Reference: Ohmer, M.E.B., Cramp, R.L., White, C.R., Harlow, P.S., McFadden, M.S., Merino-Viteri, A., Pessier, A.P., Wu, N.C., Bishop, P.J. and Franklin, C.E. 2019. Phylogenetic investigation of skin sloughing rates in frogs: relationships with skin characteristics and disease-driven declines. Proc. R. Soc. B 286: 20182378. http://dx.doi.org/10.1098/rspb.2018.2378
Skin is a barrier and a boundary. It connects and separates us from the world. It allows us to interact and perceive our environment in subtle, yet profound ways, by discerning variations in pressure, pain and temperature detection (ex. the feeling of a feather, a hot curling iron, or a cool breeze on your skin). Our skin envelopes us and provides a first line of defense against foreign invaders and it often goes overlooked until the assault is too great. Human skin is well adapted to our environment. So, too, is the skin of amphibians.
Amphibian Super Powers
In grade school we learn amphibians (i.e. frogs and salamanders) are quite unique for two reasons: they can comfortably live both on land and in water as well as have highly permeable skin which allows the passage of water, oxygen and minerals.
With approximately 8,000 known species of amphibians, this animal group lives in a wide variety of habitats with extremely different temperatures and moisture levels. Among amphibian species, many have developed highly specialized skin structures that help them survive in the environmental conditions of their habitat. For example, some species in arid climates have waxy lipids in their skin to reduce water loss; other species have glands that secrete mucus to help cool them during hot temperatures. Amphibian skin is not as uniform as we may have once thought.
Like you and I, amphibians shed, or slough, the top layer of their skin. While we shed small amounts continuously, amphibians shed the entire top layer in one piece through a series of limb and body movements. Afterwards, the shed skin is eaten (that’s right …. eaten). If you would like to see what this looks like, check out this video .
This shedding event can occur anywhere from daily through to every 2 weeks. This process is thought to play a role in controlling the growth of bacterial populations on the skin and possibly diminishing the effects of skin diseases such as chytridiomycosis, a fungal infection.
Chytridiomycosis is caused by a fungus called Batrachochytrium dendrobatidis (Bd) that currently threatens over 500 amphibian species world-wide. This fungus damages the outer most layer of the amphibian’s skin, disrupts its ability to regulate respiration, water and electrolytes, which eventually leads to death. It kills frogs and salamanders slowly thus increasing the probability that it will spread its spores by touch or through water. It is poorly understood why some populations of amphibians are highly susceptible to this disease while others are not. Species like the American bull frog do not succumb to the disease easily, but do provide a home for the fungus to multiply and spread to other frog species and individuals.
Could differences in skin structure and shedding rates predict disease susceptibility?
Dr. Michel Ohmer and a team of scientists aimed to uncover the secrets of amphibian skin structure and shedding rates in the hopes to find patterns with disease susceptibility. To potentially connect these two phenomena, Ohmer et al. gathered data related to sloughing rates and skin structure from 21 frog species from 8 different families. The sloughing rates of frogs housed in zoos or captive breeding programs were observed using remote monitoring with infrared cameras. Preserved skin specimens were gathered from museums and captive breeding programs to determine skin thickness and number of layers. Literature was collected for each frog as to the recorded incidences of chytridiomycosis, their behaviours, their habitat preference for breeding (ex. streams, ponds, land), and their location on the evolutionary tree (like a family tree, but instead outlines how different animal species are related to each other based on DNA and common physical characteristics).
What did we learn?
The rate of shedding for each frog species differed significantly from every day to every other week. Also, closely related frog species shared similar skin shedding rates. The process of skin shedding appears to be an evolutionary conserved trait that is not dependent on size, skin thickness, number of skin layers, environmental temperature or breeding habits. Using this information, scientists can predict the rate at which a frog sheds its skin based on its location in the evolutionary tree.
Unfortunately, a link between disease-related amphibian declines and skin shedding rates was not found due to a limited number of species examined. However, a link was found between disease related amphibian declines and breeding behaviours. Stream and pond breeders have experienced some of the greatest declines due to chytrid fungus infections. In addition, the research hinted that species in this category were more likely to have longer periods between sloughing events.
What can be done with this information?
For the first time, scientists will be able to predict Bd growth on individual host frog species as well as the probability of their survival upon exposure to it. Conservation efforts could be focused on those species with particularly long periods between shedding events, since they are more likely to act as a reservoir for fungal spores to multiply and/or succumb to the disease more quickly. In addition, there is the potential to introduce beneficial bacteria immediately after shedding in order to act against the fungus and prevent the disease from progressing, independently of the frog’s skin type.