"Long before it's in the papers"
June 03, 2013


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.


CEPHALOPODS (octopi and squids) BLUE ONLY
BIRDS FOUR OR FIVE COLORS for day-active birds
MAMMALS (primates-apes and chimps) SAME AS HUMANS
MAMMALS (African monkeys) SAME AS HUMANS
MAMMALS (South American monkeys) CAN'T SEE RED WELL

Color-sensing abilities that biologists believe are possessed by various animal species.  (Courtesy Arizona State U., Tufts U.)

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 eye­ball shapes in mam­mals. 

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, the re­search­ers add, 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 out­stand­ing, 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, the team wrote, 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 the cor­re­spond­ing genes, be­come more com­mon, at the ex­pense of genes for less fa­vor­a­ble traits. A use­less or­gan or abil­ity gene­rally counts as un­fa­vor­a­ble 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.

In the end, only our own experience is what we can see directly.

“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.”

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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.”