The cost of fame for popular species: strong risk of biases in comparative oncology of captive species

Comparative oncology investigates variation in cancer risk across species by analysing relationships between observed tumour prevalence and factors such as body mass, longevity, life-history traits and mutation rates. These patterns underpin hypotheses on cancer defences, conservation strategies and the evolution of multicellularity, so their robustness is critical. Here, we show that species' scientific and public popularity substantially biases these trends. Reanalysing published captive vertebrate datasets, and re-evaluating previously published trends, we found that controlling for popularity eliminated the weak positive associations previously reported between body mass and both tumour and malignancy prevalence, supporting Peto's paradox. The positive link between germline mutation rate and cancer mortality also disappeared. In birds, the effect of clutch size doubled for species with a large clutch size, while in mammals, tumour prevalence was underestimated in less popular species. Finally, the endotheliochorial placentation-mortality association vanished after accounting for confounders. These findings reveal that species popularity can substantially distort comparative cancer analyses and should be systematically controlled in future studies.