Ancient DNA from marine mammals: studying long-lived species over ecological and evolutionary timescales.

Marine mammals have long generation times and broad, difficult to sample distributions, which makes inferring evolutionary and demographic changes using field studies of extant populations challenging. However, molecular analyses from sub-fossil or historical materials of marine mammals such as bone, tooth, baleen, skin, fur, whiskers and scrimshaw using ancient DNA (aDNA) approaches provide an opportunity for investigating such changes over evolutionary and ecological timescales. Here, we review the application of aDNA techniques to the study of marine mammals. Most of the studies have focused on detecting changes in genetic diversity following periods of exploitation and environmental change. To date, these studies have shown that even small sample sizes can provide useful information on historical genetic diversity. Ancient DNA has also been used in investigations of changes in distribution and range of marine mammal species; we review these studies and discuss the limitations of such 'presence only' studies. Combining aDNA data with stable isotopes can provide further insights into changes in ecology and we review past studies and suggest future potential applications. We also discuss studies reconstructing inter- and intra-specific phylogenies from aDNA sequences and discuss how aDNA sequences could be used to estimate mutation rates. Finally, we highlight some of the problems of aDNA studies on marine mammals, such as obtaining sufficient sample sizes and calibrating for the marine reservoir effect when radiocarbon-dating such wide-ranging species.