Plasma insulin‐like growth‐factor 1 ( <scp>IGF</scp> ‐1) concentrations predict early life‐history traits in a wild mammal

Abstract The hormone insulin‐like growth factor 1 (IGF‐1) is a key player in the insulin/IGF signalling (IIS) pathway. Extensive biogerontological research demonstrates that this evolutionarily conserved nutrient‐sensing pathway plays a causal role in the regulation of growth, reproduction and longevity under laboratory conditions. However, its potential role as a mediator of adaptive life‐history variation in highly variable natural environments remains unclear. We measured IGF‐1 concentrations in blood samples from approximately 4‐month‐old wild Soay sheep lambs ( n = 669), collected over nine summers. We tested whether IGF‐1 (i) was positively related to proxies of resource availability, (ii) was associated with morphological traits measured concurrently and (iii) predicted subsequent fitness‐related traits. Plasma IGF‐1 concentrations were higher in males compared to females and positively associated with measures of resource availability in both sexes. IGF‐1 was lower in years of high population density when per capita food availability was reduced; in twin lambs who have fewer available resources compared to singletons; and in lambs born to young and old mothers, who have poor maternal provisioning compared to mothers of intermediate age. Higher IGF‐1 concentrations in summer were associated with higher body mass, faster post‐natal somatic growth and increased skeletal size, measured at the same time. These associations were observed after statistically controlling for our proxies of resource availability. Lambs with higher summer IGF‐1 concentrations were more likely to survive their first winter and reproduce the following spring. The association between IGF‐1 and reproduction was present after accounting for our resource availability proxies, whereas the association with first‐winter survival was not. The association between summer IGF‐1 and reproduction was explained by positive associations with summer body mass. Our study reveals population‐level phenotypic plasticity in circulating IGF‐1. We found IGF‐1 to be positively associated with key morphological traits and to positively predict fitness traits in early life. These findings highlight IGF‐1 as a candidate physiological mechanism underpinning plastic responses to variation in food availability and influencing life‐history traits in a wild mammal. Read the free Plain Language Summary for this article on the Journal blog.