Table 1 Hypotheses proposed in literature to explain the geographic variation of body size in amphibians

Water availability

Mean annual precipitation (BIO12)

Negative

A lower area-volume relation given by greater body size will produce less surface for water loss ([22])

Converse water availability

Mean annual precipitation (BIO12)

Positive

Amphibian activity is strongly related to high water availability and humid periods, allowing more foraging time that promotes greater body size in areas with more precipitation ([51])

Water conservation

Potential evapotranspiration (PET)

Negative

Water loss leads to heat loss, thus thermal balance is intimately linked to water balance in amphibians. Thus, a lower area-volume relation given by greater body size reduces the capacity of the environment to remove water and heat from the body surface ([29])

Seasonality

Temperature seasonality (BIO4)

Negative

The individuals of populations with longer times of favorable activity are larger than those with shorter and more fluctuating times ([25])

Starvation resistance

Temperature seasonality (BIO4)

Positive

Greater body size allows greater energy reserves to withstand periods of lower resource availability compared to smaller body size ([10]), since the rate of energy storage is greater than that of consumption (see discussion in [52])

Primary productivity

Normalized Difference Vegetation Index

NDVI

Positive

Greater food availability provides the possibility to reach larger body size ([21, 24])

Heat balance

Mean annual temperature (BIO1)

Negative

Greater body size allows more thermal inertia, providing advantages to thermoregulating ectotherms with larger body size in cold climates. Inversely, in thermoconforming ectotherms small body size is favored in colder zones, since they heat more quickly ([21])

Size-temperature rule

Mean annual temperature (BIO1)

Negative

The maturation times in most ectotherms are longer in cold climates, which results in greater body size ([53])