An early-life perspective is needed to explain the impact of gut microbiota on wild vertebrate phenotypes.

Vertebrates house dense and diverse communities of microorganisms in their gastrointestinal tracts. These communities shape host physiological and ecological phenotypes in diverse ways, with implications for animal fitness in nature. Exposure to microbes during the earliest stages of life is particularly important because, during critical developmental windows, the microbiome is exceptionally plastic and interactions with microbes can have long-lasting physiological impacts on the host. Despite our understanding that early-life microbial interactions are important to host function broadly, the majority of research in this area has been performed in human or model organisms that are not representative of animals in the wild. Specifically, most gut microbiome studies in wildlife are cross-sectional and compare microbial communities across life stages using different individuals, as opposed to tracking the microbial communities and phenotypes of the same individuals from early to later life. This knowledge gap may hinder wildlife microbiome research, as the current model lacks an early-life perspective that can contextualize host phenotypic and fitness differences observed between animals at later life stages. Further, considering early-life microbial dynamics may offer insights to applied research, such as determining the optimal age to manipulate microbiomes for desired conservation outcomes. In this Commentary, we consider current understanding of the importance of early-life host-microbe interactions to vertebrate physiology across the lifespan, discuss why this perspective is necessary in wildlife studies, and provide practical recommendations for experimental designs that can address these questions, including field and laboratory approaches.