When does weather synchronise life history traits? Spatiotemporal patterns in juvenile body mass of two ungulates

1. Theory predicts that animal populations will be synchronised over large distances by weather and climatic conditions with high spatial synchrony. However, local variation in population responses to weather, and low synchrony in key weather variables or in other ecological processes may reduce the population synchrony. 2. We investigated to what extent temperature and precipitation during different periods of the year synchronised juvenile body mass of moose and reindeer in Norway. We expected high synchronising effect of weather variables with a high and consistent explanatory power on body mass dynamics across populations, and a weaker synchronising effect of weather variables whose effect on body mass varied among populations. 3. Juvenile body mass in both species was related to temperature and precipitation during several periods of the year. Temperature had the strongest explanatory power in both species, with a similar effect across all populations. 4. There was higher spatial synchrony in temperature compared to precipitation, and accordingly temperature had the strongest synchronising effect on juvenile body mass. Moreover, periods with strong explanatory power had stronger synchronising effect on juvenile body mass in both species. However, weather variables with large variation in the effects on body mass among populations had weak synchronising effect. 5. The results confirm that weather has a large impact on the spatial structure of population properties, but also that spatial heterogeneity for instance in environmental change or population density may affect how and to what extent populations are synchronised. 

Methods
In both species body mass was measured as carcass mass, which is body mass minus head, skin, metapodials, bleedable blood, and viscera (Sæther et al., 1996). In the uploaded data it is reported as the mean body mass of all harvested calves in a municipality (moose) or herding district (reindeer) in a given year.

Moose juvenile body mass was based on calves harvested and weighed during the hunting season in autumn, from which body mass is collected as part of the National monitoring program for wild cervids in Norway (Solberg et al., 2017). Hunters also recorded sex, place (municipality) and the date the animal was shot and slaughtered. The hunting season lasted from 25th of September or 5th of October, to the end of October, but has recently been extended to December. Because calf body mass increases during the autumn (Herfindal et al., 2006), we only use calves harvested in September and October, and adjusted it according to the date of harvest and to sex assuming similar weather effects on males and females (see procedure in Herfindal et al., 2006). We then calculated the mean body mass per municipality and year.

Body mass of semi-domestic reindeer calves are reported by slaughterhouses. Data are reported as mean carcass mass and number of calves slaughtered per reindeer herding district and year. Slaughtering normally occurs at the same time each year within a district, typically two weeks prior to the rut which starts in ultimo September.

Data on temperature and precipitation were obtained from the Norwegian Meteorological Institute as downscaled 1×1 km gridded daily data covering all of Norway (Engen-Skaugen et al., 2002). For each municipality or reindeer district we calculated monthly mean temperature and sum of precipitation by averaging the value of all pixels falling inside the population borders. For moose, we restricted the calculation to pixels below the climatic tree line, obtained from Moen (1999), to exclude alpine habitat that are rarely used by moose. This was not necessary for reindeer, as all areas within districts potentially can be used by reindeer during a year.

References:

Engen-Skaugen, T., Hanssen-Bauer, I., & Førland, E. J. (2002). Adjustment of dynamically downscaled temperature and precipitation data in Norway. Oslo: Norwegian Meteorological Institute Report 20/02.

Herfindal, I., Solberg, E. J., Sæther, B.‑E., Høgda, K. A., & Andersen, R. (2006). Environmental phenology and geographical gradients in moose body mass. Oecologia, 150(2), 213–224. https://doi.org/10.1007/s00442-006-0519-8

Moen, A. (1999). National atlas of Norway: vegetation. Hønefoss, Norway: Norwegian Mapping Authority.

Solberg, E. J., Strand, O., Veiberg, V., Andersen, R., Heim, M., Rolandsen, C. M., . . . Eriksen, R. (2017). Hjortevilt 1991-2016. Oppsummeringsrapport fra Overvåkingsprogrammet for hjortevilt. NINA Rapport, 1388.

Sæther, B.‑E., Andersen, R., Hjeljord, O., & Heim, M. (1996). Ecological correlates of regional variation in life history of the moose Alces alces. Ecology, 77(5), 1493–1500. https://doi.org/10.2307/2265546