before it's in the papers"
June 03, 2013
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Could dinosaurs have shaped the way
mammals see the world?
Nov. 1, 2012
Courtesy of University of Texas at Austin
If youâ€™ve evÂer wonÂdered why dogs and cats have good night viÂsion but poor colÂor viÂsion, the anÂswer may have someÂthing to do with diÂnoÂsaurs, new reÂsearch claims.
AcÂcordÂing to the stuÂdy, mamÂmals lost the abilÂity to see colÂorâ€”perÂhaps nevÂer to reÂgain it comÂpleteÂlyâ€”durÂing a long-aÂgo age when they were mostly acÂtive at night. That hapÂpens to have been the same
age when huge, fearÂsome lizards roamed the planÂet, the MesÂoÂzoÂic era.
One doesÂnâ€™t have to reach far for a plauÂsiÂble acÂcount of why our mamÂmal anÂcesÂtors took to the dark: hunÂgry diÂnoÂsaurs may have forced them inÂto hidÂing, acÂcordÂing to evÂoÂluÂtionÂary biÂolÂoÂgist MarÂgaÂret Hall, who led the stuÂdy.
OF SELECTED ANIMALS
COLORS THEY SEE
(octopi and squids)
SEE JUST TWO COLORS
SEE SOME COLOR
COLOR AND INFRARED
OR FIVE COLORS for day-active birds
COLORS BUT WEAKLY
COLORS BUT WEAKLY
(primates-apes and chimps)
(South American monkeys)
SEE RED WELL
that biologists believe are possessed by various animal species.
(Courtesy Arizona State U., Tufts U.)
Hall, with MidÂwestÂern UniÂversÂityâ€™s ArÂiÂzoÂna ColÂlege of OsÂteÂoÂpathÂic MedÂiÂcine, and her colÂleagues conÂducted a wide-rangÂing study of eyeÂball shapes in mamÂmals.Â
AcÂcordÂing to the acÂcount they proÂpose, while many mamÂmals are now day-acÂtive, their eye strucÂture still reÂtains cerÂtain traits typÂiÂcal of night-acÂtive anÂiÂmalsâ€”which have poor colÂor viÂsion beÂcause colÂors arenâ€™t
realÂly visÂiÂble at night anÂyÂway.
The exÂcepÂtion among mamÂmals, the reÂsearchÂers add, is the lineÂage that inÂcludes peoÂple, apes and monÂkeys, called anÂthroÂpoids. These someÂhow re-eÂvolved an eye strucÂture typÂiÂcal of day-acÂtive creaÂtures, and like huÂmans tend to have good, though not
outÂstandÂing, colÂor viÂsion.
Most othÂer mamÂmals have weakÂer, or no, abilÂity to disÂcern colÂors.
â€œMamÂmals lost many adaptaÂtÂions for phoÂtopÂic [day] viÂsionâ€ durÂing the time spent sharÂing the planÂet with the diÂnoÂsaurs,
the team wrote, reÂportÂing their findÂings onÂline Oct. 24 in the reÂsearch jourÂnal
ProÂceedÂings of the RoyÂal SoÂciÂeÂty B.
â€œItâ€™s a bit surÂprisÂing to still see the efÂfects of this long peÂriÂod of nocÂturÂnalÂity on livÂing mamÂmals more than 65 milÂlion years afÂter non-avian diÂnoÂsaurs went exÂtinct, but thatâ€™s exÂactly what we found,â€ said E. ChrisÂtoÂpher Kirk of the UniÂversÂity of TexÂas at AusÂtin, one of the reÂsearchÂers. â€œNÂearly all livÂing mamÂmals have eye shapes that apÂpear â€˜nocÂturÂnalâ€™ by comÂparÂiÂson with othÂer amÂniotes,â€ the evÂoÂluÂtionÂary lineÂage that enÂcomÂpasses mamÂmals, repÂtiles and birds.
BiÂoÂlogÂiÂcal traits that go unÂused for a long time typÂicÂally disapÂpear, like the tail in huÂmans and the eye in cerÂtain cave fish. This is beÂcause speÂcies are conÂstantly unÂder presÂsure to change in a way that best suits their enÂviÂronÂment. If they posÂsess some unÂneeded strucÂture, the reÂsources that susÂtain it are gradÂuÂally rediÂrected elseÂwhere.Â
The proÂcess beÂhind all this is evÂoÂluÂtion: inÂdiÂvidÂuÂals with more faÂvorÂaÂble traits surÂvive and reÂproÂduce more than othÂer memÂbers of their populaÂtÂion. Thus their traits, and
the corÂreÂspondÂing genes, beÂcome more comÂmon, at the exÂpense of genes for less faÂvorÂaÂble traits. A useÂless orÂgan or abilÂity geneÂrally counts as
unÂfaÂvorÂaÂble and fades away.
