Amphibians are considered natural sentinels. A sentinel animal is an animal that is “sensitive to a particular environmental factor- e.g., a toxin, pathogen, radiation- whose illness or death serves to warn humans of an impending danger,” (Segen’s Medical Dictionary, 2012). Frogs possess this unique role as environmental indicators because they exhibit susceptibility to environmental changes on land and water (Badzinski, 2008).
Over the last thirty years, amphibian populations have been in mass declines due to various diseases (Christin et al, 2012). Frogs have also been appearing with abnormalities such as missing or extra limbs.
The northern leopard frog, Rana pipiens, is considered an excellent environmental indicator because it is highly adapted to its habitat; therefore it is able to provide information on the conditions of wetlands (Species Profile: northern leopard frog). In 1995 in Minnesota, Cindy Reinitz and her students found that northern leopard frogs were having difficulty jumping (Lanoo, 2008). It was uncovered that the frogs were missing legs or parts of their legs, which made jumping difficult, and this caused them to be more vulnerable to predators, because they would fall and expose their bellies (Lanoo, 2008). These abnormalities are described as malformations in frogs.
Malformation is the term used to describe a lack of symmetry, an imbalance of structure, colour or other quality, which is unintended in animals (Lanoo, 2008). Typically there are three main mechanisms by which malformations in frogs occur. The first type is genetic, when the expressions of genes are flawed during development (Lanoo, 2008). An example of a genetic malformation is albinism in frogs, which causes white bodies and pink eyes.
The second type is epigenetic, which is when there is some deviation due to an external circumstance at the time of development (Lanoo, 2008). In epigenetic cases of malformation, there may be nutrients missing or the presence of chemicals that disrupt development in frogs. Epigenetic mechanisms are the most common cause of frog malformations. The third cause of malformations is trauma, which includes injuries and/or diseases that occur after the developmental stage (Lanoo, 2008).
I will be focusing on the epigenetic causes of malformed frogs. More specifically, what effect do agricultural chemicals such as pesticides or herbicides have on the northern leopard frog?
The northern leopard frog derives its name from its medium sized spots. It is also called the “meadow frog” because it often prefers grassy habitats (Species profile: northern leopard frog). Northern leopard frogs are semi-terrestrial amphibians, meaning that they breed and hibernate in water but spend most of their time in grassy, open or wooded areas (Species profile: northern leopard frog).
Northern leopard frogs breed in ponds with mild temperatures, avoiding larger bodies of water which may be cooler in temperature, and avoiding shallow ponds which may warm up too quickly (Species profile: northern leopard frog). Eggs laid remain in the ponds during development, often resting on the bottoms of ponds. The eggs may hatch between 13 and 20 days, and the tadpoles metamorphose into frogs 60-80 days after hatching (Species profile: northern leopard frog). Young frogs stay close to water, which they use to escape from predators, and for regulating their body temperatures (Species profile: northern leopard frog). The period during which the eggs and tadpoles are developing in the pond water is crucial for epigenetic causes of malformations in frogs.
Amphibians, or anurans, spend a large amount of their time in or around water, where pollutants that don’t mix with water (hydrophobic) accumulate (Shenoy et al, 2009). Exposure to these pollutants while an amphibian is still developing can have long-term effects on the fitness of the frog (Shenoy et al, 2009).
Exposure to some chemical pollutants can lower the immune systems of frogs (Christen et al, 2012). A pesticide concoction of altrazine, metribuzine, endosulfan, lindane, dieldrin and aldicarb was used in a laboratory to test the effects on frogs’ immune systems. The reason there was a mixture of chemicals is because wild frogs are usually exposed to multiple chemicals at once in low doses (Christin et al, 2012).
The results of the experiment demonstrated that exposure to pesticides could negatively impact the frogs immune system, causing phagocytosis by splenocytes and the growth of lymphocytes from the spleen (Christin et al, 2012). Phagocytosis by splenocytes means that white blood cells ingested bacteria or other materials. When white blood cells increase rapidly in numbers on the spleen it indicates that they are trying to kill a disease or infection.
Endosulfan, an insecticide most commonly used on food crops, affects the nervous system and can alter sexual development in males (Shenoy et al, 2009). Endosulfan becomes endosulfan sulfate when it breaks down in water, and concentrations in waters near agricultural sites can reach high levels (Shenoy et al, 2009).
Another widely used agricultural is mancozeb. It can cause physical deformities (Shenoy et al, 2009). When mancozeb breaks down, it can alter thyroid function, which is key to development and metamorphosis in amphibians (Shenoy et al, 2009). Because these chemicals attach themselves to soil particles, developing larvae may be exposed to them for longer periods of time (weeks) due to soil run-off during periods of rain (Shenoy et al, 2009).
A study using northern leopard frog tadpoles (Rana pipiens) gave the tadpoles three different treaments; high concentrations of endosulfan and mancozeb, medium concentrations and low concentrations (Shenoy et al, 2009). The study also included a control, which was no treatment. Tadpoles were exposed to the treatments for seven weeks, which is about half of their larval period (Shenoy et al, 2009).
The results of the study illustrated that even low concentrations of endosulfan and mancozeb could be lethal and can inhibit growth of tadpoles when it is not lethal (Shenoy et al, 2009). However, mancozeb altered the rate of growth much more than endosulfan did. Mancozeb caused the tadpoles to grow slower than those treated with endosulfan (Shenoy et al, 2009).
The implications of this study suggest that agrochemicals like endosulfan and macozeb could play a large part in the decline in frog populations. However, the results also raise concerns about the growth of tadpoles. Slow growing larva take longer to metamorphose, or may be much smaller in size when they do (Shenoy et al, 2009). Smaller amphibians are less fit for survival because they are less able to escape from predators, more likely to be born with deformities, reproductive problems and neurological disorders (Shenoy et al, 2009).
Badzinski, D. S. Assessment of Trends in Frog and Toad Populations in Ontario using Citizen Science Monitoring Data. Ecological Monitoring and Assessment Network Coordinating Office, Environment Canada. Retrieved from https://www.naturewatch.ca/english/frogwatch/assessments/trends_in_ontario_populations/trends_in_ontario_populations.pdf
Christin, M. S., Gendron, A. D., Brousseau, P., Ménard, L., Marcogliese, D. J., Cyr, D., … & Fournier, M. (2003). Effects of agricultural pesticides on the immune system of Rana pipiens and on its resistance to parasitic infection.Environmental Toxicology and Chemistry, 22(5), 1127-1133.
Lanoo, M.J. (2008). Malformed frogs: the collapse of aquatic ecosystems. Berkeley, Calif. ; London : University of California Press.
sentinel animal. (n.d.) Segen’s Medical Dictionary. (2011). Retrieved July 20 2014 from http://medical-dictionary.thefreedictionary.com/sentinel+animal
Shenoy, K., Cunningham, B. T., Enfroe, J. W., & Crowley, P. H. (2009). Growth and survival of northern leopard frog (Rana pipiens) tadpoles exposed to two common pesticides. Environmental Toxicology and Chemistry, 28(7), 1469-1474.
Species profile: Northern leopard frog. (n.d.). . Retrieved , from http://www.epa.gov/housatonic/thesite/restofriver/reports/final_era/B%20-%20Focus%20Species%20Profiles/EcoRiskProfile_leopard_frog.pdf