Many mammals time their reproduction to take advantage of early spring growth in plants. The newest shoots are the most nutritious so timing is everything. In recent study in Ecosphere, Maik Rehnus and colleagues examine the dates of almost roe deer in Switzerland between 1971 and 2015. They found that parturition dates changed on average at a rate between 7.5 times slower than the growing season start and 5 times slower than the flowering start.
Ecologists often contrast capital breeders and income breeders. Capital breeders are organisms that gain a store of energy in order to reproduce. Income breeders in contrast need to time reproduction to match the supply of food. Roe deer, Capreolus capreolus, appear to be very much tied to the income breeding strategy. What Rehnus and colleagues did was build upon this work to see how deer reproduction and vegetation growth tallied across a range of gradients in Switzerland.
The scientists didn’t need forty-five years to run their experiment. Instead, the data set came from some reuse of other data gathered in Switzerland. Rehkitzmarkierung Schweiz has been marking roe deer nationwide since the early 1970s in Switzerland.
“Although the application of an ear mark is simple, only people experienced in handling wildlife were allowed to participate in this project,” write Rehnus and colleagues in their article. “Fawns were found by repeatedly observing the mother during late pregnancy until the day of parturition. For each fawn marked, the following information was recorded: the date of marking, the estimated age in days, the geographic coordinates, and the herbaceous layer height at the marking site (<20, 20–50, >50 cm). Fawn age was assessed by examining the umbilical cord and observing fawn behavior during marking (Jullien et al. 1992). Parturition date was then calculated by subtracting the estimated age of the fawn (in days) from the date of capture.”
“We used parturition dates instead of date of birth of individual fawns to avoid pseudo‐replication problems (sensu Hurlbert 1984) caused by the presence of twins or triplets (i.e., two or three fawns from the same mother in the same year). To ensure that parturition dates were calculated accurately, we only used information from fawns aged ten days or younger because aging precision decreases with increasing fawn age (Jullien et al. 1992; mean fawn age 4.69 ± 2.79 d). We only included marking sites with a data resolution ≤1 ha. Ultimately, we used information from 8986 parturition events recorded during the period 1971–2015.”
The team found that these parturition events were moving earlier in the year, but only by 0.06 days per year. Over the forty-five years of the study that means that fawns were born, on average, around two and a half days earlier in the year at the end of the study than at the start. In contrast the growing season started almost half a day earlier per year of the study and flowering around a third of a day earlier, meaning those dates had shifted forwards by twenty days, and over fourteen days respectively. That extra growth allowed the plants to strengthen their stems and become more fibrous, by the time the mothers were ready to raise their young.
“Unfortunately, in comparison with other Northern Hemisphere large herbivores studied, roe deer is the species that show the smallest response to climate change (−0.06 d per year). West Greenland caribou, another income breeder, advanced −0.11 (± 0.03 SE) days per year over 33 yr (Kerby and Post 2013a). Capital breeders like red deer have advanced parturition by −0.42 (± 0.08 SE) days per year over 26 yr in Scotland (Moyes et al. 2011). In Finland, semidomesticated reindeer, which are also described as capital breeders, advanced parturitions −0.15 (± 0.04 SE) days per year over 45 yr (Paoli et al. 2018). For Pyrenean chamois in France, a 10‐d advance in the onset of autumn or spring plant phenology led to an advance of four and one day in birth dates, respectively (Kourkgy et al. 2016),” write Rehnus and colleagues.
“For this species, migration and dispersion movements may constitute a good strategy to match parturition dates and peak resource availability during the breeding season.”For plants that are themselves migrating to higher slopes to escape rising temperatures, this raises the possibility of increased grazing by deer providing for their fawns. For lowland plants, a departure of deer could lead to a shift in balance of the ecology, if some plants are free to compete in the absence of grazing. It’s not merely insects that are slipping out of time with plants, but large herbivores too. The Bambis of the future might no longer be found in the meadows.