Study species and data collection:
Yellow-bellied marmots are large (adult females weighing on average 2.5
kg and adult males weighing on average 3 kg; Armitage, 2014) hibernating
rodents living up to 15 years. They have a four-month active season,
from late-April/early-May to late-September, during which they need to
reproduce and accumulate fat reserves to survive the eight months of
hibernation (Armitage, 2014). A population of yellow-bellied marmots has
been studied at the Rocky Mountain Biological Laboratory (RMBL) in
Gothic, CO, USA since 1962.
Marmots were live trapped in Tomahawk traps regularly during the active
season. When they were caught, data on their weight, sex, and
reproductive status were collected. Upon first trapping, marmots were
assigned a unique identifier and given a permanent ear tag for
identification. For observations at a distance, Nyanzol-D, a
semi-permanent dye, was applied in a unique pattern on each marmot.
Starting in 2000, parentage has been determined using genetic assignment
(for detailed methodology on genetic assignment see Blumstein et
al., 2010). Prior to this, maternal identity could be reliably
determined via behavioural observations while paternal identity remained
unknown since males do not contribute to parental care. Daily climate
data were collected by an on-site weather station since 1975. Data
collected included daily minimum and maximum temperatures, daily
precipitation, and depth of the snowpack. Mass on June
1st and August 15th were estimated
for each individual every year using best linear unbiased predictors
from age- and sex-specific linear mixed models (for detailed methods see
Kroeger et al., 2018). For pups, mass was estimated for emergence
date and not June 1st. Age was calculated using birth
year and the year of capture. Since 83% of females are captured for the
first time when they are juveniles, they are of known age. The study is
divided into an up-valley and a down-valley that differ in elevation by
165 m (Ozgul et al., 2010) resulting in a delay in the phenology
of the up-valley by about two weeks compared to the down-valley (Monclúset al. , 2014). For our analysis, we restricted our dataset to
include only main colonies which are observed with a higher frequency
(near daily) compared to satellite colonies and we could thus have a
more accurate estimation of emergence dates.
To estimate the timing of reproduction, we used the date pups first
emerge from their burrow after being weaned as a proxy. Since the length
of gestation and lactation are considered fixed in the marmots, with 30
days spent gestating and 25 days spent lactating (Armitage, 2014), and
pups emerge immediately following weaning (Monclús et al. , 2014)
this is an excellent proxy for the timing of reproduction. Given that
adult emergence in the spring is related to average spring temperature
and average spring snowpack, we focused on these two environmental
variables for our analysis of pup emergence date. Since seasonal
averages of environmental variables can be estimated between any two
arbitrary timepoints (Bailey & van de Pol, 2016), we used a statistical
approach to determine which phenological window of these two variables
had the greatest association with pup emergence date. This was done
using the statistical approach built in the R package climwin (Bailey &
van de Pol, 2016; van de Pol et al., 2016). This package allows
the fitting of multiple models with different phenological windows used
to estimate environmental averages and determine, using AIC based model
comparison, which window has the strongest relation with the biological
variable of interest.