Due to a wealth of skeletal remains,
Diplodocus is one of the best-studied dinosaurs. Many aspects of its lifestyle have been subjects of various theories over the years.
Diplodocus is estimated to have walked at a speed of . Marsh and then Hatcher assumed that the animal was aquatic, because of the position of its nasal openings at the apex of the cranium. Similar aquatic behavior was commonly depicted for other large sauropods, such as
Brachiosaurus and
Apatosaurus. A 1951 study by
Kenneth A. Kermack indicates that sauropods probably could not have breathed through their nostrils when the rest of the body was submerged, as the water pressure on the chest wall would be too great. Since the 1970s, general consensus has the sauropods as firmly terrestrial animals, browsing on trees, ferns, and bushes. Scientists have debated as to how sauropods were able to breathe with their large body sizes and long necks, which would have increased the amount of
dead space. They likely had an
avian respiratory system, which is more efficient than a
mammalian and reptilian system. Reconstructions of the neck and thorax of
Diplodocus show great
pneumaticity, which could have played a role in respiration as it does in birds.
Posture The depiction of
Diplodocus posture has changed considerably over the years. For instance, a classic 1910 reconstruction by
Oliver P. Hay depicts two
Diplodocus with splayed lizard-like limbs on the banks of a river. Hay argued that
Diplodocus had a sprawling, lizard-like gait with widely splayed legs, and was supported by
Gustav Tornier. This hypothesis was contested by
William Jacob Holland, who demonstrated that a sprawling
Diplodocus would have needed a trench through which to pull its belly. Finds of sauropod footprints in the 1930s eventually put Hay's theory to rest. A
nuchal ligament may have held the neck in this position. If
Diplodocus relied on a mammal-like nuchal ligament, it would have been for passively sustaining the weight of its head and neck. This ligament is found in many hoofed mammals, such as bison and horses. In mammals, it typically consists of a funiculus cord that runs from the external occipital crest of the skull to elongate vertebral neural spines or "withers" in the shoulder region plus sheet-like extensions called laminae run from the cord to the neural spines on some or all of the cervical vertebrae. However, most sauropods do not have withers in the shoulders, so if they possessed a similar ligament, it would differ substantially, perhaps anchoring in the hip region. Another hypothesized neck-supporting ligament is an avian-like elastic ligament, such as that seen in
Struthio camelus. This ligament acts similarly to the mammal-like nuchal ligament but comprises short segments of ligament that connect the bases of the neural spines, and therefore does not need a robust attachment zone like those seen in mammals. A 2009 study found that all
tetrapods appear to hold the base of their necks at the maximum possible vertical extension when in a normal, alert posture, and argued that the same would hold true for sauropods barring any unknown, unique characteristics that set the soft tissue anatomy of their necks apart from other animals. The study found faults with Stevens' assumptions regarding the potential range of motion in sauropod necks, and based on comparing skeletons to living animals the study also argued that soft tissues could have increased flexibility more than the bones alone suggest. For these reasons they argued that
Diplodocus would have held its neck at a more elevated angle than previous studies have concluded. However, this idea might be contradicted due to the inner ear of diplodocoids actually being in alignment for a horizontal neck pose. Also, it is not necessarily accurate to say that the alert pose is the osteologically normal position. As with the related genus
Barosaurus, the very long neck of
Diplodocus is the source of much controversy among scientists. A 1992
Columbia University study of diplodocid neck structure indicated that the longest necks would have required a 1.6-ton heart – a tenth of the animal's body weight. The study proposed that animals like these would have had rudimentary auxiliary "hearts" in their necks, whose only purpose was to pump blood up to the next "heart". as it explains the unusual wear patterns of the teeth (coming from tooth–food contact). In unilateral branch stripping, one tooth row would have been used to strip foliage from the stem, while the other would act as a guide and stabilizer. With the elongated preorbital (in front of the eyes) region of the skull, longer portions of stems could be stripped in a single action. Also, the palinal (backwards) motion of the lower jaws could have contributed two significant roles to feeding behavior: (1) an increased gape, and (2) allowed fine adjustments of the relative positions of the tooth rows, creating a smooth stripping action. Diplodocine teeth were also continually replaced throughout their lives, usually in less than 35 days, as was discovered by Michael D'Emic et al. Within each tooth socket, as many as five replacement teeth were developing to replace the next one. Studies of the teeth also reveal that it preferred different vegetation from the other sauropods of the Morrison, such as
Camarasaurus. This may have better allowed the various species of sauropods to exist without competition. The flexibility of
Diplodocus neck is debated but it should have been able to browse from low levels to about when on all fours. this means that
Diplodocus could rear up into a bipedal posture with relatively little effort. It also had the advantage of using its large tail as a 'prop' which would allow for a very stable tripodal posture. In a tripodal posture
Diplodocus could potentially increase its feeding height up to about . The neck's range of movement would have also allowed the head to graze below the level of the body, leading some scientists to speculate on whether
Diplodocus grazed on submerged water plants, from riverbanks. This concept of the feeding posture is supported by the relative lengths of front and hind limbs. Furthermore, its peg-like teeth may have been used for eating soft water plants. The conclusions of Cobley et al. were disputed in 2013 and 2014 by Mike Taylor, who analyzed the amount and positioning of intervertebral cartilage to determine the flexibility of the neck of
Diplodocus and
Apatosaurus. Taylor found that the neck of
Diplodocus was very flexible, and that Cobley et al. was incorrect, in that flexibility as implied by bones is less than in reality. In 2010, Whitlock et al. described a juvenile skull at the time referred to
Diplodocus (CM 11255) that differed greatly from adult skulls of the same genus: its snout was not blunt, and the teeth were not confined to the front of the snout. These differences suggest that adults and juveniles were feeding differently. Such an ecological difference between adults and juveniles had not been previously observed in sauropodomorphs.
Dental microwear patterns of
Diplodocus suggest that it partitioned its resources with
Camarasaurus; the former ate softer foods than the latter. However, juvenile
Camarasaurus had similar microwear to adult
Diplodocus, suggesting that adult
Diplodocus may have competed with juvenile
Camarasaurus for food.
Reproduction and growth While the long neck has traditionally been interpreted as a feeding adaptation, it was also suggested that the oversized neck of
Diplodocus and its relatives may have been primarily a sexual display, with any other feeding benefits coming second. A 2011 study refuted this idea in detail. While no evidence indicates
Diplodocus nesting habits, other sauropods, such as the titanosaurian
Saltasaurus, have been associated with nesting sites. The titanosaurian nesting sites indicate that they may have laid their eggs communally over a large area in many shallow pits, each covered with vegetation.
Diplodocus may have done the same. The documentary
Walking with Dinosaurs portrayed a mother
Diplodocus using an
ovipositor to lay eggs, but it was pure speculation on the part of the documentary author. Based on bone
histology studies in the early 2000s, it was suggested that
Diplodocus and other sauropods grew at a very fast rate, reaching
sexual maturity at just over a decade, and continuing to grow throughout their lives. However, a 2024 study estimated that the holotype of
D. hallorum was around 60 years old in maximum age of death, over 20 years older than the oldest known sauropod specimens, and that it "had 'recently' reached skeletal maturity before death". This would make it one of the oldest known dinosaur specimens. The study also suggested that
D. hallorum may have had a relatively slower and more prolonged rate of growth than
D. carnegii, as the latter reached maturity within just 24 to 34 years of age. ==Paleoenvironment==