Data from: Weaning age and supersuckling in Galápagos Sea Lions

Childs, Alexandra K.1 ; Feldmann, Carlina C.1; Dyck, Julia1; Stoehr, Svenja1; Twiss, Sean D.2; Krüger, Oliver1

Research facility: Charles Darwin Foundation

Published Jul 27, 2026 on Dryad. https://doi.org/10.5061/dryad.b8gtht7rx

Abstract

Weaning marks a critical life-history transition in mammals, shaped by maternal investment strategies, offspring condition, and environmental constraints. In Galápagos sea lions (Zalophus wollebaeki, GSL) weaning age is highly variable, with some individuals continuing to suckle well into adulthood (supersucklers). Using 20 years of both suckling observations and mark-recapture data from 1890 individuals, we applied multi-state capture-recapture models (MSCRM) to investigate weaning age, and logistic regression models to explore the drivers of supersuckling in GSL. We examined age at weaning from both the maternal (age, birth interval, and pup condition at birth) and offspring perspective (sex, early growth) and environmental variability (Oceanic Niño Index [ONI]). We found that weaning is most likely to occur between the ages of 1.5-4.5 years with 95% of offspring weaned by 4.5 years of age. El Niño conditions during early life influence weaning age and the probability of offspring becoming supersucklers in later life. Longer birth intervals were associated with later weaning, while offspring sex had no influence. No intrinsic maternal or offspring predictors of supersuckling were identified, warranting further investigation to better understand this behaviour. Our results show that MSCRMs are an effective means of identifying the occurrence of more cryptic and anecdotal behaviour and highlight the complexity of weaning behaviour in an income-breeding mammal with a prolonged lactation period. Finally, we propose that weaning age in GSL may not be wholly explained by mother–offspring conflict, and that further investigation considering factors not included in this study is needed.

Dataset DOI: 10.5061/dryad.b8gtht7rx

Description of the data and file structure

This data repository consists of 4 data files, 1 R code script, 2 R Markdown code scripts and their HTML output, 16 simulates, 15 models, and this README document, with the following data and code filenames and variables. BEASREADME.txt contains the same information as this README.

Files and variables

data_seasonal.RData

includes data on suckling events (each row is a suckling observation) along with all variables incorporated in the MSCRMs

48418 obs. of 16 variables

$ ID: identifier of the suckling individual

$ age: age of the suckling individual in reproductive half-seasons (i.e. 6 month intervals)

$ obs_behaviour: the observed behaviour of an individual where 1 = suckling (S), 2 = observed not suckling (N), 3 = no observation (0) used to identify transitions between states

$ sex: sex of the suckling individual (empty or 'NA' cells are where no positive sex was recorded (n = 36))

$ season: the reproductive season (i.e. year, Aug-Jul) during which the suckling event was observed. The Galapagos Sea Lion reproductive year runs from August 1st - July 31st

$ spring_autumn: the reproductive half-season (i.e. 6 month interval where autumn = Aug-Jan; and spring = Feb-Jul) during which the suckling event was observed

$ seasonofbirth: the reproductive season (i.e. year) of birth of the suckling individual, where 1 represents the first year of data collection. The Galapagos Sea Lion reproductive year runs from August 1st - July 31st

$ seasonofdeath: the reproductive season (i.e. year) of death of the suckling individual (empty or 'NA' cells are where no positive death was recorded because the individual has 1. not died, 2. death cannot be confirmed because no body was recovered (n = 47905)), where 1 represents the first year of data collection. The Galapagos Sea Lion reproductive year runs from August 1st - July 31st

$ motherAge: age of the nursing individual in reproductive seasons (i.e. years) (empty or 'NA' cells are where either 1. no positive ID was was recorded, 2. date of birth of mother was "unknown" and so no accurate age could be calculated (n = 43132))

$ SeasonsUntilNextSibling: the number of reproductive seasons (i.e. years) between the observed individual and their first sibling (empty or 'NA' cells are where 1. no sibling births were recorded after the birth of the suckling individual 2. the suckling individual's mother died 3. the suckling individual's mother was not observed for more than 2 years (n = 40551))

$ scaled_mass_index: scaled mass index of the individual at birth (<= 10 days old) calculated according to Peig & Green (2009) (empty or 'NA' cells are associated with missing morphological measurements so no SMI could be calculated (n = 13892))

