Demographic consequences of early primiparity: importance of food availability and individual heterogeneity

Demographic consequences of early primiparity Age at which females first reproduce Early maturation should be selected for because:  period during which animals risk dying before reproducing early breeders compound their genetic interest before those delaying their first reproduction effect on fitness of individuals effect on population growth rate

Demographic consequences of early primiparity But lots of species differ their first reproduction after physiological maturity => Reproducing early in life may be costly Trade-offs: early reproduction - growth early reproduction - survival early reproduction - future reproduction

Demographic consequences of early primiparity Importance of the food availability in the expression of reproductive costs Fitness costs of early reproduction are important when food availability is low and negligible when food availability is high

Demographic consequences of early primiparity Few studies about the demographic consequences of early primiparity Some examples on ungulates, rare on small mammals

The Kluane Red squirrels population Red squirrels at Kluane: long-term dataset, data on reproduction, age of individuals etc, etc food availability index <40% of females breed at 1 year of age (age of physiological maturity)

First hypothesis: females who bred at 1 year of age after a cone failure have lower fitness components than females delaying their first reproduction after a cone failure. Opposite difference after a mast year. The Kluane Red squirrels population

First hypothesis and results Hypothesis Results

First hypothesis and results Early primiparity Delayed primiparity

First hypothesis and results Results opposite to the prediction: higher fitness for females who bred at 1 year of age after a cone failure than females who delayed their first reproduction after a cone failure

Alternative (non exclusive) hypothesis Only females of high phenotypic quality reproduced when low food availability (i.e. after a cone failure). (previous examples on birds)

New HypothesisResults Alternative (non exclusive) hypothesis

Prediction 1: higher survival of females breeding at 1 year of age after a cone failure than females breeding at 1 year of age after a mast year Early primiparity after a cone failure Early primiparity after a mast year

Alternative (non exclusive) hypothesis Prediction 2: for the first reproductive event, higher mass and/or larger litter size and/or higher mass of juvenile produced for females breeding at 1 year of age after a cone failure than females breeding at 1 year of age after a mast year

Alternative (non exclusive) hypothesis Prediction 3: lower survival for females breeding at 1 year of age after a mast year than females who delayed their first reproduction after a mast year Early primiparity Delayed primiparity

Conclusion Our alternative hypothesis is supported by our data After a cone failure, only females of high quality breed at 1 year old and they achieve a better fitness (despite the cost of early reproduction) After a mast year, no confounding effect of the individual quality. The costs of early reproduction (on survival and breeding success) can be detected despite the high energy available

Conclusion Long-term cohort effects Other alternative hypothesis to explain our results?