From “You are What You Eat” to “You See What You Eat”: Shedding Light on Ecologically Attuned Vision
If you have ever been temporarily blinded by sunlight after emerging from a building, or have stubbed a toe in the middle of the night, then you realize first-hand that sensitivity to light is a key element of success in one's environment. Animals vary dramatically in their visual abilities under different light conditions: birds and bees use UV vision to see colors that we cannot even perceive, some cave animals forgo vision completely, and the rest of us fall somewhere in between on the spectrum of color perception and visual acuity.
Although there is a general trend for nocturnal species to have good night vision and for diurnal species to have more acute color vision, light detection ability (also known as spectral sensitivity) does vary within groups. A pair of researchers from the University of Texas at Austin recently investigated the role that ecological context may play in influencing the visual pigment types of mammals. They conducted a series of experiments involving forest light conditions and nocturnal mammals, and their results are published in a recent issue of the Journal of Experimental Biology (Veilleux & Cummings 2012).
The researchers started by asking a very basic question: does nocturnal light vary in different forest habitats? They chose to study two regions of Madagascar with disparate forest types: Kirindy Mitea National Park, featuring an open-canopied, dry woodland habitat, and Ranomafana National Park, which consists of a denser, humid rainforest habitat. Foliage density and nocturnal irradiance were measured in each park to quantify differences in light conditions between the two habitat types. As might have been expected, they found that although the wavelength of the maximum flux (maximum nighttime "brightness") was that same, overall nocturnal irradiance was lower in the dense rainforest locations than in the more open, dry woodland habitats.
Having confirmed that there were indeed differences in light conditions between habitats, the researchers confronted the next question: what specific factors influence this variation in irradiance? They generated nocturnal irradiance spectra for each site, and compared these to measures of variables such as lunar phase, habitat type, lunar altitude, canopy structure, and cloud cover. These data were used to develop predictive models, and it became apparent that the additive effects of lunar altitude, lunar phase, and canopy openness predicted nocturnal irradiance levels at a given site. Interestingly, lunar variables were very strong, a good example of the profound and sometimes unexpected effects that planetary dynamics can have on our ecosystems.
Finally, we get to the animals and the most exciting question of the study (unless you are a lunar altitude enthusiast, of course): do ecological preferences—habitat type and diet selection—seem to be related to visual ability? The researchers compared photoreceptor sensitivity for 32 different nocturnal species displaying a range of habitat and diet preferences. They combed the literature to find the peak spectral sensitivities of the visual pigments for each animal (remember that rods are responsible for seeing in low light and cones allow color perception). The pigments are divided into three categories: rods, short-wave sensitive cones (SWS), and medium- to long-wave sensitivity cones (LWS). Due to phylogenetic constraints on potential pigment phenotype, only mammals were included in the analyses.
The selection of mammals for a study on visual pigments warrants a digression: mammals are different than other vertebrates when it comes to visual pigments—in some sense, you might even say our color vision is a bit degraded. For a significant portion of our early evolutionary history, mammals were nocturnal, and as a result we lost two genes, SWS2 and Rh, which encode additional types of cones, leaving most of us with only SWS1 and LWS.
As with many phenomena in nature, there are some interesting exceptions. Eutherians lack SWS2 and Rh genes completely, although it is possible that some marsupials retain a form of the Rh gene. The Monotremata—which contains species that are largely characterized as basal “versions” of mammals—represents a noteworthy outlier. Monotreme species still possess the SWS2 gene that eutherians lack, and their SWS1 gene—which is used for color vision in other mammals—appears to be inactive (Jacobs 2009). In addition, primates have independently acquired three cones types, although the number of cone types within this order still varies between one and three (Jacobs 2009).
Thus, the mammalian complement of visual pigments is--in general--relatively depauperate compared to those of other vertebrates. Mammals function with a reduced set of visual pigments in addition to having a strong evolutionary history of nocturnality, making them a particularly interesting group to consider in a study of ecological correlates to visual ability. This study included primarily eutherians, with a handful of marsupials in the mix. Only one species, the quokka (Setonix brachyurus), was determined to possess more than two cone types. (Note: Perhaps you find it a bit confusing that this animal was in the dataset, as it occurs exclusively in Australia, not Madagascar. I was thrown by this on the first read. There is an explanation, although it isn’t clearly explained in the article: it appears that the set of nocturnal animals analyzed was not restricted to Malagasy species. The data from Kirindy and Ranomafana was used primarily to establish a generalized linkage between habitat type and nocturnal conditions, and the animal data was not restricted to local species, likely due to the lack of photopigment data in the literature for many of Madagascar's understudied animals).
So, back to the analyses: photoreceptor sensitivities were compared between species, and it was apparent that there is little difference in rod sensitivity, yet cone sensitivities did vary a good deal. These peak sensitivities were compared to light conditions in forested habitats, and were also analyzed for relationships to dietary preference. The analyses controlled for phylogenetic relatedness between species.
There were two intriguing takeaways from the comparison of pigment sensitivities to ecological preferences. First, the LWS sensitivity of nocturnal species matched peak flux measured for forests and woodlands, suggesting that the pigments are specifically adapted to detecting nighttime light under the canopy. Second, consumption of fruit and/or flowers significantly predicted the sensitivity of SWS pigments. In other words, it appears as if species have specifically tuned their light perception to accommodate colors that they “need” to see in order to gather these resources. Together, these results strongly suggest that a species’ vision is specifically tuned to aspects of the environment that are important for its survival.
These results should lead to more fascinating research in the future. Marine mammals appear to have adapted their visual pigments to the wavelengths available beneath the ocean surface, reindeer can perceive UV light to help detect lichens and predator fur through snow glare, and it’s possible that most animals see the world through ecologically attuned filters. Nature is the master of “waste not, want not,” and it appears that the loss of unnecessary visual sensitivity may be more common than previously thought. This has profound implications for future evolutionary trends within a lineage, as once a gene is lost, a new mutation is required to re-acquire the lost trait, and the chances of that happening can be vanishingly low (though certainly not impossible).
If we were to swap places with a bee, a bird, a quokka, or a seal, our visual experience of their habitat could be entirely different than theirs. Food for thought, no?
Jacobs, G. H. (2009). Evolution of colour vision in mammals. Philosophical Transactions of the Royal Society B: Biological Sciences, 364 (1531), 2957–2967.
Veilleux, C., & Cummings, M. (2012). Nocturnal light environments and species ecology: implications for nocturnal color vision in forests Journal of Experimental Biology, 215 (23), 4085-4096 DOI: 10.1242/jeb.071415
Animal eye mosaic: http://redwood.berkeley.edu/wiki/VS298:_Animal_Eyes
Kirindy Mitea National Park: http://www.asturi.as/noticias/46807/conozca_parque_kirindy_mitea_madagascar/
Ranomafana National Park http://envirogyro.tumblr.com/post/29199740116/biodiversity-in-ranomafana-national-park-we-need
Opsin genes: Jacobs (2009)
Peacock feather: http://www.bbc.co.uk/nature/18580667