The sperm whale eats primarily squid and some fish. Sperm whales have excellent hearing, and because they do not rely on eyesight, they locate prey through sound. It is difficult to determine their method of catching prey because they forage at such great depths. It has been hypothesized that they swallow fish and squid in large gulps, but from evidence of scarring from squid suction cups, catching prey may be more challenging than this (Slijper, 1958). Because their prey is found in all oceans, sperm whales are not required to migrate regularly between feeding groups, but males do swim to cooler waters in the summer months (Minasian et al, 1984). Males may be more likely to feed in polar waters during the summer because they have an easier time finding squid that prey on the abundance of plankton found there (Matthews, 1974). Aggregations of bachelor nonbreeding males and females were often found away from the continental shelf edges and in deep, open waters; the larger males were found close to the shelf edges. This may be due to the more predictable resources that can be found in underwater canyons. Females are more likely to forage here to ensure that their offspring will have plenty to eat (Lettevall et al, 2002). Decreases in food availability cause sperm whales to make short term changes in their foraging habits. Most will increase the duration of their dives and decrease the amount of time spent at the surface. This results in a large reduction of socialization time with other females at the surface (Jaquet et al, 2003).
Photo courtesy Rolf Hicker-Nature Stock Photography http://www.hickerphoto.com
The population of jumbo squid off the coast of California can vary over the duration of a few days or a few years. Because sperm whales require between a few hundred and a few thousand kilograms of squid a day, they sometimes have to make longer-term changes to their foraging patterns, or in rare cases, make long distance moves. Sperm whale abundance along Baja California has increased since 1992 and decreased in the Galapagos Islands since 1995, suggesting that sperm whales do make long distance movements in search of food. What is not known is if these whales made the move because they explored until they found an area with an abundance of food or if there was some sort of transfer of information among these whales (Jaquet et al, 2003). There are differences in diet and foraging success among clans. Relatively similar DNA suggests that these differences are not due to genetics. Because of these different foraging behaviors, shortages in food are likely to affect clans differently and thus, make some clans more fit than others. Over time, the clans that are more fit are likely to spread and this could reduce the overall diversity of the species, a process known as cultural hitchhiking (Marcoux, 2005).