Australia: The Land Where Time Began

A biography of the Australian continent 

Dinosaurs - Senses¹ 

Vision

As is the case with birds of the present, vision was apparently the primary sensory system among most dinosaurs, as evidenced by the characteristically well-developed optical lobes and large eyes. Full colour vision extending into the ultraviolet is characteristic of birds and reptiles, leading to the belief that it was also probably present in dinosaurs, as they were ancestral to birds. The author¹ suggests that the colour vision of most mammals, that is comparatively poorly developed, is a result of the ancestral mammals being nocturnal, the vision of some groups being reduced to the point where it is not the primary sense, as in the case of dogs where blood hounds will follow the scent of a person or animal even when the person is in full view. The vision of reptiles is similar to that of mammals, but birds tend to have vision that is very high resolution, partly because of the relatively large size of their eyes, and also because their eyes have higher densities of cones and rods compared to those of mammals, as well as being spread over a wider area of the retina than occurs in mammals, and being more concentrated in the fovea, allowing the region of sharp vision to be wider than occurs in mammals. And some birds also have a second fovea. In raptors active during the day their vision is about 3 times better than that of humans, and they have a much wider sharp field of vision which allows birds to be able to see clearly without pointing their eyes as precisely as do mammals. The range over which birds can focus is also grater than in humans, 20 diopters compared to 13 in young adult humans.

The vision of the bigger-eyed dinosaurs are suggested by the author¹ to possibly have rivaled that of birds. The large size of dinosaurs have been suggested by some authors to be evidence for these dinosaurs being active during the day, while others have suggested it is evidence they were active at night. According to the author¹ large eye size on its own cannot determine whether they were used at day or night, the critical factors being the structure of the retina and pupil that determine the sensitivity to light of a particular eye so indicating what it is suited for.

Birds have eyes that are so large relative to the size of the head that they are close to being fixed in the skull, so they have to turn the entire head to look at a specific object. According to the author¹ this is likely to have also been true of the dinosaurs with smaller heads, those with larger heads could be expected to have had eyeballs that were more mobile allowing them to scan a greater area without turning their heads by moving their eyeballs. Most of the dinosaurs had eyes that faced to the sides to maximise the area that could be scanned without turning the head, though this came at the expense of  the view directly in front.

Vision with overlapping visual fields are present in some birds and mammals, especially primates, that have forward-facing eyes, and in at least some cases vision includes a binocular, stereo effect that results in the perception of depth for better judgment of distant to the target. Vision that was partly forward facing with overlapping visual fields was present in some dinosaurs such as tyrannosaurids, ornithomimids, as well as many avepectoran theropods. The author¹ suggests that it is not certain if any or all of the dinosaurs with potentially stereo vision actually had it, suggesting that the forward-facing eyes of dinosaurs such as tyrannosaurids could possibly have been the result of the back of the head expanding to accommodate larger jaw muscles.

Smell

One result of the reduction of the size of birds' heads in the process of reducing weight to aid in flying has been that the space available for extra sense organs such as smell, which is of much less importance in a flying animal, has been greatly reduced from that of their ancestral dinosaurs. One exception is some vultures that have retained the ability to smell at a high enough level to help them find decaying carcasses that could be hidden in dense vegetation. Another exception is the kiwi bird of New Zealand, that uses its sense of smell to find grubs.

Many reptiles and mammals have a well-developed ability to smell, in some it is so well-developed it is the primary sensory system, especially in canids. Many dinosaurs had well-developed  nasal passages with plenty of room for olfactory tissues at the back of these passages. Many dinosaurs also had large olfactory lobes indicating that they were using these olfactory tissues, and the combination of these large lobes and large area potentially available for the olfactory tissues they probably had a highly developed sense of small. See T. rex. The author¹ suggest that herbivores needed to be approached from downwind, as do those of the present that are being hunted by predators, suggesting that in the smaller-eyed ankylosaurs the sense of smell was probably as important as vision. He suggests that tyrannosaurs and dromaeosaurs probably had an excellent sense of smell, that they probably used for finding carcasses as well as living prey.

Hearing

In mammals hearing is developed to a high level, in part because of their outer ears that have large pinnae that are often moveable, to catch sound vibrations and direct them to the inner ear via the ear opening, as well as having an inner ear comprised of 3 elements that evolved from bones of the jaw in early ancestral forms. In mammals such as bats and cetaceans hearing is the most important sense. The combination of the outer ear and the inner ear allows mammals to detect sounds at low volume levels. Among the mammals humans can detect 20 kHz, dogs up to 60 kHz, and bats 100 kHz.

Fleshy outer ears are not present on birds or reptiles, both also having a single bone in the inner ear. Birds have more auditory cells per unit length of the cochlea, the result being that in birds and mammals the hearing sharpness are broadly similar. At high frequency mammal hearing becomes superior over that of birds and reptiles. The auditory range is 1-5 kHz in many reptiles and birds, owls being an exception, having a range of 250 Hz-12 kHz. Geckos have a range up to 10 kHz. Cassowaries, that use low frequency sound to communicate over long distances, can hear frequencies down to 25 Hz. There is a suggestion that the pneumatic head crests of cassowaries is used to detect low frequency sounds. Pigeons can hear even lower frequencies, down to 2 Hz. It has been suggested that they use this low frequency to detect approaching storms.

As dinosaurs lacked the large pinnae and complex inner ears of mammals their hearing is suggested by the author¹ to be in the range of reptiles and birds, probably not being able to detect very high frequencies. There is also the consideration that the auditory lobes of dinosaur brains were not especially enlarged, though not poorly developed. As the owls are the only birds that can hear fairly high-frequency sounds suggests that most, if not all dinosaurs, could also not hear them. The hollow head crests of oviraptorosaurs have been suggested to have possibly assisted in the detection of low frequency sound as has been proposed for the head crests of cassowaries, suggesting that they may also have been capable of detecting similarly low frequency sounds.

Among the larger dinosaurs the ears were proportionally large, had the potential to hear very low frequency sounds,  that would allow them to communicate over long distances. The author¹ suggests that it is unlikely that in any dinosaurs hearing was the most important sense, though it was probably important in detecting both predators and prey, as well as in communication in all species.

See Tyrannosaurus rex for more details on its senses.

Sources & Further reading

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