Ecological and social conditions have the potential to influence the ontogenetic development of behavioural phenotypes. However, there has so far been little experimental work aiming at determining and disentangling these early influences on the development of behavioural phenotypes. We chose photoperiod – as a predictor of expected ecological conditions – and sibling interactions within a litter as factors that are likely to influence the development of behavioural phenotypes in a wide spectrum of species. We have shown that photoperiod, as expected, strongly influences the development of behavioural phenotypes in cavies (Cavia aperea), even though the effects may be reversible. We will now study the resulting fitness effects through match-mismatch experiments where we place animals, reared under an artificial spring photoperiod in the lab, into the same or into an autumn photoperiod outdoors and vice versa with animals reared under autumn conditions. Besides photoperiod, we found size-rank in the litter and sex also influential in determining behavioural phenotypes. Such sibling effects are widely known among animals (and humans), but have hardly been studied experimentally. We now suggest to investigate these influences through cross-fostering experiments. We will exchange the largest or smaller pups in a litter to other litters in such a way that these pups either find themselves in the same size-rank as in their litter of origin or in the other position and will determine how this influences the development of their behavioural phenotypes. These experiments will enable us to disentangle how the interactions between photoperiod, size-rank in litter, and sex contribute to the developing behavioural phenotype. Additionally, these experiments should enable us to assess the influence of maternal and genetic effects on the developing behavioural phenotype.