Hi Gemma, Interesting question indeed. I think, the significance of having different types of photoreceptors in different animals is for their survival in nature. Having many photoreceptors will increase the visible spectrum which might enhance their ability to catch food, run away from other hunting animals, adaptability to the environment. As I am not into evolution field, I don't have much knowledge about it but would love to learn more on scientific evidences regrading your question.

Regards.

Hi Gemma,

There are paired cones of a variety of constellations, e.g. identical twin cones (same morphology and same pigment), non-identical twin cones (same morphology, but different pigments) and identical double cones (different morphology but same pigments. They may have different functions, also depending on the question whether the members are electrically coupled or not. Proposed functions are chromatic discrimination (e.g. when the members have different absorbation spectra), luminosity detection, and even (somewhat hypothetically) detection of the polarization plane of light (see e.g. http://www.nature.com/nature/journal/v353/n6340/abs/353161a0.html).

But experimental data are rather scarce, unfortunately.

Best regards

QK

There have been other proposed roles, in addition to the various functional roles that Quirin mentioned above, including facilitation of an optimum mosaic distribution (to presumably improve photon catch) and changes during development, see e.g. http://jcs.biologists.org/content/s3-98/42/189.full.pdf+html

These are references you have likely seen but good overviews of some of the proposed functional roles of the cone types including some experimental studies:

http://www.vision-in-cichlids.com/why_double_cones.htm

http://rsbl.royalsocietypublishing.org/content/early/2010/01/28/rsbl.2009.1010.full

http://web.qbi.uq.edu.au/ecovis/Images/double%20cones/ARCDiscovery2004f_Esection.pdf

Cheers-cjb

This has been a long-standing question among visual ecologists and to be honest there is no good answer yet. Ironically, whlle many vision scientists think double cones the exception since we and over mammals do not have them, they are infact the rule in the majority of vertebrates. Previous literature had suggested that double cones were not involved in colour discrimination but there is now some evidence that this is not always true (See Pignatelli et al.2010 biology letters).

Thanks for some really great answers. There are so many proposed purposes out there but none of them seem to be particularly well understood. Obviously, each species will have its own uses for these cone types depending on its ecology (which adds to the complexity). There must be shared evolutionary trajectories that has resulted in the formation of the multiple-cones in related taxa which would presumably have the same (if not similar) functions (especially considering that they are not present/common in mammals). Is phylogenetics a feasible way of providing answers or is there a need for more behavioural/ecological based evidence?

Gemma

Justin Marshall's group at UQ have been interested in this question for some time, so it would be worth touching bases with him if you want to explore it further. I look forward to one day reading about your finding and insights into this intriguing question.

Purpose suggests that evolutionary process is teleological. Let us stick with advantage, which suggests adaptation by natural selection.

This might help answer the question:

Preprint A Model of Color Vision for Remote Sensing 1