Charnov and Ernest (2006) present data on offspring number per year and neonatal mass for 532 species of mammals. The two are related by the linear regression equation:
ln(offspring/year) = 2.4 - ( 0.3 * ln(neonate mass) )
Giant panda neonates weigh 100-200 g and are weaned at 46 weeks.
So, according to the regression, pandas should have, on average, 2.8 to 2.2 offspring per year (for 100 and 200 g respectively). With a weaning time of 46 weeks, they could have 1.13 (52/46) litters per year. If every litter were exactly twins, that would be 1.13 * 2 = 2.26 offspring per year, which is within the predicted range.
Humans (neonatal mass of 3400 g) are predicted to have 0.96 offspring/year.
Charnov EL and SKM Ernest. 2006. The Offspring‐Size/Clutch‐Size Trade‐Off in Mammals. American Naturalist 167:578-582.
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