that enable the fittest males to monopolize matings, 

 should lead to the development of conspicuous male 

 secondary sex characteristics. For example, males en- 

 dowed with larger size and weapons are most likely 

 to prevail, and any such traits that are heritable will 

 be passed along to their male descendants, leading to 

 pronounced differences between males and females 

 (so-called sexual dimorphism). 



But that's not what happens among wildebeests. 



cease to be male sex symbols. As a consequence, 

 horn development can (and does) begin much ear- 

 lier than in species with hornless females. 



In most species in which both sexes have horns, 

 the females' horns do not grow as large or robust as 

 the males' do. The reason, I presume, is that the fe- 

 males' mimicry of male horn development proceeds 

 only to the stage at which young males are ready to 

 leave the female herds and assert themselves in com- 



The sound and fury of the Serengeti rut comes from bulls competing 

 to mate. Some 600,000 cows are inseminated in less than a month. 



After years of pondering the puzzle, I developed my 

 own hypothesis, that natural selection suppresses de- 

 velopment of conspicuous male secondary sex char- 

 acteristics in species that regularly associate in mixed 

 herds. To be sure, competition among males of the 

 same age and stage of development (peer competi- 

 tion) promotes the evolution of such characteristics. 

 But as soon as young males look different from fe- 

 males, breeding males begin to treat them as poten- 

 tial rivals. Sooner or later, young males subjected to 

 such despotic competition are driven out of the fe- 

 male herd and the familiar home range, and hence 

 exposed to greater danger. So the longer 

 adolescent males look and act like females, 

 the better their chances of postponing evic- 

 tion and surviving to reproduce. 



There is a hitch, however, to how male 

 sexual development plays out in bovids, 

 the large family of ungulates that com- 

 prises the antelopes and other hollow- 

 horned ruminants, such as cattle, goats, and 

 sheep. Horns are the essential weapons 

 with which bovid males compete for 

 dominance; horns are also the most basic 

 male secondary sex characteristics. Unlike 

 the bony antlers of deer, they are perma- 

 nent structures. Peer competition pro- 

 motes their growth as early as possible, pre- 

 senting a dilemma for the young male. 

 A factor that comes into play here is the 

 females' interest in protecting their male 

 offspring from the aggression of breeding 

 males. How can they help? One way is by 

 growing horns themselves — and, among 

 many bovids, including wildebeests, that 

 is just what they do. Presto! Horns of sim- 

 ilar shape and size carried by both sexes 



Wildebeests share the tall grasses of Kenya's 

 Masai Mara Reserve with zebras. 



petition with other males. But in species where nat- 

 ural selection favors the continued association of 

 adults of both sexes within herds, the females mimic 

 horns and other male secondary sex characteristics all 

 the way to maturity. Among western white-bearded 

 wildebeests, females have even developed faux penile 

 tufts that make it harder to tell the sexes apart. 



In spite of years of observation, biologists still have 

 much to learn about wildebeest ecology. For ex- 

 ample, one way adult bulls express their aggressive- 

 ness is to use their horns to thrash small trees of a 



34 



NATURAL HISTORY September 2006 



