DOI: 10.5061/dryad.jq2bvq8jh
title: Suckling observations of Galapagos sea lions

description: Milk is not cheap. Mammalian offspring are expected to wean during the
 early juvenile stages, whether through their own mechanisms or their
 mothers’ instigation. To not do so would result in a reproductive
 trade-off for the mother: continued investment in the health and growth of
 her current offspring, over a shorter birth interval, and the
 possibility of begetting a fitter pup. Here we show, for the
 first time, using 20 years of data, repeated suckling events between
 female Galápagos Sea Lions (GSL) Zalophus wollebaeki and
 their fully adult biological offspring well beyond the expected
 age of independence and when the offspring are themselves already
 reproductively active. This behavior, ‘supersuckling’, suggests that GSL
 mother-offspring relationships are more complex and longer-lasting than
 previously thought. To our knowledge, this behavior has not been
 previously documented in any marine mammal species.
description: Data were collected between September 2003 and April 2023 on Isla
 Caamaño (0° 45’ S, 90° 16’ W) during 2 annual field seasons from
 October-December and February-April that cover the main reproductive
 period. Suckling observations were made through minimum twice daily rounds
 using non-invasive means. Morphometric measurements were collected during
 captures of individuals. All data collection was carried out with the
 knowledge and permission of the Galápagos National Park.
description: # Suckling observations and morphometric measurements of Galapagos Sea
 Lions Dataset DOI: [10.5061/dryad.jq2bvq8jh](10.5061/dryad.jq2bvq8jh) ##
 Description of the data and file structure GENERAL INFORMATION This
 README.txt file was updated on 24 September 2025 A. Paper associated with
 this archive Citation: Milking it: repeated post-weaning suckling events
 in Galápagos Sea Lions *Zalophus wollebaeki* Data were collected between
 September 2003 and April 2023 on Isla Caamaño during 2 annual field
 seasons from October-December and February-April that cover the main
 reproductive period. The islet is located off the southern coast of Santa
 Cruz in the centre of the Galápagos archipelago and is home to a breeding
 colony of GSL that has been intensively studied since 2003. All
 individuals that are born on, or regularly use the islet, are marked and
 assigned a unique identification ('ID'). During the marking
 morphometric measurements are taken, some individuals have been captured
 multiple times over their life span either to replace missing markers or
 as part of another contained study. Identification rounds ('resight
 rounds') were conducted at least twice daily. During each round the
 ID, location, and behavior (including suckling events) of every individual
 present on the islet is recorded. Suckling observations are only recorded
 when the individual at the teat is clearly suckling.  B. Originators
 Alexandra K. Childs; Department of Animal Behaviour, Faculty of Biology,
 Bielefeld University, Konsequenz 45, 33501, Germany; Galápagos Sea Lion
 Project, Bielefeld 33501, Germany. Carlina Feldmann; Department of
 Business Administration and Economics, Bielefeld University,
 Universitätsstraße 25, 33615 Bielefeld, Germany. Svenja Stoehr; Department
 of Animal Behaviour, Faculty of Biology, Bielefeld University, Konsequenz
 45, 33501, Germany; Galápagos Sea Lion Project, Bielefeld 33501, Germany.
 Rémi Demarthon; Galápagos Sea Lion Project, Bielefeld 33501, Germany.
 Paolo Piedrahita; Facultad de Ciencias de la Vida, Escuela Superior
 Politécnica del Litoral, Guayaquil P.O. Box 09-01-5863, Ecuador. Sean D.
 Twiss; Department of Biosciences, Durham University, Durham DH1 3LE, UK.
 Oliver Krüger; Department of Animal Behaviour, Faculty of Biology,
 Bielefeld University, Konsequenz 45, 33501, Germany;Joint Institute for
 Individualisation in a Changing Environment (JICE), Bielefeld University
 and University of Münster, Konsequenz 45, Bielefeld 33615, Germany. C.
