Epigenetic clocks provide powerful tools to evaluate nutritional, hormonal, and genetic effects on aging. What can we learn from differences between species in how these clocks tick?Please see related Research articles: http://genomebiology.biomedcentral.com/articles/10.1186/s13059-017-1203-5 , http://genomebiology.biomedcentral.com/articles/10.1186/s13059-017-1186-2 , http://genomebiology.biomedcentral.com/articles/10.1186/s13059-017-1187-1 and http://genomebiology.biomedcentral.com/articles/10.1186/s13059-017-1185-3.