NaÂture can reÂbuild lost charÂacÂterÂisÂtics if freshly changed conÂdiÂtions call for it, but how long that might take deÂpends on many facÂtors.
Hall and colÂleagues arÂgue that for huÂmans and their close relÂaÂtives, the basÂic eye shape typÂiÂcal of day viÂsion reÂturned; for othÂer mamÂmals, it didÂnâ€™t. Why is unÂclear, they say, but perÂhaps for many mamÂmals, othÂer senÂsoÂry abilÂiÂties evolved durÂing the MesÂoÂzoÂic era that as a group turned out to be adÂeÂquate subÂstiÂtutes for good day viÂsion. These abilÂiÂties inÂclude exÂcelÂlent sense of smell, and whisker-based touÂch. HuÂmans and their relÂaÂtives may have enÂgaged in more small-anÂiÂmal huntÂing that on the othÂer hand deÂmanded betÂter colÂor viÂsion, the reÂsearchÂers proÂpose.
They studÂied 266 mamÂmal speÂcies checkÂing the width of the corÂnea, the transÂparÂent layÂer covÂerÂing the eye, relÂaÂtive to eye length. These are two key measÂures of the eyeâ€™s abilÂity to adÂmit light and form sharp imÂages. Their raÂtio difÂfers beÂtween day-acÂtive and night-acÂtive creaÂtures, biÂolÂoÂgists say, beÂcause there are tradeÂoffs beÂtween the two types of viÂsion; both canâ€™t work opÂtiÂmally in the same eye. Day viÂsion calls for good colÂor senÂsiÂtiÂvity, but night viÂsion calls for more emÂphaÂsis on simply disÂcerning obÂjects.
Hall and colÂleagues found that most of the mamÂmals, exÂcept the huÂmans and close relÂaÂtives, had the relÂaÂtively wide corÂneÂas typÂiÂcal of nocÂturÂnal anÂiÂmals.
The MesÂoÂzoÂic, from about 250 milÂlion to 65 milÂlion years ago, witÂnessed both the reign of the diÂnoÂsaurs and the first mamÂmals. HalÂlâ€™s group arÂgues that these mamÂmals lost an abilÂity to disÂcern probably four colÂors, enÂjoyed by anÂcesÂtors of theirs beÂfore this peÂriÂod. These anÂcesÂtors thereÂfore might not have been mamÂmals themÂselves, though they likely had some mamÂmal-like traits.
In fact, Hall and colÂleagues arÂgue, this four-colÂor viÂsion probably exÂisted in a creaÂture that was an anÂcesÂtor not just of mamÂmals, but repÂtiles and birds too. Such an anÂiÂmal is beÂlieved to have lived tens of milÂlions of years beÂfore the MesÂoÂzoÂic. This beast would have been a primÂiÂtive amÂnioteâ€”an anÂiÂmal that, like all these deÂscenÂdants, enÂcloses its emÂbryÂo in a memÂbrane called the amÂniÂon.
If this anÂcesÂtor could disÂcrimÂiÂnate four colÂors, that would sugÂgest that even peoÂple still havenâ€™t reÂcovÂered colÂor viÂsion fulÂly. We can see three basÂic colÂors; all othÂers are mixÂtures of those three. Day-acÂtive birds are thought to see four or more colÂors, alÂthough such conÂcluÂsions usuÂally deÂrive from studÂies of the types of eye pigÂments rathÂer than on what the anÂiÂmal acÂtuÂally exÂpeÂriÂences.
In the end, only our own experience is what we can see directly.
â€œHuÂmans and othÂer anÂthroÂpoid priÂmaÂtes are so deÂpendÂent on viÂsion for evÂerything that they do,â€ Kirk said. â€œIn this case, we are radicÂally difÂferÂent from othÂer mamÂmals.â€
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If youâ€™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 colorâ€”perhaps never to regain it completelyâ€”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 doesnâ€™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 Universityâ€™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 animalsâ€”which have poor color vision because colors arenâ€™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.
â€œMammals lost many adaptations for photopic [day] visionâ€ 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.
â€œItâ€™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 thatâ€™s exactly what we found,â€ said E. Christopher Kirk of the University of Texas at Austin, one of the researchers. â€œNearly all living mammals have eye shapes that appear â€˜nocturnalâ€™ by comparison with other amniotes,â€ 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 didnâ€™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 eyeâ€™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 canâ€™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. Hallâ€™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 amnioteâ€”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 havenâ€™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.
â€œHumans and other anthropoid primates are so dependent on vision for everything that they do,â€ Kirk said. â€œIn this case, we are radically different from other mammals.â€