Forum for Science, Industry and Business

Sponsored by:     3M 
Search our Site:

 

Survey explains why some animals have smaller eyes: Lifestyle matters more than size

09.08.2004


If brain size is proportional to body size in virtually all vertebrate animals, Cornell University biologists reasoned, shouldn’t eye size and body size scale the same way? While they failed to find a one-size-fits-all rule for eyes, what they learned about the 300 vertebrates they studied helps to explain how animals evolved precisely the orbs they need for everyday life.

The biologists reported their findings in the journal Vision Research (August 2004, "The allometry and scaling of the size of vertebrate eyes"). Howard C. Howland, Stacey Merola and Jennifer R. Basarab say they did find a logarithmic relationship between animals’ body weight and eye size for all vertebrates, in general: Bigger animals do tend to have bigger eyes, on average.

But breaking vertebrates into smaller groups -- such as birds, fishes, reptiles and mammals -- and trying to predict their eye size gets more complicated. And dividing all mammals into groups -- such as rodents and primates -- could make a scientist cross-eyed: Compared with all vertebrates, rodents’ eyes are only 61 percent as large as they "should be" if all animals obeyed the general rule, while primates’ eyes are 35 percent larger than those of vertebrates as a whole. Except for gorillas, that is.



Says Howland, a Cornell professor of neurobiology and behavior who led undergraduate students Merola and Basarab through the survey: "A carnival barker who correctly guesses your weight could also derive the weight of your brain with this allometric equation: log (organ size) = slope constant * log (body weight) + intercept constant . Allometry refers to the scaling of size of animals and their parts, and it works amazingly well for brain size and body weight.

"But the carny who wants to guess your eye size would have to know something more about how you use your vision and your other senses. Larger eyes are better for gathering light. Birds in general and owls in particular have relatively large eyes because their vision is critically important to them. So do most nocturnal mammals," Howland says. "But a shrew’s eyes are tiny because it

spends most of its time underground, where things are dark, but where the shrew’s senses of smell and touch are more important [than vision] for making a living."Or the carnival barker could simply measure eye size, which is what Merola did for some dissected specimens, while she and Basarab scanned the scientific literature for weight-and-size information already gathered by other biologists on other species. Then the student biologists turned to their graphing calculators to enter the values in the allometric equation and compared the results for various animal groups to the average for all vertebrates.

Some results were reasonably consistent. Most reptiles, for example, tend to have eyes only 70 percent as large as vertebrates in general. But relative eye size in fishes was all over the map, in part because their body weight in the water is less constrained by gravity than the weight of an animal out of water. One aquatic creature that fails to obey the bigger body-bigger eyes rule is the reedfish, a lanky, well-camouflaged fish with no apparent head and little room for eyes.

The Cornell biologists introduced their Vision Research report with a brief comment on perception and reality, noting that "cartoonists often exploit the fact that the eyes of babies are much larger in proportion to body size than the eyes of adults." They point out that many of the smaller non-human primates -- the ones we humans think are cute -- tend to have much larger eyes than their body size might suggest, whereas the biggest primate, the gorilla, has especially small eyes for its body size. "I imagine gorillas don’t have to worry whether we think they’re cute," Howland says.

He hopes the eye-size research, which was sponsored by the National Institutes of Health, will encourage other investigators to ask the questions: Is that a big eye or a small eye, and why? What is it about environments in which animals live that determines eye size?

For his part, the Cornell neurobiologist has moved on to a new study of eye size throughout various stages of development. He observes that human eyes grow rapidly in the womb and for the first three months after birth. That explains why babies are so adorably cute: Their disproportionately big eyes gaze out from those little round faces.

By three months, our eyes are as big as they’ll ever be -- at least from outward appearances: The corneas have reached their full width, although inside the eyes, the neurobiologist notes, the front-to-back length will increase somewhat. Then our eyes move farther apart in the face as the head grows. And we’ll never be that cute again.

Roger Segelken | EurekAlert!
Further information:
http://www.cornell.edu

More articles from Life Sciences:

nachricht Zap! Graphene is bad news for bacteria
23.05.2017 | Rice University

nachricht Discovery of an alga's 'dictionary of genes' could lead to advances in biofuels, medicine
23.05.2017 | University of California - Los Angeles

All articles from Life Sciences >>>

The most recent press releases about innovation >>>

Die letzten 5 Focus-News des innovations-reports im Überblick:

Im Focus: Turmoil in sluggish electrons’ existence

An international team of physicists has monitored the scattering behaviour of electrons in a non-conducting material in real-time. Their insights could be beneficial for radiotherapy.

We can refer to electrons in non-conducting materials as ‘sluggish’. Typically, they remain fixed in a location, deep inside an atomic composite. It is hence...

Im Focus: Wafer-thin Magnetic Materials Developed for Future Quantum Technologies

Two-dimensional magnetic structures are regarded as a promising material for new types of data storage, since the magnetic properties of individual molecular building blocks can be investigated and modified. For the first time, researchers have now produced a wafer-thin ferrimagnet, in which molecules with different magnetic centers arrange themselves on a gold surface to form a checkerboard pattern. Scientists at the Swiss Nanoscience Institute at the University of Basel and the Paul Scherrer Institute published their findings in the journal Nature Communications.

Ferrimagnets are composed of two centers which are magnetized at different strengths and point in opposing directions. Two-dimensional, quasi-flat ferrimagnets...

Im Focus: World's thinnest hologram paves path to new 3-D world

Nano-hologram paves way for integration of 3-D holography into everyday electronics

An Australian-Chinese research team has created the world's thinnest hologram, paving the way towards the integration of 3D holography into everyday...

Im Focus: Using graphene to create quantum bits

In the race to produce a quantum computer, a number of projects are seeking a way to create quantum bits -- or qubits -- that are stable, meaning they are not much affected by changes in their environment. This normally needs highly nonlinear non-dissipative elements capable of functioning at very low temperatures.

In pursuit of this goal, researchers at EPFL's Laboratory of Photonics and Quantum Measurements LPQM (STI/SB), have investigated a nonlinear graphene-based...

Im Focus: Bacteria harness the lotus effect to protect themselves

Biofilms: Researchers find the causes of water-repelling properties

Dental plaque and the viscous brown slime in drainpipes are two familiar examples of bacterial biofilms. Removing such bacterial depositions from surfaces is...

All Focus news of the innovation-report >>>

Anzeige

Anzeige

Event News

AWK Aachen Machine Tool Colloquium 2017: Internet of Production for Agile Enterprises

23.05.2017 | Event News

Dortmund MST Conference presents Individualized Healthcare Solutions with micro and nanotechnology

22.05.2017 | Event News

Innovation 4.0: Shaping a humane fourth industrial revolution

17.05.2017 | Event News

 
Latest News

Scientists propose synestia, a new type of planetary object

23.05.2017 | Physics and Astronomy

Zap! Graphene is bad news for bacteria

23.05.2017 | Life Sciences

Medical gamma-ray camera is now palm-sized

23.05.2017 | Medical Engineering

VideoLinks
B2B-VideoLinks
More VideoLinks >>>