In the humid lowlands of Tabasco, Mexico, the early morning mist rises over a checkerboard of emerald pastures and isolated patches of tropical forest. Suddenly, a low, resonant drone reverberates through the humid air—not the sound of agricultural machinery, but the primeval roar of the Yucatan black howler monkey ( Alouatta pigra ). This booming vocalization, amplified by an enlarged hyoid bone in the male's throat, once rolled unimpeded across vast, unbroken tracts of Mesoamerican rainforest. Today, it echoes across a highly fragmented landscape dominated by cattle ranches and agricultural fields. For decades, biologists assumed that these strictly arboreal primates were marooned in their shrinking forest islands, too vulnerable to cross the open pasturelands that separate them. However, a series of groundbreaking behavioral studies and conservation monitoring initiatives over the past year have revealed a surprising story of behavioral plasticity. Armed with thermal-...