Acknowledgements--Research awards from the CSIR and the University of Pretoria for field work in South Africa and Botswana are gratefully acknowledged. We especially thank M. P. Coward for discussions which inspired this work. J. M. Barton;
G;
Brandl;
E;
A;
Retief;
C;
Roering;
Smit;
D. D. van Reenen and J. F. Wilson are thanked for discussions. Text improvements by J. M. Barton;
P;
Eriksson;
J;
Hatton;
M;
R;
Sharpe;
D. D. van Reenen and K. L. Walsh are gratefully acknowledged. S. Vasconcelos drafted the figures;