Orcas generally live in pods (groups) consisting of several females, calves, one or more males, and/or juveniles. Some pods consist of a mother and her offspring who stay with her for life. This type of matrilineal family structure has been observed in the U.S. Pacific Northwest where resident pods have been documented as stable, consistent matriarchal family groups with several generations traveling together. Transient pods appear to be more fluid; individuals come and go, groups often contain unrelated females with offspring, offspring do not stay with their mother and pods may form solely as a temporary foraging pack. The social structure of other populations, including offshore orcas, is being studied to document whether certain family groups always stay together or return to each other after periods of time. Mothers are very protective of their calves, and orcas are known to protect and care for sick and injured companions. Sparked by the increase in live capture for aquaria and public concern, scientists have been studying resident pods along the northern Pacific coast of the United States and Canada since 1970. By 1973, photographs were being used to identify individuals based on differences in saddle color pattern, dorsal fin shapes and other identifying marks and scars. Identified orcas have all been numbered and careful records are kept of their re-sightings. Recordings of the sounds made by these orcas have revealed that each pod has its own "dialect." Each pod has some sounds in common with other pods, and other sounds that are unique to its own pod. Through these scientific studies, much has been learned about population, travel patterns, reproduction, behavior and social habits of orcas.