You are what you eat: Parasite transfer in cannibalistic cane toads

Authors: Ligia Pizzato and Richard SHine
Journal: Herpetogica, 67(2), 2011, pp.118-123
Link: http://www.hljournals.org/doi/full/10.1655/HERPETOLOGICA-D-10-00051.1

Introduction:
Cannibalism can affect ecological dynamics at population and community levels.  While it has many advantages such as additional nutrition, enhancing individual survivorship, and enabling population persistence during food scarcity, it does can come with some costly disadvantages.  The mostly costly, and likely of these disadvantages is the risk of acquiring parasites and diseases.  Because of the host specificity of many pathogens, pathogen transfer is much more likely from eating conspecifics than from eating heterospecifics.  The research performed studied the role of cannibalism by Cane Toads (Rhinella marina) on the transmission of the lungworm Rhabdias pseudosphaerocephala to adress the questions: (1) Can the lungworms be transferred through cannibalism? (2) If so, then do parasites taken up this way influence the cannibal's viability.

Materials and Methods:
Cane toads are native to South and Central America, but were introduced to Australia in 1935 and have since spread and inflicted major ecological consequences.  Some populations contain the nematode lung worm (R. pseudosphaerocephala). 
Sixteen small toads were collected and treated with the antiparasite drug ivomectin.  They regular checked the feces of the animals to confirm that they had been "de-wormed" by the drug.  The researchers scored the time each animal took to hop 200 times, after which mot became exhausted and reluctant to move.  Eight of the toads were fed noninfected live conspecific metamorphs and the other eight were fed infected metamorphs.
Repeated-measures analysis of variance to compare control vs. treatment groups were performed for (1) the times taken to perform 200 hops, and (2) body sizes at beginning and end of the experiments.

Results:
Five of the eight toads fed on infected metamorphs had adult worms in their lungs. None of the controls had any worms.  Control toads took less time to hop 200 times when compared to the treatment group.  Growth rates did not differ significantly between the two groups.

Conclusion:
In Cane Toads, cannibalism can confer a cost in terms of macroparasitic transfer.  This may be the first example of macroparasite transmission via intraspecific predation in amphibians.  Inside the cannibal, the worms were able to mature in the lungs and affect the locomotor performance of their new host. Because survival rates of metamorphic toads are very low and size dependent, the large cannibals may be the only animals that survive long enough to recruit into the adult population.  This may cause an increase in the proportion of infected toads through time by selective mortality of small infected individuals that are very unlikely to survive.  The end result is that cannibalism-mediated pathogen transfer could increase prevalence, intensity, and parasite persistence in a population.  Further ecological modeling and additional field work are needed to test their hypothesis about how cannibalism could affect disease dynamics at the population level.