Data from: Causes and consequences of individual variation: Linking state-dependent life histories to population performance
Data files
Mar 05, 2025 version files 6.66 KB
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Calf_Survival.txt
2.62 KB
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README.md
2.94 KB
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Starkey_Elk_Data.txt
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Abstract
The tradeoff between investment in current reproduction versus future survival is central to life-history theory, and long-lived, iteroparous mammals disproportionately favor their own survival. Previous work has demonstrated that adjustment of reproductive effort in long-lived mammals often occurs after parturition, owing to the greater cost of lactation relative to gestation. Under the right conditions, however, this difference in the relative costs of reproduction may also facilitate another, arguably less intuitive, strategy. Those conditions, which are relatively common among capital-breeding ungulates, include: (1) Females have the capacity to adjust gestation length; (2) Neonatal mortality occurs mostly during the first month of life and is inversely related to birth mass; and (3) The influence of birth mass on the probability of surviving the first month of life is stronger than the influence of autumn body mass on the probability of surviving the first winter of life. Under these circumstances, a female in poor condition in early spring could potentially increase fitness by delaying parturition and increasing investment in gestation, giving birth to a correspondingly larger neonate that has a higher probability of survival during its first month of life, and subsequently reducing investment in lactation to help rebuild somatic reserves. We developed and empirically parameterized a state-dependent model of maternal resource allocation that reflected this strategy. We tested the prediction that population growth would be faster when resource allocation was state-dependent than when gestation length was decoupled from dam condition and adjustment of reproductive investment was largely post-natal. Our results supported this prediction: state-dependent resource allocation by maternal females increased lambda by an average of 4%, leading to larger population sizes after 30 years. Population growth was consistent across a range of winter severities, suggesting that state-dependent resource allocation could help buffer ungulate populations against climatic variation. Our results reveal a potentially general mechanism underpinning intraspecific variation in life-history strategies of long-lived, capital-breeding mammals, and suggest that such variation at the individual level can influence performance outcomes at the population level.
The file “Starkey_Elk_Data.txt” contains data for the subset of dam/juvenile pairs for which we had data on (1) birth mass of the neonate, (2) mass of the juvenile in the winter following birth, (3) nutritional condition of the dam both in the spring prior to parturition and the winter following. It was exploratory analyses of these data that suggested the bet-hedging strategy in the first place. The following meta-data pertain to columns in that dataset:
- Sex: Sex of the neonate (female or male).
- Birth mass: Birth mass of the neonate in kg.
- Birth date: Birth date of the neonate.
- Julian birth date: Julian date on which the neonate was born.
- Winter mass date: Date on which mass of the neonate (now
juvenile) was measured during capture operations in the winter
following birth. - Julian winter mass date: Julian date on which the winter mass
measurement was obtained. - Winter mass: Winter mass of the juvenile in kg.
- Rate of mass change: Rate of mass change of the juvenile between
birth and winter in kg/d. - Change in BC: Difference in nutritional condition (%
ingesta-free body fat) of the dam between spring and winter. - Spring BC: Nutritional condition of the dam (% ingesta-free body
fat) in spring. - Add. To Gestation (kJ): Estimated additional energy devoted
to gestation by the dam (in kJ) based on the difference between
actual birth mass of the neonate and the minimum birth mass of 12.6
kg. - Invested in Lactation (kJ): Estimated energy invested in
lactation (in kJ) by the dam based on growth rate of the juvenile
between spring and winter. - Winter BC: Nutritional condition of the dam (% ingesta-free body
fat) in winter.
The file “Calf-Survival.txt” is the encounter history file used to estimate survival of neonatal elk during the first four months of life as a function of their sex, birth mass, and growth rate. This encounter history was analyzed as presented using the known-fate model in Program MARK. The following meta-data pertain to columns in that dataset:
- ID: Unique ID of each monitored elk calf.
- EncHist: The encounter history for the calf, coded according to
requirements for analysis using the known-fate model in Program
MARK. - Grp: A required grouping variable, set to 1 in this dataset so
that all calves were included as part of the same analysis. - Sex: Sex of each calf (1 = male, 0 = female).
- BM: Birth mass of each calf in kg.
- Growth: Estimated growth rate of each calf.
The file “StateDependentModel_Code.R” contains annotated code for running the state-dependent bet-hedging model of maternal investment.
The file “PostNatalModel_Code.R” contains annotated code for running the post-natal model of maternal investment.
A detailed description of data-collection methods can be found in the following publications:
Long, R.A., R.T. Bowyer, W.P. Porter, P. Mathewson, K.L. Monteith, S.L. Findholt, B.L. Dick, and J.G. Kie. 2016. Linking habitat selection to fitness-related traits in herbivores: the role of the energy landscape. Oecologia 181:709-720.
Long, R.A., R.T. Bowyer, W.P. Porter, P. Mathewson, K.L. Monteith, and J.G. Kie. 2014. Behavior and nutritional condition buffer a large-bodied endotherm against direct and indirect effects of climate. Ecological Monographs 84:513-532.
The two data files are in .txt format. The code is contained in .R files that require program R (or associated GUI like R Studio) to open.