... Developmental effect Sibling competition Phenotypic plasticity Body mass Mus musculus To overcome various adverse environmental changes, animals must often react both physiologically and behaviourally. Usually, more than one reaction is possible; therefore, animals ideally respond in such a way...

... ; Braude et al., 1999 ; Hasselquist et al., 1999 ),which is also suggested for maternal androgens (T. G. G. Groothuis, C. M. Eising, C. Dijkstra and W. Müller, manuscript submitted; Müller et al. , in press). High yolk androgens may also induce detrimental levels of sibling competition or metabolic costs...

..., which can be interpreted as an escalation of sibling competition. Thus, developmental and behavioural plasticity may be an important factor in the evolution of sibling rivalry. *Author for correspondence (e-mail: brzekpaw@cksr.ac.bialystok.pl ) 26 6 2001 © The Company of Biologists...