Scientists in Colombia’s Amazon are capturing and testing endangered pink river dolphins to monitor mercury contamination from illegal gold mining and deforestation. The laborious operation, carried out only a few times a year, involves health checks, ultrasounds and sampling for diseases and toxins. Mercury levels in the dolphins have been found far above safe limits, reflecting risks to wildlife and people. Populations have sharply declined, and researchers say protecting the dolphins is inseparable from protecting the Amazon’s rivers and ecosystems.