$ SMI_older: mean scaled mass index of the individual between 30 - 90 days old, calculated according to Peig & Green (2009) (empty or 'NA' cells are associated with missing morphological measurements so no SMI could be calculated (n = 20356))

$ sibling_ID1: identifier of the suckling individual's first sibling (empty or 'NA' cells are where 1. no sibling births were recorded after the birth of the suckling individual 2. the suckling individual's mother died 3. the suckling individual's mother was not observed for more than 2 years (n = 40551))

$ mean_ANOM: the rolling 3-month average temperature (Celsius) anomaly used to classify the occurrence and strength of ENSO events

$ ONI_Category: the ONI category (as defined by NOAA) according to the mean_ANOM

$ rounds_suckling: the number of rounds per reproductive season (i.e. year) where suckling observations were recorded

data_seasonal_wdeaths.RData

includes data on suckling events (each row is a suckling observation).

54737 obs. of 18 variables

$ ID: identifier of the suckling individual

$ age: age of the suckling individual in reproductive half-seasons (i.e., 6-month intervals)

$ obs_behaviour: the observed behaviour of an individual where 1 = suckling (S), 2 = observed not suckling (N), 3 = no observation (0) used to identify transitions between states

$ sex: sex of the suckling individual (empty or 'NA' cells are where no positive sex was recorded (n = 36))

$ season: the reproductive season (i.e., year, Aug-Jul) during which the suckling event was observed, where 1 represents the first year of data collection. The Galapagos Sea Lion reproductive year runs from August 1st - July 31st

$ spring_autumn: the reproductive half-season (i.e. 6 month interval where autumn = Aug-Jan; and spring = Feb-Jul) during which the suckling event was observed

$ motherAge: age of the nursing individual in reproductive seasons (i.e. years) (empty or 'NA' cells are where either 1. no positive ID was recorded, 2. date of birth of mother was "unknown" and so no accurate age could be calculated (n = 48936))

$ sibling_ID2: identifier of the suckling individual's second sibling when their mother gave birth to twins (empty or 'NA' cells are where there was no twin (n = 54737))

$ SST: the average sea surface temperature (Celsius) for the reproductive half-season, i.e. 6-month interval where autumn = Aug-Jan; and spring = Feb-Jul) during which the suckling event was observed

$ mean_ANOM: the rolling 3-month average temperature anomaly (Celsius) used to classify the occurrence and strength of ENSO events

$ ONI_Category: the ONI category (as defined by NOAA) according to the mean_ANOM

$ rounds_suckling: the number of rounds per reproductive season (i.e. year) where suckling observations were recorded

growthdata.RData

includes data on all handling conducted during the data collection period, used to verify accuracy of age estimation for individuals with "unknown" date of birth at their first handling event

10130 obs. of 6 variables

$ Animal_ID: identifier of the individual being handled

$ Age_at_Capture_Days: age of the individual being handled in days, on the day of handling

$ Acctual_Mass_Kg: mass in Kg of the individual being handled (empty or 'NA' cells are where no mass was recorded during handling (n = 945)

$ Body_Length_cm: length in cm of the individual being handled, measured from the tip of the snout to the tip of the tail (empty or 'NA' cells are where no length was recorded during handling (n = 487))

$ Animal_Sex: sex of the individual being handled (empty or 'NA' cells are where no positive set is associated with the individual (n = 8))

$ SMI: scaled mass index of the individual being handled calculated from the mass and body length according to Peig & Green (2009) (empty or 'NA' cells are associated with missing values in Acctual_Mass_Kg and/or Body_Length_cm so no SMI could be calculated (n = 1074))

susu_FemaleRepro.RData

includes information on observations of suckling events including details of the suckling offspring and the identifier of their biological other (each row is a unique observation)

48805 obs. of 11 variables

$ ResightRound_Date: date of the observation

$ Offspring_ID: identifier of the suckling individual

$ Offspring_Sex: sex of the suckling individual

$ Offspring_DateOfBirth: date of birth of the suckling individual

$ Offspring_ExactBirth: how exact the birth information is ("1": exact, the day is known precisely, "2": weak, +-2 weeks, "3": rough, the individual was handled for the first time at under 6 months of age and so could be assigned to the correct cohort of birth). NOTE: all individuals with an 'unknown' birth were removed from this data as they were not of interest to this study

$ BiologicalMother_ID: identifier of the biological mother of the suckling individual. NOTE: this is not the identifier of the nursing individual at the event

