Insulin-like growth factor 1 and life-history evolution of passerine birds

1. Natural selection has generated a diversity of ways in which vertebrates allocate their resources between fundamental life-history traits. The availability of possible evolutionary trajectories of these traits is limited by various genetic, physiological and phylogenetic constraints. This causes trade-offs due to shared resource pools for, or genetic linkage of, competing traits. The majority of these trade-offs are mediated by hormones and create the variability in phenotypes that can be -observed in nature. 2. Insulin-like growth factor 1 (IGF-1) is an evolutionarily conserved peptide, which has been shown to be essential in the regulation of body size, the pattern of reproductive investment and life span across a broad taxonomic range of model species in laboratory and domesticated conditions. However, studies addressing corresponding evolutionary hypotheses on a broader scale and in free-living vertebrates are very rare. 3. In this phylogenetic comparative study on free-living passerines (Passeriformes), we explore the way in which plasma IGF-1 levels underlie the evolution of body size and demographic fitness correlates (clutch size, egg weight, life span). 4. We showed firstly that IGF-1 levels were positively associated with the body size of passerines, although smaller birds had larger IGF-1-mediated investment into building up their body faster. IGF-1 levels were negatively associated with life span of passerines, and more so in birds with smaller body weight. Finally, IGF-1 levels were negatively associated with clutch size in heavier species, and positively associated with egg weight in species with higher body weight and longer duration of parental care. The pattern was opposite in species with smaller body weight and shorter duration of parental care. 5. The described evolutionary framework indicates that variation in IGF-1 levels can be regarded as an important mechanism that may underlie life-history evolution in passerines. IGF-1 could act as a physiological link mediating the inter-regulatory growth-reproduction-lifespan "life-history triangle" on the pace-of-life continuum. Interestingly, body weight and investment into parental care have likely imposed a constraining effect on the IGF-1-mediated co-evolution of demographic fitness traits, such as life span or reproductive investment. This has limited the availability of adaptive pathways via which those traits could evolve as passerines diversified.