The timing of reproduction is responding plastically, not genetically, to climate change in yellow-bellied marmots (Marmota flaviventer)

Abstract

With global climates changing rapidly, animals must adapt to new environmental conditions with altered weather and phenology. The key to adapting to these new conditions is adjusting the timing of reproduction to maximize fitness. Using a long-term dataset on a wild population of yellow-bellied marmots (Marmota flaviventer) at the Rocky Mountain Biological Laboratory (RMBL), we investigated how the timing of reproduction changed with changing spring conditions over the past 50 years. Marmots are hibernators with a 4-month active season. It is thus crucial to reproduce early enough in the season to have time to prepare for hibernation, but not too early, as snow cover prevents access to food. Importantly, climate change in this area has, on average, increased spring temperatures by 5°C and decreased spring snowpack by 50 cm over the past 50 years. We evaluated how female marmots adjust the timing of their reproduction in response to changing conditions and estimated the importance of both microevolution and plasticity in the variation in this timing. We showed that, within a year, the timing of reproduction is not as tightly linked to the date a female emerges from hibernation as previously thought. We reported a positive effect of spring snowpack but not of spring temperature on the timing of reproduction. We found inter-individual variation in the timing of reproduction, including low heritability, but not in its response to changing spring conditions. There was directional selection for earlier reproduction since it increased the number and proportion of pups surviving their first winter. Taken together, the timing of marmot reproduction might evolve via natural selection; however, plastic changes will also be extremely important. Further, future studies on marmots should not operate under the assumption that females reproduce immediately following their emergence.

Keywords: Marmota flaviventer; climate change; microevolution; phenotypic plasticity; quantitative genetics; reproduction.

FIGURE 1

Relationship between female emergence date and pup emergence date. Thin black line represents the predicted slope if females were reproducing immediately after emerging. Darker black line represents the observed relationship between pup emergence date and female emergence date (number of females = 88, number of litters = 171).

FIGURE 2

Relationships between climate variables (a—mean spring temperature [°C]; b—mean spring snowpack [cm]) and pup emergence date (number of females = 192; number of litters = 461).

FIGURE 3

Relationships between climate variables (a—mean spring temperature [°C]; b—mean spring snowpack [cm]) and pup emergence date. The black, bold line represents the average individual response. Each thin grey line represents a unique female, with the length of the line showing the range of weather conditions measured for that female. The plot has been filtered to include only those females with 3 or more litters to enable clearer visualization. (number of females = 73; number of litters = 303).

FIGURE 4

Relationships between pup emergence date and fitness proxies. (a) Output of our selection model, examining the relationship between pup emergence date and the weighted proportion of pups surviving their first winter. (b) Output of our selection model examining the association of pup emergence date with the total number of pups surviving their first winter. The black line represents the predictions from the models. The grey shading is the associated confidence intervals. Data points have been jittered to enable clearer visualization (number of females = 176; number of litters = 417).