1. How mothers balance the trade-off between offspring size and number to maximize maternal fitness has long been of interest to ecologists seeking to understand the evolution of offspring size.

The Indigenous peoples of the Bolivian highlands are survivors. For thousands of years they have lived at altitudes of more than two miles, where oxygen is about 35 percent lower than at sea level.

The rest of this article is behind a paywall. Please sign in or subscribe to access the full content. But by using a long-term study of one population of penguins, researchers have been able to do ...