Division of Labor 329 



the leader was shot, the band was reported to remain in a confined 

 area of the island, and did not take its usual trips to other parts of 

 the island. 



Cooperation without any special morphological modifications among 

 the individuals and with a minimum of organization is another ad- 

 vantage often resulting from the formation of groups. When at- 

 tacked, musk ox form in a circle with their horns extending outward 

 and thus secure for themselves a protection that would not exist if 

 each animal attempted to defend itself individually. A group of 

 beavers working together can dam a stream that could not be success- 

 fully dammed by animals operating singly. In a penguin colony 

 some adults guard not only their own young but also the young of 

 others while the remaining parents leave the area in search of food. 

 Feeding cormorants are observed to maintain a rough line as they 

 swim and dive and thus presumably improve the effectiveness of their 

 fishing (Bartholomew, 1942). 



A somewhat more elaborate division of activity within the group 

 is found in certain species during the breeding season. In sea lions 

 (Fig. 9.8), fur seals, elk (Fig. 9.9), deer, and other mammals, as well 

 as various game birds, the population of a region becomes organized 

 into breeding units each consisting of a male, his harem, and sub- 

 sequently their young. After the mating season the harem groups are 

 broken down, and the animals may reorganize themselves into sepa- 

 rate bands of males, females, and young for the remainder of the year. 

 Group organization in the fur seals (Callorhinus ursiniis) may be 

 taken as an example. The bulls arrive first early in May at the 

 breeding grounds on the Pribilof Islands, and each stakes out a 

 breeding site for himself which he jealously guards. When the cows 

 arrive in mid-July, each of the larger and more senior of the bulls 

 collects a harem for himself within his own territory. The remain- 

 ing bulls, kept away by ferocious fighting, form a group of disap- 

 pointed bachelors. Bulls with harems are continually being chal- 

 lenged by the bachelors, and, although each established bull drives 

 off the intruders for one or more seasons, eventually each bull ages 

 and after losing a fight or a series of fights he is forced to relinquish 

 his harem to a new master. Soon after the females are organized 

 into harems the pups resulting from the previous breeding season are 

 born, and subsequently mating with the bulls takes place. When 

 breeding has been completed, the harems disintegrate. After a 

 period of feeding and of nursing the pups, the seals reorganize them- 

 selves in groups consisting of males only, and other groups made up 

 of females and pups. In these groups the seals leave the Pribilofs 



