Maturation trends in red deer females over 39 years in harvested populations

There is increasing awareness that heavy harvesting can lead to rapid evolution towards earlier sexual maturation. With increased harvest pressure, individuals that begin reproduction at light weights have a greater chance of reproducing at least once compared with individuals that begin reproduction at heavier weights and hence later in life. Although well documented in fish, this has not been empirically tested for harvested populations of mammals in terrestrial ecosystems differing in many environmental aspects. Using data from 3856 female red deer (Cervus elaphus L.) from three harvested populations, we tested whether red deer maturity changed from 1967 to 2006. In these populations, density has increased markedly over the time period reducing body mass of deer, which has decreased the proportion of females maturing as yearlings. We therefore assessed trends in maturation as yearlings after controlling for body mass. Long-lived iteroparous ungulate females are expected to have a prudent life history not risking their future survival and reproduction under harsh conditions (e.g. at high density). An alternative to the harvest-induced evolution hypothesis is that maturation is driven mainly by phenotypic responses to increased density, and we predicted later maturation even after controlling for the reduction in body mass. There was a marked trend towards later age at maturation in one out of three populations (after controlling for the effect of reduced body mass due to increased density), while there was no marked trend in the two other populations. The harvesting-induced evolution hypothesis was therefore not supported. However, although the decline was predicted from the prudent life-history tactic hypothesis, the estimate for the density effect was positive rather than negative after accounting for the year trend. Although we did not find support for the harvest-induced earlier maturation hypothesis, evidence was not clearly in favour of the alternative hypothesis. Our study contrasts results from trophy-hunting traditions targeting large males, and points to a potential role of the cultural tradition. Harvesting in Scandinavia mainly aims for meat targeting calves and yearlings (and males), which is less likely to yield an evolutionary response in maturation of females. Our results, although being on a plastic trait and not directly on genetic make-up, are indicative that harvesting-induced evolution is weak on age at maturation in these populations under the current management regime.