By Amanda Jacobs (Scripps College) and Grace Rodriguez (Claremont McKenna College) [Edited by Lars Schmitz, as part of BIOL 167 “Sensory Evolution”, an upper division class at the W.M. Keck Science Department. Written for educational purposes only].
Scientists have always been aware of animals’ ability to detect ultraviolet (UV) light. While this is a common feature in organisms such as fish, insects, and birds, a study published in January 2014 by Glen Jeffery, a neuroscience professor at London University College; and Ronald Douglas, a biology professor at City University London, decided to research this phenomenon in an overarching study of mammals.
The range of wavelengths that different animals can perceive is dependent on the absorption capabilities of the visual pigments within the retina and the wavelengths of environmental light. Humans can see a spectrum of wavelengths between approximately 400-700 nanometers using three pigments that act as transducers, which convert light into chemical energy that stimulate impulses in neurons. Previously scientists assumed that animals who could detect UV light had types of visual pigments specially adapted to picking up shorter wavelengths and that the only way to estimate such capabilities was to study visual pigments.
Not all visual pigments are alike and can vary between different species as well as within the same species. For example, color blindness in humans is the result of one of the 3 pigments in the cones being defective. Although humans do not have specialized pigments, it has been documented that aphakic individuals (individuals who have defective lenses or have had their lenses removed via surgery) have been able to detect UV light. It is well known that the human lens actually blocks shorter wavelengths, preventing UV light from even reaching the retina where the photoreceptor pigments are. Furthermore, the cornea has also been documented to block wavelengths based on the chemical makeup and thickness, although to a lesser extent than the lens. For the first time, Jeffrey and Douglas focused on what types of light actually reach the retina by focusing on the amount of UV penetration through the lens across a wide variety of mammals.
Jeffrey and Douglas collected the eyes of 38 different mammalian species from zoos, veterinary practices, and other scientific establishments; these species encompassed 25 families in nine different orders. After removing the lenses from the eye, they were mounted in a Shimadzu 2101 UV-PC spectrophotometer for data collection. Light was applied to each lens at 1 nanometer intervals between the range of 300 to 700 nanometers. The percent transmission of each lens at each interval was measured (Percent transmission is a proportion of light intensity entering the sample to light intensity leaving the sample.) Variable numbers of eyes were obtained for each species, and for instances in which many specimens were suitable for study, Jeffrey and Douglas also compared the differences in age and size within the species. All lenses were harvested and studied immediately after death, but in some cases the specimens needed to be temporarily frozen dry, and then thawed for later study. To determine if this preservation process has any effect on light transmission, Douglas and Jeffrey also measured transmission levels before and after flash-freezing; no significant effect was determined (Figure 1) and therefore data collected from both fresh and frozen lenses are comparable.
Between the 38 studied species, the most extreme spectral ranges lie between 50% transmission at 310-320 nm in young murid rodents to 50% transmission at 424-465 nm in primates, sciurid rodents, meerkats, and tree shrews; all other mammals’ spectral capabilities fell between these two extremes. From the 38 studied species, Douglas and Jeffrey selected ten representative lenses that best captured the spectral transmission of all examined specimens (Figure 2).
As expected, the transmission of short wavelengths was highly variable between mammalian species. Young murid rodents’ lenses experience maximum transmission of all light wavelengths in the UVA range (320-400 nm) , while mature diurnal animals such as meerkats and primates have lenses that maximally transmit light in the longer and visible spectrum of wavelengths. Generally, animals that were at least partially nocturnal possessed UV-permissible lenses while the diurnal species did not; the lenses of diurnal species were also observed to have an obvious yellow-coloration. However, this trend is by no means definitive and some exceptions were found, including the okapi which is exposed to large amounts of UV radiation during the day but still possesses a relatively UV-permissible lens.
For species in which multiple specimens were suitable for study, Douglas and Jeffrey also searched for a relationship between short wavelength transmission and the age and size of the animal. In all four species studied (Livingston’s fruit bats, black rats, house mice, and meerkats), short wavelength transmission decreased with both increased age and increased size of the animal (Figure 3).
Previous studies have confirmed that short wavelength UV light can be more damaging to the retina than longer wavelength light (Van Norren & Gorgels, 2011). In conjunction with this previous knowledge, the novel finding that UV-light transmission decreases with age and size of the animal logically makes sense. Blocking UV-light in longer-lived species could protect the animal from these more damaging short wavelengths and protect the retinas during these longer lifespans by slow the degradation of photoreceptors.
The knowledge that some mammals possess the capabilities to detect short wavelengths is not a novel concept (Figure 4), but this paper suggests that UV light sensitivity is much more widespread than previously thought. Other studies have looked as single or a few species, but this is the first one that compares UV sensitivity between many different species using UV penetration of lenses.
To improve this study, more research needs to be done on the yellow pigmentation that was observed in several of the strong UV blocking lenses by identifying the compound(s) that make the lens this color. Furthermore, now that there is data on the UV sensing capabilities across many different clades, one could perform ancestral state reconstructions to determine if UV blocking lenses are independent convergent evolutions or if there is a common ancestor with the trait with multiple losses later on.
Douglas, R.H. & Jeffrey, G. (2014). The spectral transmission of ocular media suggests ultraviolet sensitivity is widespread among mammals. Proc R Soc 281: 20132995. (DOI 10.1098/rspb.2013.2995)
Van Norren D. & Gorgels T.G. (2011). The action spectrum of photochemical damage to the retina: a review of monochromatic threshold data. Photochem. Photobiol. 87, 747–753. (DOI:10.1111/j.1751-1097.2011.00921.x)