First thing I pack when I go to the beach: sunscreen. Without it, I, and many other people, get terrible sunburns, caused by the exposure to ultraviolet (UV) light from the sun. UV light is a type of light that has a shorter wavelength than visible light. This means that it is invisible to the naked eye, but UV rays are what make whites glow under a blacklight, and also what cause sunburns.
To avoid the pain and embarrassment of turning bright red, we generally put on sunscreen before lounging on the beach. However, not all animals react to UV light like we do. Many animals can actually see UV light, and some even use it to communicate. Male jumping spiders, Cosmophasis umbratica, have patches that reflect two different types of UV light—longer wave UVA and short wave UVB. Previous studies have shown that the UVA reflection is used in female mate choice and male-male interaction. However, no one has studied the potential signal in the UVB reflecting areas, so Christina Painting and her colleagues designed a study to figure it out. They hypothesized that because UVA reflectance is used in mate choice and as communication between males, the UVB is likely important for sexual signaling.
Painting and her colleagues ran two experiments: one determining whether UVB was
used for communication with other jumping spiders, and the other testing which was more important for choosing a mate: UVA or UVB. They found that females were significantly more interested in males with UVB reflectance than those without it. Additionally, they discovered that although there was no significant difference in the amount of time females spent examining males with UVA or UVB reflectance blocked, they changed their behavior depending on the type of UV reflectance available. Females courted the males that reflected just UVA light, whereas when males only reflected UVB wavelengths the females acted as if they were encountering another female. Along the same lines, male spiders tried to court other males that had their UVA reflectance blocked (so they only reflected UVB light), suggesting that they thought it was a female.
There are a few different interesting results to pull from this data. Firstly, UVB plays a role in sexual signaling, since females were less attracted to males without UVB reflectance. Secondly, the data suggest that UVA (but not UVB) reflectance is an important communication tool within the species for sex recognition, since both females and males treated males without UVA as if they were females. Treating males as females indicates that the spiders were not able to recognize them as males, so it is likely that the UVA reflectance patches identify the spiders as male. Most studies look at male ornamentation only as a sexual signal, and disregard the possibility of other messages. This study shows that coloration can have other purposes as well, such as sex recognition, and this should be taken into account when studying it in other species. Furthermore, there is a common assumption that even animals that see UVA light are generally unable to detect UVB light, but this study clearly indicates that the spiders use it as a signal. This opens up a whole new field for research, to see whether other animals can use UVB light in similar ways.
Painting, C.J., Rajamohan G, Chen, Z., Zeng, H., & Li, D. (2016). It takes two peaks to tango: the importance of UVB and UVA in sexual signalling in jumping spiders. Animal Behavior 113: 137-146.