1. Although marine mammals are considered to be disproportionately threatened compared with terrestrial mammals, and research data on marine mammals are limited, few attempts have been undertaken to analyse overall marine mammal research effort.
2. We applied the bibliometric approach to evaluate global trends in research on pinnipeds and cetaceans and attempted to evaluate the distribution of publication output with respect to spatial, taxonomic and conservation status-related patterns.
3. Results show that global marine mammal research is disproportionately directed towards less endangered species. There is an uneven scientific focus among different cetacean and pinniped groups. A spatial asymmetry in research effort was also observed: a negative relationship exists between research allocation and regional marine mammal species richness. Cetacean species with larger ranges and pinniped species with larger population abundances tend to be better studied than others.
4. The observed patterns may exist because researchers have limited access to specimens of highly endangered species and to species found in less accessible habitats. Researchers from high-income countries may also focus their research on species geographically close to them, and other economic considerations may lead to research bias. The observed unevenness of research allocation may result in a lack of information needed by policy-makers and resource managers for the development of sound conservation and restoration measures, and thus in reduced effectiveness of policies and management practices.