"Long before it's in the papers"
November 01, 2012

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Could dinosaurs have shaped the way mammals see the world?

Nov. 1, 2012
Courtesy of University of Texas at Austin
and World Science staff

If you’ve ev­er won­dered why dogs and cats have good night vi­sion but poor col­or vi­sion, the an­swer may have some­thing to do with di­no­saurs, new re­search claims.

Ac­cord­ing to the stu­dy, mam­mals lost the abil­ity to see col­or—per­haps nev­er to re­gain it com­plete­ly—dur­ing a long-a­go age when they were mostly ac­tive at night. That hap­pens to have been the same age when huge, fear­some lizards roamed the plan­et, the Mes­o­zo­ic era.

If you’ve ever wondered why dogs and cats have good night vision but poor color vision, the answer may have something to do with dinosaurs, new research claims. According to the study, mammals lost the ability to see color—perhaps never to regain it completely—during a long-ago age when they were mostly active at night. That happens to have been the same era in which huge, fearsome lizards roamed the planet, the Mesozoic era. One doesn’t have to reach far for a plausible account of why our mammal ancestors took to the dark: hungry dinosaurs may have forced them into hiding, according to evolutionary biologist Margaret Hall, who led the study. Hall, with Midwestern University’s Arizona College of Osteopathic Medicine, and her colleagues conducted a wide-ranging study of eye shapes in mammals and other vertebrates, or backboned animals. According to the account they propose, while many mammals are now day-active, their eye structure still retains certain traits typical of night-active animals—which have poor color vision because colors aren’t really visible at night anyway. The exception among mammals is the lineage that includes people, apes and monkeys, called anthropoids. These somehow re-evolved an eye structure typical of day-active creatures, and like humans tend to have good, though not the best, color vision. Most other mammals have weaker, or no, ability to discern colors. “Mammals lost many adaptations for photopic [day] vision” during the time spent sharing the planet with the dinosaurs, reporting their findings online Oct. 24 in the research journal Proceedings of the Royal Society B. “It’s a bit surprising to still see the effects of this long period of nocturnality on living mammals more than 65 million years after non-avian dinosaurs went extinct, but that’s exactly what we found,” said E. Christopher Kirk of the University of Texas at Austin, one of the researchers. “Nearly all living mammals have eye shapes that appear ‘nocturnal’ by comparison with other amniotes,” the evolutionary lineage that encompasses mammals, reptiles and birds. Biological traits that go unused for a long time typically disappear, like the tail in humans and the eye in certain cave fish. This is because species are constantly under pressure to change in a way that best suits their environment. If they possess some unneeded structure, the resources that sustain it are gradually redirected elsewhere. The process behind all this is evolution: individuals with more favorable traits survive and reproduce more than other members of their population. Thus their traits, and corresponding genes, become more common, at the expense of genes for less favorable traits. A useless organ or ability generally counts as less favorable and fades away. Nature can rebuild lost characteristics if freshly changed conditions call for it, but how long that might take depends on many factors. Hall and colleagues argue that for humans and their close relatives, the basic eye shape typical of day vision returned; for other mammals, it didn’t. Why is unclear, they say, but perhaps for many mammals, other sensory abilities evolved during the Mesozoic era that as a group turned out to be adequate substitutes for good day vision. These abilities include excellent sense of smell, and whisker-based touch. Humans and their relatives may have engaged in more small-animal hunting that on the other hand demanded better color vision, the researchers propose. They studied 266 mammal species checking the width of the cornea, the transparent layer covering the eye, relative to eye length. These are two key measures of the eye’s ability to admit light and form sharp images. Their ratio differs between day-active and night-active creatures, biologists say, because there are tradeoffs between the two types of vision; both can’t work optimally in the same eye. Day vision calls for good color sensitivity, but night vision calls for more emphasis on simply discerning objects. Hall and colleagues found that most of the mammals, except the humans and close relatives, had the relatively wide corneas typical of nocturnal animals. The Mesozoic, from about 250 million to 65 million years ago, witnessed both the reign of the dinosaurs and the first mammals. Hall’s group argues that these mammals lost an ability to discern probably four colors, enjoyed by ancestors of theirs before this period. These ancestors therefore might not have been mammals themselves, though they likely had some mammal-like traits. In fact, Hall and colleagues argue, this four-color vision probably existed in a creature that was an ancestor not just of mammals, but reptiles and birds too. Such an animal is believed to have lived tens of millions of years before the Mesozoic. This beast would have been a primitive amniote—an animal that, like all these descendants, encloses its embryo in a membrane called the amnion. If this ancestor could discriminate four colors, that would suggest that even people still haven’t recovered color vision fully. We can see three basic colors; all others are mixtures of those three. Day-active birds are thought to see four or more colors, although such conclusions usually derive from studies of the types of eye pigments rather than on what the animal actually experiences. “Humans and other anthropoid primates are so dependent on vision for everything that they do,” Kirk said. “In this case, we are radically different from other mammals.”