Whales' large bodies help them consume their prey at high efficiencies, a more than decade-long study of around 300 tagged whales now shows, but their gigantism is limited by prey availability and foraging efficiency. These results, though seemingly intuitive, have been difficult to confirm with quantitative data because of challenges studying these gargantuan mammals in the field. However, this information is a necessary beginning to efforts to preserve these endangered giants, says Terrie Williams in a related Perspective. Growing large depends on the delicate balance between energy gained from food and energy expended. On land, typically this balance works out so that small creatures feed on small prey and big creatures feed on big prey. However, this paradigm breaks down in the ocean, where the largest predators in the world feed on tiny prey. Explanations for this phenomenon remain inconclusive. Using submersible wildlife tags designed with microprocessor technology, Jeremy Goldbogen and colleagues tagged toothed and filter-feeding whales - from the smallest porpoise to the largest animal on Earth, the blue whale - and calculated their energetic efficiency (energy from captured prey divided by energy spent). They found that larger body size for both kinds of whales increased their energy efficiency by allowing greater consumption of prey and effective prey capture; however, for toothed whales of all sizes, the energy gained from deep-sea hunting was ultimately constrained by limited abundance of prey attainable during one dive. By contrast, filter feeders consistently exhibited rapid increases in energy from food, with the total biomass and energetic content of their tiny prey exceeding on average those of the largest toothed whale prey. For these toothless whales, size might be limited by their biology (ability to gulp as much krill-enriched water as quickly as possible), rather than prey availability, the authors postulated. Altogether, their analyses suggest that filter feeding fueled an evolutionary pathway to gigantism not available to toothed whales, by exploiting vast quantities of small prey at high efficiencies.