$ offspring_age_at_sighting_days: age of the suckling individual in days on the day of observation

$ season: the reproductive season (i.e. year) during which the suckling event was observed

$ offspring_sob: the reproductive season (i.e. year) of birth of the suckling individual

$ offspring_age_seasons: age of the suckling individual in reproductive seasons (i.e. years) at the time of the observation

$ Event_YR: the reproductive year (equivalent to season) during which the suckling event was observed, named according to the year (Gregorian calendar) the reproductive season started

Code scripts and workflow:

Supersucklers_drivers.Rmd: fits logistic regressions, plots weight curves

UWA_SOND_final.Rmd: fits the MSCRM, plots results

R Markdown code includes all code necessary to reproduce results: loading the data, preparing the data, running the analyses, creating the plots

Additionally, the knitted .htmls (Output_Supersucklers_drivers.html & Output_UWA_SOND_final.html) are added which are the output of these .Rmd files

Simulates:

For plotting and extracting data from UWA_SOND_final.Rmd (it is recommended that these are saved in a subfolder "html" of "UWA_SOND_final_cache" for UWA_SOND_final.Rmd to run correctly).

These contain a simulated population of 10000 individuals for analysis of covariate effects. all simulates can be generated using the data sets provided and UWA_SOND_final.Rmd, however this requires a large amount of power and is time consuming we have therefore provided them in addition to the original script for ease of replication.

\__packages

$ simulate_stationary__motherAge_ef40f6333546a93722f05fbfde94cbc3.RData

$ simulate_stationary__motherAge_ef40f6333546a93722f05fbfde94cbc3.rdb

$ simulate_stationary__motherAge_ef40f6333546a93722f05fbfde94cbc3.rdx

$ simulate_stationary__SMI_4fe95978faf3688c307c055f1331089d.RData

$ simulate_stationary__SMI_4fe95978faf3688c307c055f1331089d.rdb

$ simulate_stationary__SMI_4fe95978faf3688c307c055f1331089d.rdx

$ simulate_stationary_ONI_6e351224afa9912140feddf8070ac17f.RData

$ simulate_stationary_ONI_6e351224afa9912140feddf8070ac17f.rdb

$ simulate_stationary_ONI_6e351224afa9912140feddf8070ac17f.rdx

$ simulate_stationary_sibling_6f20a69acdf8c6089eabc5e32d9b52ba.RData

$ simulate_stationary_sibling_6f20a69acdf8c6089eabc5e32d9b52ba.rdb

$ simulate_stationary_sibling_6f20a69acdf8c6089eabc5e32d9b52ba.rdx

$ simulate_stationary_sex_0ee4d9e2abcaacc30f7be2dfa89bb68e.RData

$ simulate_stationary_sex_0ee4d9e2abcaacc30f7be2dfa89bb68e.rdb

$ simulate_stationary_sex_0ee4d9e2abcaacc30f7be2dfa89bb68e.rdx

$ simulate_stationary_61eb64143c0b392dd6317e36529559ee.RData

$ simulate_stationary_61eb64143c0b392dd6317e36529559ee.rdb

$ simulate_stationary_61eb64143c0b392dd6317e36529559ee.rdx

Models:

For UWA_SOND_final.Rmd (it is recommended that these are saved in a folder "mods" for UWA_SOND_final.Rmd to run correctly).

These models test the covariates of interest and were selected as outlined in the associated publication. All can be generated by running the script UWA_SOND_final.Rmd however this requires a large amount of power and is time consuming, we have therefore provided them in addition to the original script for ease of replication.

