Proportion of Mexican wolves (Canis lupus baileyi) in the Mexican Wolf Experimental Population Area in Arizona and New Mexico that died before reaching 2 yr of age, that died between 2 yr and 4 yr old, or that survived beyond 4 yr, by birth cohort year. Removed from the data set were wolves that were illegally or legally killed or that were permanently removed from the population before 2 yr old. Canine distemper antibodies were detected via serum virus neutralization or immunofluorescent antibody detection in 2003–06 and 2010. In 2014, one wolf died at just older than 2 yr, indicated by an asterisk.