Context dependent fitness costs of reproduction despite stable body mass costs in an Arctic herbivore

1. The cost of reproduction on demographic rates is often assumed to
operate through changing body condition. Several studies have found that
reproduction depresses body mass more if the current conditions are
severe, such as high population densities or adverse weather, than under
benign environmental conditions. However, few studies have investigated
the association between the fitness and body mass costs of reproduction.
2. Using 25 years of individual-based capture-recapture data from Svalbard
reindeer (Rangifer tarandus platyrhynchus), we built a novel Bayesian
state-space model that jointly estimated inter-annual change in mass,
annual reproductive success, and survival, while accounting for incomplete
observations. The model allowed us to partition the differential effects
of intrinsic and extrinsic factors on both non-reproductive mass change
and the body mass cost of reproduction and to quantify their consequences
on demographic rates. 3. Contrary to our expectation, the body mass cost
of reproduction (mean = 5.8 kg) varied little between years (CV = 0.08)
whereas the between-year variation in body mass changes, that were
independent of the previous year’s reproductive state, varied
substantially (CV = 0.4) in relation to autumn temperature and the amount
of rain-on-snow in winter. This mass loss led to a cost of reproduction on
the next reproduction, which was amplified by the same environmental
covariates, from a 10% reduction in reproductive success in benign years,
to a 50% reduction in harsh years. The reproductive mass loss also
resulted in a small reduction in survival. 4. Our results show how
demographic costs of reproduction, driven by inter-annual fluctuations in
individual body condition, result from the balance between body mass costs
of reproduction and body mass changes that are independent of previous
reproductive state. We illustrate how a strong context dependent fitness
cost of reproduction can occur, despite a relatively fixed body mass cost
of reproduction. This suggests that female reindeer display a very
conservative energy allocation strategy, either aborting their
reproductive attempt at an early stage or weaning at a relatively constant
cost. Such a strategy might be common in species living in a highly
stochastic food limited environment.