The authors of this
study state that most mammals have dichromatic vision as a result of having
eyes with two types of cones: short-wave sensitive (S) and middle-to-long wave
sensitive (L). Seals, however,
only have L-cones and are therefore monochromats
because the marine setting in which they live does not require them to receive
short-wave light signals. The
study was done to determine whether polar bears are dichromats
like most mammals, despite living in a relatively colorless environment and
feeding on prey that lives in the same habitat and has monochromatic
vision.
To research their question, the authors retrieved the eyes of polar bears as well as four subspecies of brown bears from zoo autopsies and fixed them in formalin, an aqueous solution of formaldehyde. They used L-opsin antiserum JH492, S-opsin antiserum JH455, and S-opsin antiserum sc-14363 to analyze the composition of the bearsÕ retinas. The results of the study were that both L-cones and S-cones were present in all the bear eyes, indicating that the brown bears as well as polar bears are dichromats. They even concluded that the S-cones represented a high proportion of overall cones, reaching 15%-25% and even 35%. Additionally, they mentioned that cone density and the proportion of S-cones is higher in the bears than in many other mammals. The researchers also double-labeled the cones for L-opsin and S-opsin to confirm that they were pure in their signal reception and did not express both types of waves.
The authors concluded that polar and brown bears have dichromatic vision based on the presence of two types of cones in their retinas. They proposed that polar bears have separate visual demands and adaptive pressures than seals since both cone types have been preserved in their eyes even though the environment they live in lacks a lot of color.
This article relates to and expands somewhat on material covered in lecture. It reinforces the presence of multiple types of cones that are sensitive to different wavelengths. However, I wish the authors had gone more in depth while discussing this portion of the paper. I also would have liked to read more about their cut-off values for classification as a high proportion versus a lower proportion of S-cones. Something I did not realize before reading this study was that cones were capable of showing opsin co-expression.