Unlike herbivores and omnivores, predators have traditionally been thought not to balance nutrient intake because of the assumption that animal tissue as a food source varies little and is nutritionally balanced. But chemical analysis of invertebrate prey reveals remarkable variation in nutrient composition among species; even within species, nutrient composition may vary considerably. Greenstone suggested that predators may select food items according to their nutrient contents. Jensen et al (2011) have shown experimentally that even sit-and-wait invertebrate predators with limited mobility can work to address nutrient deficiencies. The wolf spider, for instance, has been shown to regulate nutrient intake by extracting more dry mass from a prey item if it contains a higher proportion of a nutrient that was deficient in the previous prey.