Climate, Plant Phenology and Variation in Age of First Reproduction in a Temperate Herbivore

Abstract

Density-independent weather effects can have important consequences for the demography of terrestrial herbivores because precipitation, temperature and insolation influence plant phenology, forage quality and biomass production, which in turn affects the habitat carrying capacity. Since forage digestibility influences intake and weight gain, life-history traits of young, growing animals are likely to reflect variation in the prevailing weather. This paper specifically investigates spatial and temporal variation in age at maturation in female red deer (Cervus elaphus) in Norway in relation to climate variables known to influence primary production. Our findings are corroborated by analysing differences in age at maturation in 21 cohorts of red deer on the Isle of Rum, Scotland. In Norway the majority of females ovulated as yearlings and calved for the first time as 2-year-olds. The proportion calving for the first time at two years varied from 0.23 to 0.67 between regions and fluctuated from 0.46 to 0.76 between cohorts. On Rum, where age of maturation was delayed at least a year, the proportion calving for the first time as 3-year-olds varied between cohorts from 0.0 to 0.89. In a subset of yearlings culled in Norway at the time of conception, the spatial and temporal differences in ovulation rates were related to the geographical and annual variation in body weight. 5. Both the spatial and temporal variation in the proportion of 2-year-olds calving in Norway, and cohort differences in the proportion calving as 3-year-olds on Rum, were negatively related to variation in May-June degree days 12 months earlier. Although primary production on the preferred herb-rich Agrostis-Festuca grasslands was positively correlated with temperature in May and June on Rum, the proportion of females calving as three years old, was negatively correlated with annual differences in May June primary production. We argue that retarded phenological development, during periods of cooler weather, enhances diet quality because leaf :stem ratios and digestibility of plant parts decline more slowly. Thus, weight gain during the early summer growth spurt should be rapid during cool May-June weather, increasing the probability of conception in the autumn. Since density-independent variation in food availability also influences fitness components which commonly have a more pronounced influence on population demography, for example offspring survival, we argue that our results highlight the potential importance of variation in weather on herbivore abundance.