DOI: 10.5061/dryad.63xsj3v1p
title: Macroevolution of dimensionless life history metrics in tetrapods

description: Life history traits represent organisms’ strategies to navigate the
 fitness trade-offs between survival and reproduction. Eric Charnov
 developed three dimensionless metrics to quantify fundamental life history
 trade-offs. Lifetime reproductive effort (LRE), relative reproductive
 lifespan (RRL), and relative offspring size (ROS), together with body
 mass, can be used to classify life history strategies across the four
 major classes of tetrapods: amphibians, reptiles, mammals, and birds.
 First, we investigate how the metrics have evolved in concert with body
 mass. In most cases, we find evidence for correlated evolution between
 body mass and the three metrics. Second, we compare life history
 strategies across the four classes of tetrapods and find that LRE, RRL,
 and ROS delineate a space in which the major tetrapod classes occupy
 mostly unique subspaces. These distinct combinations of life history
 strategies provide us with a framework to understand the impact of major
 evolutionary transitions in energetics, physiology, and ecology.
description: We compiled life history trait data for birds, mammals, reptiles,
 and amphibians from multiple sources in order to calculate 3 dimensionless
 life history metrics: lifetime reproductive effort, relative reproductive
 lifespan, and relative offspring size. For the birds and mammals, we used
 data exclusively from the Amniote Life History Database (Myhrvold et al.
 2015). For the reptiles, we supplemented the data available in Amniote
 with another published set of reptile life history traits (Allen et al.
 2017) through a two-step process. First, if a reptile species present in
 the Amniote database lacked trait data for one of the life history traits
 necessary to calculate the dimensionless metrics, we filled in the
 corresponding value from Allen et al. (2017). Secondly, we added trait
 data for species present in the Allen et al. database but not in Amniote.
 For the amphibians, we obtained life history trait data from the AmphiBIO
 database (Oliveira et al. 2017). <strong>Lifetime
 Reproductive Effort</strong> The first
 dimensionless metric, LRE, is the product of reproductive effort and
 average adult lifespan. Charnov defines reproductive effort as R/m, where
 R is the organism’s average reproductive allocation per unit time and m is
 the average adult body mass. To calculate R, we multiplied litter or
 clutch size by the number of litters or clutches per year, yielding the
 number of offspring per year, and then multiplied this value by the mass
 of offspring at independence. AmphiBIO reports a minimum and maximum
 clutch size, so we averaged these two values when calculating clutch size.
 We defined independence as fledging for birds, weaning for mammals,
 hatching for reptiles, and offspring or egg for amphibians (whichever
 value for offspring mass was provided in AmphiBIO). After calculating R,
 we divided by the average adult body mass for the amniotes and the maximum
 adult body mass for the amphibians to calculate reproductive effort. While
 Charnov’s model calls for an average body mass, AmphiBIO only provides a
 maximum adult body mass, so we used this value to provide an approximate
 value of this metric for amphibians. Finally, we multiply reproductive
 effort by adult lifespan to calculate LRE. We used maximum longevity,
 rather than average longevity, for all classes due to data quality and
 availability. <strong>Relative Reproductive
 Lifespan</strong> To calculate RRL, we divided
 adult lifespan by the time to female maturity. Since Amniote reports
 longevity in years and age at female maturity in days, we converted
 longevity to days to keep the ratio dimensionless. AmphiBIO reports
 minimum and maximum age at sexual maturity, so we averaged these values to
 calculate an average. <strong>Relative Offspring
 Size</strong> In order to calculate the final
 dimensionless metric, ROS, we divided mass at independence by average
 adult body mass. We used the same criteria for independence for birds,
 mammals, reptiles, and amphibians as used to calculate R. Since AmphiBIO
 reports offspring size as minimum and maximum lengths rather than mass, we
 used allometry equations for Anura and Caudata to convert these lengths to
 offspring mass (Santini et al. 2017). We used the models predicting mass
 from SVL for Anura and Caudata, rather than those including habitat and
 paedomorphy since the majority of frog species in AmphiBIO existed in
 multiple habitats and paedomorphy was not reported for the salamander
 species. After converting the minimum and maximum lengths at independence
 to mass, we averaged these two masses to calculate mass at independence
 for the amphibians.
description: Columns: order: order
 family: family genus: genus
 species: species subspecies: subspecies
 name, if relevant common_name: common name of
 species, if found adult_body_mass_g: adult body mass,
 in grams C_E: lifetime reproductive effort (also
 called C*E by Charnov) I_m: relative offspring size
 (also called I/m by Charnov) E_alpha: relative
 reproductive lifespan (also called E/α by Charnov)
 log_bodymass: natural log of body mass in grams
 log_C_E: natural log of lifetime reproductive effort
 log_I_m: natural log of relative offspring size
 log_E_alpha: natural log of relative reproductive
 lifespan