Intraspecific variation and environmental filtering jointly and dynamically shaped individual elementomes of two sympatric small mammals

1. The biogeochemical niche hypothesis (BNH) recently proposed the multi-elemental composition (i.e., their elementome) as a promising ecological dimension. However, whether elementomes merely reflect species-specific stoichiometric constraints that vary little or randomly among individuals (the null BNH) or instead integrate structured ecological information (the alternative BNH), and, if the latter holds, how much elementome variation we can explain, remain unanswered questions. Our main hypothesis was that biological factors (species and intraspecific variation) would explain essential elementomes (Ca, Mg, Na, K, Zn, Cu, Mn, Fe, Mo, and Co), and that environmental filtering would explain non-essential elementomes (Pb, Al, V, Cr, Ni, As, Sr, Ba, Li, and B). 2. Here, we studied how species identity and intraspecific variation (body mass and sex), together with environmental filtering (season and habitat), contribute to shape the mandibular elementomes of eighty-four individuals belonging to two sympatric small mammals (Apodemus flavicollis and Clethrionomys glareolus). We further evaluated ontogenetic body-mass allometric scaling in elemental stoichiometry under the vertebrate bone hypothesis (VBH). 3. We show that elementomes embed significant ecological information, as we explained up to 41.8% of multi-elemental variance. Species identity and intraspecific variation contributed more strongly to essential elementomes (19.7% of multi-elemental variance, vs. 11.8% for the environment). In contrast, environmental filtering contributed more strongly to non-essential elementomes (18.3% vs. 13.3% for intraspecific variation). Despite their phylogenetic proximity, ecological sympatry, and omnivorous diets, the species maintained distinct but moderately overlapping elementomes. Interestingly, the elementomes of the two species diverged from spring to autumn, perhaps as a result of micro-habitat or diet partitioning to reduce competition. 4. Ontogenetic body-mass scalings of several elemental and Ca-substitution relationships were consistent with predictions of the VBH. However, these scalings were context dependent, with, for instance, hypermetric scalings in autumn that could reflect life-history processes. 5. Our findings across individual and species levels of biological organization show that animal elementomes emerge from the combined influence of intraspecific variation, species identity, and environmental filtering, and that these factors explained a substantial proportion of multi-elemental variance. To conclude, elementomes provide useful ecological information on free-ranging animals.