American Society of Naturalists

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“Estimation of individual growth trajectories when repeated measures are missing”

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Mollie E. Brooks, Christopher Clements, Josephine Pemberton, and Arpat Ozgul

Soay Sheep have growth-reproduction tradeoff. Plus, new R package for modeling growth trajectories avoids bias of LMMs

Reproduction impacts a mother’s growth trajectory

A Soay ewe with a lamb on St Kilda, Outer Hebrides.<br />(Credit: Arpat Ozgul)
A Soay ewe with a lamb on St Kilda, Outer Hebrides.
(Credit: Arpat Ozgul)

What are the consequences of reproductive investment on parental growth in wild animals? This question was investigated by Mollie Brooks, Christopher Clements, Josephine Pemberton, and Arpat Ozgul at the Universities of Zurich and Edinburgh using Soay sheep. Soay sheep, roaming wild on the Isle of Hirta in the St. Kilda archipelago off Scotland, have been tagged and recaptured each year since 1985 (see St Kilda Soay Sheep Project). This provides a long time series of the sheep’s demography including reproduction and survival, along with weights each summer. However, time series of wild animal populations frequently contain missing values because every individual cannot be captured every year.

Focusing on the body mass time series, the authors developed an R package that handles missing values better than other common methods. Using this approach, Brooks et al. found that female sheep that give birth to a lamb in any given year grow about half as much as would otherwise be expected in that year. This is due to the energetic costs of being a mother, such as pregnancy and nursing. Sheep also have reduced growth in years when the population is large, probably because of greater competition for food or increased parasite burdens. Adverse winter weather also affects the sheep’s growth.

Individually marked Soay sheep.<br />(Credit: Christopher Clements)
Individually marked Soay sheep.
(Credit: Christopher Clements)

Looking across individuals, the researchers found that sheep with consistently high growth rates are higher quality in other aspects as well; they live longer and produce more lambs across their life span; they are also more likely to start reproducing at an early age. Differences in individual quality could be caused by genetics or prolonged parasite infections.

These patterns of body mass growth will probably have consequences for population growth because survival and reproduction depend on how much energy an animal has stored in its body, and this better understanding may help to predict population fluctuations in the future. Read the Article