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Modeling the demography of species providing extended parental care: A capture-recapture approach with a case study on Polar Bears (Ursus maritimus)

Citation

Cubaynes, Sarah (2022), Modeling the demography of species providing extended parental care: A capture-recapture approach with a case study on Polar Bears (Ursus maritimus), Dryad, Dataset, https://doi.org/10.5061/dryad.fn2z34tsq

Abstract

1. In species providing extended parental care, one or both parents care for altricial young over a period including more than one breeding season. We expect large parental investment and long-term dependency within family units to cause high variability in life trajectories among individuals with complex consequences at the population level. So far, models for estimating demographic parameters in free-ranging animal populations mostly ignore extended parental care, thereby limiting our understanding of its consequences on parents and offspring life histories.

2. We designed a capture-recapture multi-event model for studying the demography of species providing extended parental care. It handles statistical multiple-year dependency among individual demographic parameters grouped within family units, variable litter size, and uncertainty on the timing at offspring independence. It allows for the evaluation of trade-offs among demographic parameters, the influence of past reproductive history on the caring parent’s survival status, breeding probability and litter size probability, while accounting for imperfect detection of family units. We assess the model performance using simulated data, and illustrate its use with a long-term dataset collected on the Svalbard polar bears (Ursus maritimus).

3. Our model performed well in terms of bias and mean square error and in estimating demographic parameters in all simulated scenarios, both when offspring departure probability from the family unit occurred at a constant rate or varied during the field season depending on the date of capture. For the polar bear case study, we provide estimates of adult and dependent offspring survival rates, breeding probability and litter size probability. Results showed that the outcome of the previous reproduction influenced breeding probability.

4. Overall, our results show the importance of accounting for i) the multiple-year statistical dependency within family units, ii) uncertainty on the timing at offspring independence, and iii) past reproductive history of the caring parent. If ignored, estimates obtained for breeding probability, litter size, and survival can be biased. This is of interest in terms of conservation because species providing extended parental care are often long-living mammals vulnerable or threatened with extinction.

Methods

Polar bears were caught and individually marked as part of a long-term monitoring program on the ecology of polar bears in the Barents Sea region (Derocher 2005). All bears one year or older were immobilized by remote injection of a dart (Palmer Cap-Chur Equipment, Douglasville, GA, USA) with the drug Zoletil® (Virbac, Carros, France) (Stirling et al. 1989). The dart was fired from a small helicopter (Eurocopter 350 B2 or B3), usually from a distance of about 4 to 10 meters. Cubs of the year were immobilized by injection with a syringe. Cubs and yearlings were highly dependent on their mother; therefore, they remained in her vicinity and were captured together with their mother. 

The file "CR.txt" contains the capture-histories of n= 158 family units captured between 1992 to 2019 arranged in a matrix with the status of each family unit (in rows) provided each year (in columns). Code 1 is for two-year-old independent juvenile female, code 2 is three-year-old independent juvenile female, code 3 is four-year-old subadult female, code 4 is four-year-old subadult female, code 5 is adult female with one cub, code 6 is adult female with two cubs, code 7 is adult female with one yearling, code 8 is adult female with two yearlings, code 9 is adult female with one depdendent two-year-old bear, code 10 is adult female with two dependent two-year-old bears, code 11 is adult female without dependent offspring, code 0 means a no-capture event. 

The file "captureday.txt" contains the date of capture, in day of the year, for each family unit (in row) each year (in column). "NA" means a no-capture event. 

The file "weaning.txt" contains the status (code 1 is independent from its mother, code 0 means still dependent from its mother), identification number and date of capture of each two-year-old bear (n=120 males and females) captured on the field.

Usage Notes

R script with guidance to analyse these data is provided at https://github.com/SCubaynes/Appendix2_extendedparentalcare

Funding

Agence Nationale de la Recherche, Award: ANR‐18‐CE02‐0011

World Wildlife Fund