Born in heterogenous landscapes: birth timing, body mass and early development of roe deer fawns in contrasting habitats

Although the widespread effects of climate change impact almost all ecosystems, we lack a detailed understanding of how wildlife that thrive in human-dominated environments are able to adjust their life history to modifications in land use of their natural habitat. In particular, spatial variation in environmental conditions is predicted to influence body development during the crucial early life phase, with marked impacts on individual performance and population dynamics for long-lived species. Large herbivores have increased substantially in number and distribution over the last half century across Europe. The synanthropic roe deer (Capreolus capreolus) has been particularly successful, gradually colonizing agricultural landscapes. However, little is known about how habitat heterogeneity in this heavily human-impacted environment impacts demographic performance. Specifically, we predicted that fawns born in predominantly cultivated local habitats would achieve faster early development due to the food subsidies obtained from agricultural crops by their mothers. Contrary to our expectations, fawns in semi-natural forest were around 10% heavier at birth than those born in more mixed (by 0.163 ± 0.058 kg) and open (by 0.169 ± 0.006 kg) agricultural habitats. However, the body mass of all fawns subsequently increased at a similar average rate (0.148 ± 0.058 kg/day) over the first 20 days of life, irrespective of their habitat. This habitat-dependent variation in early life mass appeared to be driven by reproductive phenology, as (1) early-born fawns were heavier than late-born fawns, and (2) mothers living in forest gave birth around 10 days earlier than those living in the mixed and open sectors. Semi-natural habitats might thus be the more suitable for fawn early development, despite the availability and abundance of energetically rich food resources for lactating mothers in open and mixed habitats.