<p>Insular conditions, such as reduced levels of resources, predation and competition, may affect life history traits of island mammal species, promoting a shift towards a slow life history. This pattern has been found in several species of insular dwarf deer but has not been tested for island deer that were subject to gigantism, which may evolve opposite life history shifts. The Pleistocene deer genus of Crete (Greece), <i>Candiacervus,</i> provides an ideal case to test this potential difference, because this endemic genus comprised dwarf as well as giant forms in an otherwise depauperate mammalian fauna. Here, we tested maturation patterns in this genus using a palaeohistological approach. We found that small, medium and larger forms of <i>Candiacervus</i> all demonstrated delayed skeletal maturity compared to their putative ancestor fallow deer (<i>Dama dama</i>) as well as similar-sized mainland deer. This is the first report showing that insular deer that are either subject to gigantism or retained ancestral size, also displayed adaptations to a slow life history similar to insular deer that underwent dwarfism. This research contributes to our understanding of how insular conditions may influence life history patterns in a species radiation of deer.</p>