ORIGINAL PAPER Growth rate and hatching date in ostrich chicks reflect humoral but not cell-mediated immune function Maud Bonato & Matthew R. Evans & Dennis Hasselquist & Schalk W. P. Cloete & Michael I. Cherry Received: 16 February 2009 / Revised: 12 July 2009 / Accepted: 26 July 2009 / Published online: 13 August 2009 # Springer-Verlag 2009 Abstract A tradeoff between immune response and life history traits, in particular growth rate, has been docu- mented in various bird species. Ostriches are fast-growing birds and a typical feature of cohorts is that offspring often differ greatly in size. We investigated the relationship between hatching date and growth rate of chicks and both cell-mediated (measured using a phytohaemagglutinin (PHA) injection) and humoral immune responses in ostrich chicks maintained on a research farm. Chicks with higher growth rates had intermediate responses to both diphtheria and tetanus toxoids. By contrast, no relation between growth rates and responses to PHA injection were found. We conclude that chick growth rate variation may be explained beyond a certain threshold by a tradeoff between the humoral response and growth. Both responses to PHA injection and humoral responses in chicks were found to decrease with chick hatching date. Within the context of ostrich farming, these results could partially explain size variations observed in cohorts of chicks, as well as high mortality rates during their first 3 months of age. Keywords Immune response . Growth rate . Seasonal effect . Stabilising selection . Struthio camelus Introduction The vertebrate immune system has evolved as a defence mechanism against parasites and pathogens and hence plays a crucial role in host survival and fitness (Goldsby et al. 2000). Individuals are affected to varying degrees by the negative impact of parasites because of inter-individual variations in genetic and non-genetic factors affecting general phenotypic condition (Nordling et al. 1998; Gonzalez et al. 1999). Goossens et al. (1997) reported that in farmed goats the main effect of disease was the suppression of growth, inevitably leading to reduced fitness. The control of parasites in chickens led to a major improvement in feed efficiency, weight gain, reproductive performance and survival (Stephenson 1994), underlining the crucial role of the immune system. Although Klasing (1998) has argued that the energetic costs of immune function may be minimal compared to other physiological processes such as tissue growth, there is increasing empirical and experimental evidence that a Communicated by J. Graves M. Bonato (*) : M. I. Cherry Department of Botany and Zoology, University of Stellenbosch, Private Bag X1, Matieland 7602, South Africa e-mail: mbonato@sun.ac.za M. R. Evans Centre for Ecology and Conservation, School of Biosciences, University of Exeter, Cornwall Campus, Penryn, Cornwall TR10 9EZ, UK D. Hasselquist Department of Animal Ecology, Ecology Building, 223 62 Lund, Sweden S. W. P. Cloete Institute for Animal Production: Elsenburg, Private Bag X1, Elsenburg 7607, South Africa S. W. P. Cloete Department of Animal Sciences, University of Stellenbosch, Private Bag X1, Matieland 7602, South Africa Behav Ecol Sociobiol (2009) 64:183–191 DOI 10.1007/s00265-009-0835-1