The parasite within: understanding how cancer shapes life history
Cancer and life-history traits: lessons from host–parasite interactions is the March Parasitology Paper of the Month.
Cancer is a disease associated with clonal evolution and cell competition within the body. The appearance of cancer has been linked to transitioning from a unicellular to multicellular lifestyle more than half a billion years ago, and has been observed in nearly the entire animal kingdom, from bivalves to whales. Although mounting evidence suggests that oncogenesis occurs during the life of most, if not all, metazoans (multicellular organisms), it is surprising that the impact of cancer on wildlife has so far been largely unexplored. One of the reasons for the lack of such studies may partly be due to the fact that (with few exceptions e.g. Tasmanian devil facial tumour disease (DFTD)), wildlife cancers are often difficult to study as pathological manifestations are usually not visible until the development of metastasis. As a result, most animals affected by cancer in the wild likely succumb to predation prior to developing overt clinical signs of cancer. A similar conclusion was reached in the 1970s and 1980s by Roy Anderson and Robert May with regard to infectious diseases. However, following the extensive efforts invested during the last decades into the study of host-parasite interactions no scientists presently ignore the significant effects of pathogens/parasites on life-history strategies and ecological and evolutionary processes.
Although the majority of cancers are not transmissible, the progression of cancer can, in many ways, be compared to pathogen/parasite infections as cancer often results in similar negative consequences in reduced host fitness. In our review we propose that cancer is indeed an important indirect cause of early death in wild animals (e.g. cancer carrying individuals being subjected to reduced body condition and concomitant increased risk of predation as well as becoming infected by pathogens/parasites). Consequently we discuss how natural and sexual selection should favour adaptations that preclude cancer-induced reduction in fitness. Such adaptations may include shifts in life history strategies, i.e. shifts in behaviour (e.g. reduced activity to conserve energy, reduce predation risk and overcome harsh environmental conditions), shifts in resource allocations (e.g. increased allocation to immune function relative to somatic maintenance), shifts in life history traits (e.g. early onset of reproduction) and an ability to recognize and avoid cancerous mates. We conclude, that wildlife cancer may therefore have a similar significant impact on ecological and evolutionary processes, including life history adjustments, as that of pathogens and parasites.
Unfortunately, there is only limited data available to date to support the hypotheses we have outlined in our article, therefore clearly more research is needed to draw a more substantiated parallel between cancer and infectious diseases. Furthermore, to understand the evolution of life history traits in a cancer perspective, one must consider the complete ecological context in which individuals (affected by cancer) live. Finally we encourage greater collaboration and knowledge exchange between researchers of disparate fields: evolution, ecology and medical professions, to be able to systematically explore and understand the myriad of symptoms displayed by cancerous patients in order to discover those that could be adaptive responses, vs those that illustrate pathological costs. On a personal note, the authors strongly believe that integrating evolutionary ecology theory into cancer research could reshape the conceptual and concrete approaches used to tackle cancer, a disease that touches every family on the planet.