wildebeests happens to move into a resident neigh- 

 borhood, however, the spacing may be effectively 

 halved, as outsider bulls infiltrate and temporarily set 

 up shop amid the resident bulls. 



At the height of the annual Serengeti rut the den- 

 sity is even higher: the average increases to three 

 competing bulls per hectare. And it is not unusual 

 to find two or even three bulls under the same tree, 

 moving through a crowd of a hundred cows that are 

 seeking shade from the midday sun. At first glance 

 it might seem they are tolerating each other's close 

 presence, willingly sharing the wealth of females. In 

 fact, the packed bodies are just keeping them from 



catching sight of one another. The same screening 

 effect enables noncompeting males to mingle in 

 dense concentrations and thereby gain access to the 

 best grazing, as long as they can escape the notice 

 of territorial bulls. 



The fact that males and females look so much 

 alike promotes their association in mixed herds. 

 The territorial males do stand out in the crowd, but 

 that is primarily because of their behavior. Never- 

 theless, the physical similarities present a puzzle. Ac- 

 cording to Darwin's theory of sexual selection, re- 

 productive competition between males, in systems 



September 2006 natural history 



33 



