1. Centre for Behavioural Biology, School of Biological Sciences, University of Bristol, Woodland Road, Bristol, BS8 1UG, UK.
2. Animal Demography Unit, Department of Zoology, University of Cape Town, Private Bag, Rondebosch 7701, South Africa.
3. Penguin Datasystems, 2 Cotham Place, Trelawney Road, Bristol, BS6 6QS, UK.
4. H. H. Wills Physics Laboratory, University of Bristol, Tyndall Avenue, Bristol, BS8 1TL, UK.
5. Robben Island Museum, Robben Island 7400, South Africa.
6. Marine Research Institute, University of Cape Town, Private Bag X3, Rondebosch 7701, Cape Town, South Africa.