$ UWA_nocov.RData

$ UWA_roundsZ_agecattil7Y.RData

$ UWA_roundsZ_agepoly1Y.RData

$ UWA_roundsZ_agepoly2expdecayY_agexANOMY_agexmotherAgeY.RData

$ UWA_roundsZ_agepoly2expdecayY_agexANOMY_agexsiblingY.RData

$ UWA_roundsZ_agepoly2expdecayY_agexANOMY_agexsmiY.RData

$ UWA_roundsZ_agepoly2expdecayY_agexANOMY_datamotherAge.RData

$ UWA_roundsZ_agepoly2expdecayY_agexANOMY_datasibling.RData

$ UWA_roundsZ_agepoly2expdecayY_agexANOMY_datasmi.RData

$ UWA_roundsZ_agepoly2expdecayY_agexmotherageY.RData

$ UWA_roundsZ_agepoly2expdecayY_agexsiblingY.RData

$ UWA_roundsZ_agepoly2expdecayY_agexsmiY.RData

$ UWA_roundsZ_agepoly2expdecayY_ANOMY_agexANOMY.RData

$ UWA_roundsZ_agepoly2expdecayY_ANOMY.RData

$ UWA_roundsZ_agepoly2expdecayY_ANOMY_agexANOMY_sexY_agexsexY_ANOMxsexY.RData

$ UWA_roundsZ_agepoly2expdecayY_ANOMY_agexANOMY_sexY_agexsexY.RData

$ UWA_roundsZ_agepoly2expdecayY_sexY_agexsexY.RData

$ UWA_roundsZ_agepoly2expdecayY_datamotherAge.RData

$ UWA_roundsZ_agepoly2expdecayY_datasibling.RData

$ UWA_roundsZ_agepoly2expdecayY_datasmi.RData

$ UWA_roundsZ_agepoly2expdecayY_sexY.RData

$ UWA_roundsZ_agepoly2expdecayY.RData

$ UWA_roundsZ_agepoly2Y.RData

$ UWA_roundsZ_agepoly3Y.RData

$ UWA_roundsZ.RData

In order to run the script Supersucklers_drivers.Rmd independently, the following files are required:

$ likelihood_UWA_SOND.R: functions for fitting the MSCRM, these are sourced in the Rmd files

$ UWA_roundsZ_agepoly2expdecayY_ANOMY_agexANOMY.RData

Includes model output of nlm() of the selected MSCRM which is produced in UWA_SOND_final.Rmd and processed for the regression models in Supersuckler_drivers.Rmd (This output is created when running UWA_SOND_final.Rmd, but is uploaded so it is available for running Supersuckler_drivers.Rmd independently).

Code/software

Code scripts and workflow:

$ Supersucklers_drivers.Rmd: fits logistic regressions, plots weight curves

$ UWA_SOND_final.Rmd: fits the MSCRM, plots results

R Markdown code includes all code necessary to reproduce results: loading the data, preparing the data, running the analyses, creating the plots

Additionally, the knitted .htmls (Output_Supersucklers_drivers.html & Output_UWA_SOND_final.html) are added which are the output of these .Rmd files

$ likelihood_UWA_SOND.R: functions for fitting the MSCRM, these are sourced in the Rmd files

SOFTWARE VERSIONS

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## - Bates, D., Maechler, M. & Jagan, M., 2025. Matrix: Sparse and dense matrix classes and methods. R package version 1.7-3. Available at: https://CRAN.R-project.org/package=Matrix. https://doi.org/10.32614/CRAN.package.Matrix

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## - Pinheiro, J., Bates, D. and R Core Team, 2025. nlme: Linear and Nonlinear Mixed Effects Models. R package version 3.1-168. doi:10.32614/CRAN.package.nlme.

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## - R Core Team (2024). R: A Language and Environment for Statistical Computing. R Foundation for Statistical Computing, Vienna, Austria. https://www.R-project.org/.

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## - Wickham, H., Hester, J. and Bryan, J., 2024. readr: Read Rectangular Text Data. R package version 2.1.5. http://doi:10.32614/CRAN.package.readr

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REFERENCES

## - Jeglinski, J. W. E., Mueller, B., Pörschmann, U., & Trillmich, F. (2010). Field-Based Age Estimation of Juvenile Galapagos Sea Lions (Zalophus wollebaeki) Using Morphometric Measurements. Aquatic Mammals, 36(3), 262–269. https://doi.org/10.1578/AM.36.3.2010.262

## - Peig, J., & Green, A. J. (2009). New perspectives for estimating body condition from mass/length data: the scaled mass index as an alternative method. Oikos, 118(12), 1883–1891. https://doi.org/10.1111/j.1600-0706.2009.17643.x

RELATED WORKS

## - Childs, Alexandra Kate; Feldmann, Carlina; Stoehr, Svenja et al. (2025). Suckling observations of Galapagos sea lions [Dataset]. Dryad. https://doi.org/10.5061/dryad.jq2bvq8jh

## - Childs, Alexandra K.; Stainfield, Claire; Stoehr, Svenja et al. (2026). Pupping and yearling survival in Galápagos Sea Lions: impact of maternal investment, environmental fluctuations and population decline [Dataset]. Dryad. https://doi.org/10.5061/dryad.zs7h44jr0