What about birds?
The pecten oculi is an unusual and poorly understood organ in the eyeball of most birds. The only bird to have a much diminished organ is the New Zealand kiwi which has abandoned vision in favour of touch.
The pecten oculi is a comb-like structure of blood vessels belonging to the choroid. It is a non-sensory, pigmented structure that projects into the vitreous body from the point where the optic nerve enters the eyeball. It therefore minimizes obstruction of the birds vision The pectin is present in the eyes of all birds and some reptiles. Most flying birds have a pleated topology with an almost rectangular profile consisting of about 12 pleats, held together apically by a heavily pigmented ‘bridge‘. Ascending and descending blood vessels of varying calibre are joined by a profuse network of capillaries reported to be about 13-20 μm in diameter. It has been proposed as an organ to nourish the vitreous humor. However the organ has very high levels of carbonic anhydrase and other enzymes compatible with the catalysis of blood reactions. The working assumption is that birds use this organ to control blood chemistry.
There is a strong similarity of the internal structures of the pecten oculi and the dermal papilla. We believe the dermal papilla is an organ for cooling the blood by a catalysed endothermic reaction. If the pecten oculi has a similar function then we can speculate that the purpose of this organ is to control the temperature of the retina. Thermal images of nocturnal birds often show cold eyes as shown below.
It also fits with slower nocturnal birds having smaller pectens than more energetic daytime birds and the general trend of the pecten size with the weight of the bird. The capilliary spacing of 13-20 micron already hints that there is some tuning to the bioluminescent wavelengths. The main problem with this theory is that the refractive indices of blood and aquaeous humor are too similar to enable any effective optical beam steering (to direct the infrared through the front lens). For this reason the purpose may be to simply control the temperature of the eyeball and in particular the retina.
It is interesting to develop the infrared radiation theory. There are reports of a distinct distribution of spherical melanosomes of 1.6 to 2.2 microns diameter, densest on the bridge and upper end of the structure and strongly associated with the capillary network. This is assumed to be for protection from UV because unlike mammals birds do not have UV filtering in the front lens. The melanosome spheres however could have another function. The melanosomes can be considered as dielectric, sub-wavelength, nanospheres with the correct diameter range to exhibit anomalous scattering including control of the directionality. It would be of great interest to the nano-optic community to get a 3D distribution map of the melanosomes in the pecten to see if birds are exploiting quantum optical effects to achieve beam control.
Experts can review their research on the pecten oculi in the light of this new theory. In particular does the pecten size relate to the birds metabolic energy requirements?
M.I. Tribelsky, J-M. Geffrin, A. Litman et al, “Small Dielectric Spheres with High Refractive Index as New Multifunctional Elements for Optical Devices”, Scientific Reports 5, Article number: 12288 (2015), doi:10.1038/srep12288
M.O. Dayan and T. Ozaydin, “A Comparative Morphometrical Study of the Pecten Oculi in Different Avian Species”, The Scientific World Journal, Volume 2013 (2013), Article ID 968652, http://dx.doi.org/10.1155/2013/968652
S.G. Kiama, J. Bhattacharjee, J.N. Maina and K.D. Weyrauch, “A scanning electron microscope study of the pecten oculi of the black kite (Milvus migrans)”, J, Anat, 1994 Dec; 185(Pt 3): 637–642.
Last updated – 28th February 2018