As the world’s oceans become warmer, noisier, and more acidic, how will marine animals adapt? Researchers have come up with a novel proxy for these stresses: They’ve glued lead-filled wooden cubes to the backs of 12 young elephant seals, seen here after their release at Hopkins Marine Station on Monterey Bay in Pacific Grove, California. The blocks added drag that forced the animals to exert more energy to swim. In a study published today in The Journal of Experimental Biology, the team reports that the seals didn’t change their flipper rhythms or their diving habits to increase efficiency, and it took them longer to dive and to surface. Data loggers and water samples revealed that the animals worked 1.6 times harder than untreated seals and spent 46% longer at the surface resting after their dives, unusual behavior for animals who normally surface briefly to avoid predators like sharks and orcas. The findings suggest that the otherwise hardy elephant seal may be unable to easily alter its behavior in response to higher energy demands. The same could hold true for other predators and for more sensitive species such as the northern fur seal. Less efficient foraging could diminish animals’ health, with possible negative effects on their ability to reproduce.