Physiology - Comparison between mammals and birds

Comparison between mammals and birds

Table 1. Comparison of number of cones, visual information process, and proportion of nerve fibres that do not cross over to the opposite side in the optic chiasm between mammal and avian

	Mammal	Bird
Number of cones	Most diurnal mammals have 2 cones (dichromatic).	Diurnal birds have 3 to 4 types. Most birds are tetrachromatic. The cones in birds contain small, coloured oil droplets which absorb certain wavelengths before light reaches the photopigment. This “light filtration” results the sensitivity of the cones in more finely tuned to the optimal wavelength than in mammals.
Visual information process	Occurs in the paired nuclei superior colliculus in the midbrain. In mammals, the optic tectum is a small sized part in the midbrain.	Occurs in the optic tectum in the midbrain. In birds, the optic tectum is a dominant part of the midbrain.
Proportion of nerve fibres that do not cross over to the opposite side in the optic chiasm	Human: 50% Cat: 30% Dog: 25% Horse: 15%	Almost 100%

Reference

1. Sjaastad O.V., Sand O. and Hove K. (2010) Physiology of domestic animals, 2nd edn., Oslo: Scandinavian Veterinary Press.