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.

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).

Lifetime Reproductive Effort

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.

Relative Reproductive Lifespan

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.

Relative Offspring Size

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.

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