Why do different species have different colour vision? Flashcards
Intro
Explain opsin
4 distinct opsin pigments coding for 4 different photopigments
These 4 classes have been around for more than 450 million years (as indicated by the presence of 4 distinct photoreceptors in jawless fish like Lampreys)
Therefore, plenty of time for these 4 photoreceptors to undergo genetic mutation, driven by selective pressures in animals’ environments.
This essay will provide examples of intuitive visual adaption in some species, as well as seemingly counterintuitive colour vision systems in others.
Bowmaker et al (?)
Study of Lake Baikal (1994)
- Water is a monochromator, meaning that light is restricted to progressively shorter wavelengths with depth.
Littoral species (surface dwellers) have cone rich retinas with three spectral classes, to accompany the spectral variety of light entering this habitat.
However, species occupying deeper waters have lost LWL sensitive cones, and a shift of MWL cones progressively shorter wavelengths of light is observed.
Therefore, each species’ visual systems are perfectly adapted for it’s depth-determined light environment within the water.
Bowmarker & Martin ()
(1984) - The most abundant cone pigment in every examined bird species absorbs LWLs of light, around the 570nm region. However, for penguins, an evolutionary adaption means this is not the case…
- In Penguins, this LWL pigment has shifted it’s maximal sensitivity so that it is most sensitive to light in the 543nm region. (Bowmaker & Martin, 1984)
- This renders the penguins with poor discrimination in the LWL (red-green) part of the spectrum
- However, it enables the penguin to have good discrimination in the blue-green part of the spectrum, making the animal well adapted to the spectral qualities of its aquatic environment.
OW vs NW wonkeys
OW moneys trichromatic
NW monkeys polymorphic (male monkeys often dichromatic)
Why did OW monkeys evolve to posses trichromacy?
Frugivory (Sumner & Mollon, 2002)
Folivery (Dominy & Lucas, 2001)
General advantage hypthesis seems more likely (Gegenfurtner, 2003)
- This suggests that any advantage provided by trichromatic vision, such as leaf and fruit finding, was instrumental in the maintenance of the trait
Govardovskii et al (?)
A number of rodents have been shown to possess S-cones with maximal sensitivity in the near UV (360nm). (Including the rat)
(Govardovskii et al, 1992)
Most of the rodent species which have retained UV-sensitive cones are nocturnal which makes UV damage less of an issue.
However, some of the UV-sensitive species are diurnal including the Degu.
The selective pressure which has led to UV sensitivity in these rodents is unclear.
However, one possibility is that the urine which rodents use to scent-mark their territories and trails is highly UV-reflective and that rodents might profit from seeing their scent marks as well as smelling them (Chavez et al, 2003)
This hypothesis still awaits behavioural testing.
Smith et al ()
(2002) UV vision has been well documented for other species of vertebrates, including the fish species ‘guppies’.
In a study by Smith et al (2002), they manipulated the visual appearance of potential mates using either UV transmitting or UV blocking filters.
They found that female guppies significantly preferred males under UV transmitting filters.
Interestingly, they found that males preferred females under UV blocking filters although this was not significant.
They conducted further experiments to control for the role of luminance and concluded that UV wavelengths are more likely being used for colour discrimination than for detecting differences in brightness.
These experiments suggest that UV is being used by this species in mate selection.
Bowmaker & Hunt (?) [whales and dolphins]
In cases described above, the reasons for specific visual adaptations are clear.
In marine mammals like whales and seals however, there is a universal loss of the S-cone pigment, and their retinas are dominated by rods and a small number of LWL cones.
As water transmits primarily SWL light this retinal adaptation in whales and seals is somewhat counterintuitive.
Therefore, the reason why seals and whales have LWL sensitive retinas remains unclear. (Bowmaker & Hunt, 2006).
Marshall and Arikawa (?) Daphnia
(2014) Ideal visual adaptations are not always of paramount importance….
e. g. Although the waterflea Daphnia displays 4 well spread spectral channels, its low resolution compound eye and simple lifestyle renders this tetrachromatic adaption relatively unnecessary for survival.
‘Colour dances’ in response to UV and MWL light?
Conclusion
As mentioned in the first half of the essay, some animals have visual systems that are well adapted to visual environment, for food foraging, territory marking and mate selection.
However, the visual system of some animals seem uncoupled to any of these aspects.
- Perhaps for some animals e.g Daphnia, their colour visual systems is simply an evolutionary artifact, once used by evolutionary ancestors but now rendered relatively redundant.
- Instead, these animals may utilize other senses to ensure survival.
Therefore, the colour of vision of some species serves a functional advantage for their day to day survival while other species’ colour vision may simply reflect a series of unknown past and now relatively superfluous selective pressures.