Abstract: The distinctive Somali Arid and the East African grassland faunas approach each other to the north of the Kenya Highlands and along the Tana River. Both have unique endemics and many counterparts at the species and race level. Mostly these latter remain isolated but there are instances of intergradation and range overlap without interbreeding. An interesting case of the latter, that of the zebras Equus grevyi and E. burchelli, is analyzed here. The unusual feature of the zebra overlap is that mixed herds are formed. These maintain a level of constancy from day to day, and are not disrupted during flight. Mutual tolerance is marked. While mixed herds occur all year, the composition varies from season to season and district to district. Migrations may take place conjointly. Outside of the overlap zone habitat differences are marked. The mixed zebra herds of Kenya are contrasted with the zebra-wildebeest associations of the Kruger Park, South Africa, and it is shown that the former have greater cohesion and constanc

Abstract: Feral horses (Equus caballus) in the Venezuelan savannas (Ilanos) formed bands of 3-35 individuals; mean group size varied between 15 and 21. Some bands had up to three adult males, and there was a positive correlation between the number of males and the number of females in bands. Adult males also were seen solitary or formed bachelor groups. One association of three females was observed for 3 months. We noted a correlation between number of females in bands and number of foals per female born during our study (January-July 1992). The advantages of being in larger groups were unclear, because natural predators were rare. Perhaps females in larger groups have more time available for foraging due to reduced individual vigilance or that larger groups live in better-quality home ranges.