Schools, groups, pods. How do they exist? It seems reasonable to imagine that there is a coexistence of symbiosis and competition between the individuals in the group, and that the advantages and disadvantages exist in some sort of balance. Does the maintenance of a balance have enough of an effect to influence the traits of members of the group? How do the individuals respond if the balance is shifted? Would the individuals stick to the symbiosis or fend for themselves as a response to the shift? These are all questions researchers may ask themselves as they think about animals that live in groups, such as killer whales (Orcinus orca, figure 1).
Killer whales live with their maternal family for their entire lives. Pods have both young and old, male and female members (figure 2). Group living enables teamwork for hunting, raising young, and overall pod survival. On the other hand, living in a group may get stressful when resources of food and space are limited, and competition between individuals may ensue.
Groups evolve to work together. Each individual must compromise their own needs to assure the success of the group. Such behavior can result in the evolution of slight differences between individuals. For example, if the success of the group requires that food sources are available to satisfy the lot, then being able to spread spatially and hunt greater areas is advantageous. Thus, whales that have the capability to dive deeper are going to be a greater asset to the group than those that cannot, and may be favorably selected for reproduction.
It is found that in some killer whale pods, males hunt in deep waters while the females and calves tend to stick to the shallow, near-shore areas. Scientists suspect these observations could either be related to males avoiding food that is easily accessible to the females and calves, or possibly just that males are physically able to dive deeper and longer.
In British Columbia, scientists sought to learn about why male and female northern resident killer whales hunt in different areas. They hypothesized that whale pods stick together for all activities, except for when they are feeding. They also investigate if changes in the food source can influence the behavior of individuals of the group.
Researchers monitored the movement paths and activity of northern resident killer whales in the western end of Johnstone Strait, BC, Canada (figure 3), an area designated as a critical habitat for the species. The data collection was between July and August each year for a period of six years. Information such as age, sex, and pod affiliation for the monitored whales were determined based on previous records. Information was statistically analyzed to decipher significant relationships between gender, activity (resting, traveling, or hunting), and depth.
Over the period of six years forty-four whales from ten different pods produced data for 354 tracks.
During times of rest and travel the pods of whales were found all together, but when they were hunting, there was a significant relationship between feeding location and gender.
Female whales in deep water were either resting or traveling, not hunting. They were only found hunting the shallow, near-shore areas. Male whales, however, did not have a signification relationship between depth and activity; they rested, traveled, and hunted at all depths.
In addition, it was estimated from fishery catches that prey availability (Chinook salmon, Oncorhynchus tshawytscha, figure 4) in the area varied by a factor of six between each monitoring season. However, the supply must have remained enough to satisfy the whale population because no differences in whale behavior were observed between years in this study.
When traveling or resting killer whales of either gender were distributed independent of gender. They do however hunt in different areas. The females were found to hunt in shallow, near-shore areas, while the male whales seemed to not have a preference between shallow or deep water. Scientists postulate that this is evidence that the males do not avoid shallow areas as a means to have more food available for females and calves. Instead, they suspect the difference in behavior is a function of physical abilities. Either the females stick to shallow waters because they themselves cannot withstand the deep dives or because their dependent calves cannot.
To test the physical capability hypothesis researchers could observe other populations with separate foraging habitats and congruent dimorphism; in theory, a greater degree of difference in traits between the genders in a species could mean a greater divide in foraging habitats.
Researchers could also study whale dynamics during years when food availability is limited, if the male whales hunt in deep water just because they can, and not because they are avoiding food available of the females and calves, than one might suspect that if food was limited the males would be competing with the females for food, and the group dynamic could be damaged.
Although the northern resident killer whale population is a small, focused sample size, a study like this is important when making guidelines of marine mammal projection areas. For example, if protected areas are only in shallow near shore areas, it may influence the ability of the male whales to hunt and feed, because they may have to navigate unprotected waters.
It is important to remember that this is just one orca whale population, many more studies of killer whales in different areas would need to be conducted to make solid conclusions about the entire species.