Global patterns in phenology

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Bonnet, T., Morrissey, M.B., Morris, ... Kruuk, L.E.B., 2019. The role of selection and evolution in changing parturition date in a red deer population. PLOS Biology 17, e3000493. https://doi.org/10.1371/journal.pbio.3000493 Lane, J.E., McAdam, A.G., ... J.C., Boutin, S., 2018. Phenological shifts in North American red squirrels: disentangling the roles of phenotypic plasticity and microevolution. Journal of Evolutionary Biology 31, 810–821. https://doi.org/10.1111/jeb.13263 Renaud, L.-A., Pigeon, G., Festa-Bianchet, M., Pelletier, F., 2019. Phenotypic plasticity in bighorn sheep reproductive phenology: from individual to population. Behavioral Ecology and Sociobiology 73, 1–13. https://doi.org/10.1007/s00265-019-2656-1

de Villemereuil, P., Charmantier, A., ... J., Chevin, L.-M., 2020. Fluctuating optimum and temporally variable selection on breeding date in birds and mammals. Proceedings of the National Academy of Sciences 117, 31969–31978. https://doi.org/10.1073/pnas.2009003117

Macphie, K., Samplonius, J., Pick, J., Hadfield, J., Phillimore, A., 2023. Modelling thermal sensitivity in the full phenological distribution: A new approach applied to the spring arboreal caterpillar peak. Functional Ecology 37. https://doi.org/10.1111/1365-2435.14436

Pakeman, R.J., Stevenson, I., Pilkington, J., Bal, X., Pemberton, J., Fenton, A., Childs, D., Crawley, M. and Nussey, D., 2024. Plant community‐specific greening patterns predict population size increases in a temperate herbivore. Oikos, 2024(12), p.e10551. Hayward, A.D., Behnke, J.M., Childs, D.Z., Corripio-Miyar, Y., Fenton, A., Fraser, M.D., Kenyon, F., McNeilly, T.N., Pakeman, R.J., Pedersen, A.B. and Pemberton, J.M., 2022. Long-term temporal trends in gastrointestinal parasite infection in wild Soay sheep. Parasitology, 149(13), pp.1749-1759.

Phenology is the annual pattern of life-history traits. Examples include the timing of migration, reproduction, hibernation, etc.

Tell me more about the sheep.

The Soay sheep are a primitive breed of sheep that originate on the islands of St. Kilda, Scotland. Specifically, we study a population of unmanaged sheep on the island of Hirta in the Village Bay area. The core study population size fluctuates between years but is usually around 500 individuals. We have collected data on morphmetrics, space use, vegetation, genetics, parasites and disease, and other research areas since 1985. See the project website for more details: https://biology.ed.ac.uk/soaysheep

What is fieldwork like?

There are three core field seasons per year with a slightly different focus. Team sizes vary between 3-16 people depending on the trip. During every field season, 10 censuses of the study area are done with the aim of identifying all tagged sheep, who they are associated with, and where they are. We also do vegetation surveys, and mortality searches as standard procedure during all field visits. Any time a new sheep is caught they are tagged and a small tissue sample is taken so that we can follow them throughout their lives and include them in our genetic pedigree. Spring: catching and tagging lambs when they are a few days old Summer: catching as many individuals in the study area as possible and taking measurements Fall: catching males during the rut to aid paternity assignment

What drives Soay sheep reproductive phenology?

Maternal identity explains that largest amount of variance in birth date (~29%) so ewes are more likely to lamb at around the same time every year. We know that there is a genetic basis for birth date in this system (h2 ~ 18%). Space use also explains ~10% of the variance where individuals who live in the NW of the study area give birth several days earlier and the rest of the population. This is likely because males enter the study area from this side during the rut. Otherwise, very few demographic or environmental factors influence birth date. Maternal age has no impact on birth date, nor does the size of the litter or the population density in a given year. Weather during the rut (so when conception occurs) also doesn't impact birth dates. In short! Still an unanswered question.

How accurate is your birth data?

In spring, there are thorough daily checks for lambs in the Village Bay study area and any previously unseen lamb is recorded along with the identity of the mother. Lambs are then caught a few days after birth for genetic sampling and weighing. This means that for most of the core study population we have an exact birth date. More peripheral individuals may not have a precise birth date as they were first seen as older lambs or adults. Only sheep with exact birth dates are included in these analyses.

How is your fitness measured?

We measure fitness as lamb survival to Oct 1st. By this time, lambs have grown substantially with both females and males able to reproduce in their first year of life (with females getting pregnant at 6 months old). Lamb mortality is calculated based on whether an individual was found deceased and when that individual was last seen alive.

How did you calculate your fitness functions?

We ran these models in a Bayesian framework and allowed all parameters to vary and co-vary among each other and among years. For more details on the methodology we adapted for our questions, see Macphie et al 2023 Functional Ecology for more details.

How does this system compare to other systems?

The Soay sheep have been studied for 40 years with individuals followed throughout their life. Similar studies in mammals include: the Red deer on the Isle of Rum, the Bighorn sheep in Western Canada, Red squirrels in Kluane National Park, Ground squirrels in Colorado, to name just a few. Mammal phenology is relatively understudied compared to other taxa, namely birds, plants, and invertebrates. This is largely to do with the logistical challenges associated with studying wild mammal populations.