Phenological changes in response to climate change have been documented across taxa throughout the Northern Hemisphere. The rate of advancement often decreases with increasing trophic level, resulting in mismatch between consumers and their resources. The match-mismatch hypothesis has been investigated in bighorn sheep (Ovis canadensis) at Ram Mountain, Alberta (52ºN, 115ºW), a population which has been monitored since 1971. The gestation period of bighorn sheep is 172 ± 2 days, and our team previously showed that ewes have plastically responded to environmental changes in fall by advancing their conception date and giving birth on average 15 days earlier in recent years. However, spring conditions have changed at a much faster rate, resulting in lambs being born an average of 10 days later with respect to spring green-up. Ewes do not seem to pass on costs of mismatch onto lambs, leading us to hypothesize that mismatch may increase maternal reproductive costs. Our goal was to determine whether maternal summer mass gain is affected by increasing mismatch. We use parturition dates recorded since 1992, and spring green-up dates obtained from the Enhanced Vegetation Index available since 2000, to test the effects of phenological mismatch on bighorn ewe summer mass gain.
Results/Conclusions
A total of 152 females were measured, between 1-11 times each, with a total of 449 parturition dates recorded since 1992, 279 of which have been recorded since 2000. Mismatch between spring green-up and parturition, (parturition date - green-up date, in Julian days) ranges from -24 days (early parturition) to +120 days (late parturition), and population mismatch, (individual parturition date - population median parturition date, in Julian days), ranges from -18 to +95 days. Preliminary results indicate that ewes with a greater (positive) mismatch, both green-up and population, gain less mass over summer; those with the greatest mismatch gain up to 6kg less during summer than those whose parturition dates are matched with spring green-up or median parturition date. This suggests that ewes who are missing the highest quality forage have the greatest reproductive cost, using their fat reserves for lactation. Current knowledge of the effects of climate change on phenological plasticity are primarily from studies on birds, which are income breeders and thus more sensitive to daily changes in food abundance. This study improves our understanding of large mammal phenological plasticity, and the potential costs of phenological mismatch, which are expected to increase with climate change.