 Contact information Alexandra K. Childs; ORCID 0009-0003-4150-6745
 Department of Animal Behaviour, Faculty of Biology, Bielefeld University,
 Konsequenz 45, 33501 Bielefeld, Germany;
 [alexandra.childs@uni-bielefeld.de](mailto:alexandra.childs@uni-bielefeld.de) D. Dates of data collection September 2003 and April 2023 E. Geographic Location(s) of data collection Isla Caamaño, Galápagos, Ecuador ### Files and variables DATA & CODE FILE OVERVIEW This data repository consist of 4 data files, 1 code script and its HTML output, and this README document, with the following data and code filenames and variables Data files and variables #### File: SucklingPairs.csv **Description:** includes data on suckling events (each row is a suckling observation) 54777 obs. of 11 variables ##### Variables * $ resight_round_date date of the suckling event $ sighting_ID identifier of the suckling event $ offspring_ID identifier of the suckling offspring (empty or 'NA' cells are where no positive ID was was recorded and can be considered the same as an negative value in the variable, which indicates that the offspring is unmarked, n = 4) $ offspring_sex sex of the suckling offspring (empty or 'NA' cells are where no positive ID was was recorded and therefore the sex is unknown, n = 2957) $ offspring_dob date of birth of the suckling offspring (unidentified offspring (negative values) are automatically assigned a DoB which holds no value; empty cells are a result of no value is present in offspring_ID and a DoB could not be automatically assigned, n = 4) $ offspring_exact_birth how exact the birth information is („exact“: the day is known precisely, „weak“: +-2 weeks, „unknown“: less exact or completely unknown (empty cells are a result of no value is present in offspring_ID and categorisation of exactness could not be made, n = 4) $ suckled_female_ID identifier of the nursing female (empty or 'NA' cells are where no positive ID was was recorded, nor any information regarding the presence/absence of tags, n = 2) $ suckled_female_dob date of birth of the nursing female (empty or 'NA' cells are where no positive ID was was recorded, and so no DoB could be automatically assigned, n = 2) $ suckled_female_exact_birth how exact the birth information is (empty or 'NA' cells are where no positive ID was was recorded, and so the exactness of the DoB could not be categorised, n = 2) $ offspring_birth_mother_ID identifier of the suckling offspring’s biological mother (empty or 'NA' cells indicate that the identity of the offsprings biological mother is unknown and that no information regarding the presence/absence of tags was recorded, n = 5357) $ offspring_age_at_sighting_days age of the suckling offspring in days on the day of observation (empty or 'NA' cells indicate that there is too much uncertainty around the observation of the offspring i.e. it's DoB is categorised as „unknown“ or the ID is unknown, for such a precise age to be reliably calculated) #### File: GrowthData.csv **Description:** 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 9968 obs. of 6 variables ##### Variables * $ Capture_ID identifier of the handling event $ Capture_Date date of the handling event $ Animal_ID identifier of the individual being handled $ Animal_Date_of_Birth date of birth of the individual being handled $ Animal_Exact_Birth how exact the birth information is $ Age_at_Capture_Days age of the individual being handled in days on the day of handling #### File: AllRounds.csv **Description:** includes information on the observation effort (each row is an observation round) 5905 obs. of 3 variables ##### Variables * $RR_ID identifier of the observation round $RR_date date of the observation round (empty or 'NA' cells are where no date was recorded for the round, n = 2) $RR_RTID type of round, where 3 and 4 indicate rounds where observed suckling was noted (empty or 'NA' cells are where no tide was recorded for the round, n = 2) #### File: detrend.nino34.ascii.txt **Description:** includes information on the ONI anomalies effort (each row is an observation round) 892 obs. of 5 variables: ##### Variables * $ YR year of the observation $ MON month of the observation $ ANOM the rolling 3-month average temperature anomaly used to classify the occurrence and strength of ENSO events ### ## Code/software Code scripts and workflow ``` Code_Supersuckling.Rmd 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 html (Output_Supersuckling.html) is added which is the output of this Rmd file ``` SOFTWARE VERSIONS \## - Auguie B (2017). *gridExtra: Miscellaneous Functions for "Grid" Graphics*. R package version 2.3, [https://CRAN.R-project.org/package=gridExtra](https://CRAN.R-project.org/package=gridExtra). \## - Grolemund G, Wickham H (2011). "Dates and Times Made Easy with lubridate." *Journal of Statistical Software*, *40*(3), 1-25. [https://www.jstatsoft.org/v40/i03/](https://www.jstatsoft.org/v40/i03/). \## - Makowski D, Lüdecke D, Patil I, Thériault R, Ben-Shachar M, Wiernik B (2023). "Automated Results Reporting as a Practical Tool to Improve Reproducibility and Methodological Best Practices Adoption." *CRAN*. [https://easystats.github.io/report/](https://easystats.github.io/report/). \## - R Core Team (2024). *R: A Language and Environment for Statistical Computing*. R Foundation for Statistical Computing, Vienna, Austria. [https://www.R-project.org/](https://www.R-project.org/). \## - Wickham H (2016). *ggplot2: Elegant Graphics for Data Analysis*. Springer-Verlag New York. ISBN 978-3-319-24277-4, [https://ggplot2.tidyverse.org](https://ggplot2.tidyverse.org). \## - Wickham H, François R, Henry L, Müller K, Vaughan D (2023). *dplyr: A Grammar of Data Manipulation*. R package version 1.1.4, [https://CRAN.R-project.org/package=dplyr](https://CRAN.R-project.org/package=dplyr). \## - Wickham H, Hester J, Bryan J (2024). *readr: Read Rectangular Text Data*. R package version 2.1.5, [https://CRAN.R-project.org/package=readr](https://CRAN.R-project.org/package=readr). \## - Wickham H, Vaughan D, Girlich M (2024). *tidyr: Tidy Messy Data*. R package version 1.3.1, [https://CRAN.R-project.org/package=tidyr](https://CRAN.R-project.org/package=tidyr). 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](https://doi.org/10.1578/AM.36.3.2010.262) ## Access information Other publicly accessible locations of the data: none Data was derived from the following sources: Climate data according to the ONI index obtained from NOAA ([https://origin.cpc.ncep.noaa.gov/products/analysis_monitoring/ensostuff/detrend.nino34.ascii.txt](https://origin.cpc.ncep.noaa.gov/products/analysis_monitoring/ensostuff/detrend.nino34.ascii.txt))