colonies, and as the breeding hadrosaurs were much smaller than the giant sauropods they could remain in an area to care for their their eggs and hatchlings without stripping the local vegetation. The egg shells show signs of being trampled over an extended period, and skeletons of juveniles much larger than hatchlings have been found in hadrosaur nests suggesting the parents didn't abandon their nests as soon as the eggs hatched. Another feature that suggests they were cared for by the parents for a period after hatching was the short snouts and large eyes of  the hatchlings, both features that tend to be associated with parenting behaviour in the adults, the author1 suggesting that it indicates that the parents may have opened the mounds as the young were hatching, bringing food to the altricial juveniles as long as they remained in the nest. If this was indeed the case it would have avoided problems such as accidently stepping on the hatchlings, and protecting them from predators, with the added advantage that their growth rates could have been maintained at maximum level by food being brought to the young in the nests. It has been suggested that juveniles may have formed independent pods when they left the nest until they were big enough to join the adult herds, though this is only speculation as it is not known what really happened when they left the nest.

According to the author1 not much is known of the nesting behaviour of the large predatory theropods. He suggests the young of tyrannosaurids were vulnerable to being either accidently killed or eaten by their parents as they had elongated snouts, the opposite of the usual short-faced juveniles of animals that are cared for by their parents, suggesting the juveniles hunted independently. It has been suggested that the gracile juveniles tyrannosaurids hunted for the parents, but the author1 believes this is implausible, saying that when food is exchanged between the juveniles and parents it is always the juveniles that receive the food.

As the juveniles of the smaller dinosaurs weren't in as much danger from being trampled by giant parents they were more likely to be cared for by their parents. The avepectoran theropods, that were birdlike, have provided the best evidence for brooding among dinosaurs and the incubation of the eggs, especially among the oviraptors. As there were a number of eggs in a nest, that could be up to a few dozen, it has been suggested that they were unlikely to have all been produced by a single female, the nests probably being communalistic, as occurs among the big ratites. The eggs of oviraptors were arranged in 2 -layer rings with an opening in the centre. Laid flat, the eggs were partly buried and partially exposed. As eggs that were not protected from the elements would die, either from exposure or predators it is indicated that the adults protected and incubated them. Some of the oviraptor nests that have been found had an adult that had died in the classic brooding posture of birds on top of the eggs, with the legs tucked up alongside the hips and the arms spread over the eggs, and the deep pelvis could rest on the ground in the clear space in the centre of the rings of eggs. Among the birds this clear space is not required as they have flatter bellies.

It has been assumed that the feathers on the wings, as well as elsewhere on the body, would have completely covered the eggs of raptorosaurs to protect them from weather as well as keep them warm by the body heat of the adult. The presence of dead adults on top of eggs in a nest has been assumed to indicate they have been killed by sudden events, such as a sandstorm, or more likely by a dune duneslide.  Nests containing giant eggs that have been found that are believed to have been laid by oviraptors, the eggs being arranged in rings, though in their case the nests were of enormous proportions up to 3 m (10 ft) in diameter. The largest known incubated nests, they are believed to have been brooded by oviraptors in the weight range of 1-2 tonnes. The nests of troodonts also had the eggs arranged subvertically in a partial spiral ring with the centre clear for the pelvis of the brooding adult. When the adult troodont has been found in brooding posture have been as small as 1 pound. The author1 suggests that among the avepectorans the half-buried, half-incubated nesting habits ideally represent the bird-like arrangement that would be expected of the dinosaurs that are closest to birds.

Hatching from hard-shelled eggs, a problem faced by all embryos developing in such eggs is getting out of the eggs when they are mature enough to hatch, a problem that would be made worse with large eggs that would have a thicker shell than in smaller eggs. The author1 suggests the embryo absorbs some of the calcium from the egg shell to use in its developing skeleton. The young of birds have an egg tooth to break out of the egg. Such an egg-tooth has been found on the nose of titanosasur sauropod embryos, and was possibly present in other dinosaurs.

It is not known if the small birdlike theropods, as well as many other dinosaurs, cared for their young after they hatched, though is has been suggested to be plausible. The small ornithischians have provided the best known evidence so far of the parenting of juvenile dinosaurs. A compact clutch of almost 3 dozen juveniles of Psittacosaurus, about a tenth of a kilogram, or about a fifth of a pound, has been found in which the skeletons of the juveniles were articulated, in intimate association with an adult that was more than a dozen times heavier. A similar situation occurs among ratites, the offspring of a number of females being gathered into a large crèche that is cared for by a set of adults. The author1 suggests that  the tightly packed collection of psittacosaurs were entombed in a fossil burrow.

Among dinosaurs the degree of parental care probably ranged from minimal to extensive, in some exceeding that of reptiles and crocodilians, possibly rivaling that of birds. It is believed that the level of parenting seen in mammals was not reached by any of the dinosaurs, and as no dinosaurs nursed their young it seems most could grow up independently.

Oldest known dinosaur nesting site in the world

A nesting site has been found in South Africa, dating to about 190 Ma in the Early Jurassic, of Massospondylus, a mid-sized dinosaur that was 4-6 m long. The eggs in the nests are 6 cm in diameter. The nest site is in the Golden Gate National Park. It has been found from evidence found in the nests that the juveniles remained in the nest until they doubled their hatching size, and that they walked quadrupedally while young but probably were bipedal as adults. It is the oldest known instance of this behaviour among the dinosaurs.

According to Dr. Eric Roberts of James Cook University, a co-author of a study carried out by a team and published in the National Academy of Sciences (link 1) the nests are a source of information on the reproductive behaviour of early dinosaurs and the environment in which they built their nests. In the nests were found tracks of dinosaur hatchlings, as well as fossilised ripple marks and evidence of cracking of the substrate resulting from desiccation that suggests the nest sites were in a shoreline environment that was dynamic with fluctuating climatic conditions.

At least 10 nests have been uncovered from various levels in the site, each of which contained up to 34 eggs in close-knit clutches. It is suggested by their distribution throughout the deposit that these dinosaurs returned  repeatedly to the same site to breed. The author suggests that it is indicated by the nature of the nests, that were highly organised, that the mother dinosaur probably carefully arranged the eggs after they were laid.

The nests, containing embryos and eggs, were found in a 25 m long road cutting that was nearly vertical, according to Dr Reisz, head of Vertebrate Palaeontology at the University of Toronto Mississauga, who predicts that many more nests will become available for study as erosion continues at the site.

According to David Evans, associate curator of Vertebrate Palaeontology, Royal Ontario Museum, it is important as very little is known of the reproductive behaviour of dinosaurs, especially of the early dinosaurs. He says these nests are the first to give some idea of dinosaur reproduction at an early stage of their evolution, and it documents the antiquity of the nesting strategies that were previously known only from much later in the fossil record of dinosaurs.


  1. Oldest know dinosaurian nesting site and reproductive biology of the Early Jurassic sauropodomorph Massospondylus

Sources & Further reading

  1. Paul, Gregory S., 2010, The Princeton Field guide to Dinosaurs, Princeton University Press.



Author: M. H. Monroe
Last updated 31/01/2012 





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