DOI: 10.5061/dryad.44j0zpct6
title: An early-life survival and reproductive trade-off shapes selection on body size

description: Evolutionary trade-offs - opposing trait effects on total fitness via
 different fitness components - are likely to be widespread. Some key
 trade-offs are expected to be the result of chains of causation acting
 across an organism's lifetime. For example, a trait imparting
 reproductive benefits early in life may trade off against reduced survival
 to attain later-life reproductive opportunities. Tools in evolutionary
 quantitative genetics have recently been developed to formally
 characterise selection acting through different causal pathways throughout
 the life cycle and, therefore, to formally characterise evolutionary
 trade-offs. We use these methods to investigate a trade-off between early
 life reproduction and survival, and how that trade-off affects selection
 on body size in the Soay sheep population inhabiting St Kilda (Outer
 Hebrides, Scotland). We decompose and quantify the total effects of
 first-year female body mass on lifetime fitness, with particular attention
 to the effect of body mass on early-life reproduction, and the potential
 survival cost of early-life reproduction. Our results establish that the
 total effect of body mass on lifetime fitness is positive, despite the
 strong negative contribution acting via early life reproduction. Moreover,
 we show that the magnitude of the selection on body mass acting through
 different causal paths highly depends on population density. At higher
 densities, the cost of early-life reproduction is higher and, therefore,
 it contributes a strong negative component to the total selection of body
 mass - i.e., at higher population density, selection on body mass is
 weaker than it is when the population size is smaller. By decomposing
 total selection and quantifying selection acting through different causal
 paths, we expose the underlying mechanics shaping body mass in Soay sheep
 female lambs, and we provide a meaningful contribution to the
 understanding of the evolution of body size in this population.
description: # An early-life survival and reproductive trade-off shapes selection on
 body size Dataset DOI: [10.5061/dryad.44j0zpct6](10.5061/dryad.44j0zpct6)
 ## Description of the data and file structure Four datasets have been made
 available: (a) data_ewes1.csv: it comprises data for female Soay sheep
 lambs, and includes the variables used in the statistical models (twin,
 mass, pregnancy, survival, fyres, and slres. (b) data_ewes_all_ages.csv:
 data used in Figure 2f. It includes information to derive survival curves
 of Soay sheep according to maternal age at conception. (c) data_ped.csv:
 the pedigree used in model (1) and in section *Alternate formulations of
 the bivariate relationship between lamb mass and early pregnancy
 (bivariate formulation)* (d) data_pop.csv: population density in the study
 site for the years 1995-2015. ### Files and variables #### File:
 data_ewes1.csv **Description:**  ##### Variables * cohort: year of birth *
 birth_month: month of birth * death_year: year of death * twin: indicator
 of twin status (0 = singleton, 1 = twin) * mass: body mass measured in
 August (kg) * pregnancy: indicator of pregnancy (0 = no, 1 = yes) *
 survival: indicator of first-year survival (0 = no, 1 = yes) * fyres:
 indicator of first-year rearing success (0 = no, 1 = yes) * surv_off_1Nov:
 indicator of offsprin survival to November 1st * slres: subsequent
 lifetime rearing success (number of offspring produced after the first
 year of life) * measure_date: Julian day of mass measurement * birth_date:
 Julian birthday * mum_age: maternal age at birth (years) * density:
 population size * animal: individual ID * mum_id: maternal ID #### File:
 data_pop.csv **Description:**  ##### Variables * year: year * density:
 population size #### File: data_ped.csv **Description:**  ##### Variables
 * ID: individual ID * sireID: paternal ID * mumID: maternal ID #### File:
 data_ewes_all_ages.csv **Description:**  ##### Variables * time: offspring
 age (years) * cens: censorship indicator (0 = not alive, 1 = alive)  *
 age: maternal age at conception (years)