Mates but not sexes differ in migratory niche in a monogamous penguin species
Jean-Baptiste Thiebot, Charles-André Bost, Nina Dehnhard, Laurent Demongin, Marcel Eens,
Gilles Lepoint, Yves Cherel and Maud Poisbleau
Electronic Supplementary Material
S1: Methods – Geolocation process
Light-based geolocation loggers from British Antarctic Survey, Cambridge, UK (model MK4,
MK5, MK7 or MK15) were used. These loggers measure light level every minute and record
the maximum value at the end of every 10 min period together with the timer value. In
addition, saltwater immersion was tested every 3 s and ambient seawater temperature was
recorded once after every 20 min period of continuous immersion with an accuracy of ±
0.5°C. All the loggers were let for at least one week at the colony, before and after the winter
deployments, in order to calibrate the light response of each logger at a known geographic
place. The start and end dates of the at-sea period were determined from the time of the first
and last immersion records from each GLS, respectively. Winter movements were estimated
from downloaded data following the maximum-likelihood approach of the package
‘tripEstimation’ in R 2.9.0 [1,2], and using the R script provided in [3] for similar logged data
files. Movement parameters within the model were constrained by a speed of 3 km/h derived
from a satellite tracking study of wintering southern rockhopper penguins [4], and by the
location of the colony at the departure and return dates. The matching procedure with seasurface temperature satellite data allowed refining latitude estimates throughout the at-sea
period of the penguins, with a tolerance window of 2°C around the recorded value. Weekly
satellite data were downloaded from the LAS Level-3 gridded data visualization and
subsetting tool (http://podaac-tools.jpl.nasa.gov/las), with a spatial resolution of 1°. The use
of a “land mask” precluded any location estimate to occur on land for our marine study
species. Thanks to this geolocation approach, no location estimate was discarded for the
analyses, although the at-sea period of penguins (April–October) included the equinox
periods when the calculation of latitude based on light and time only is unreliable for 2–3
weeks (because day length is equivalent everywhere on Earth at that time).
References:
1.
R Development Core Team 2009 R: a language and environment for statistical
computing. Vienna, Austria: R Foundation for Statistical Computing (http://www.Rproject.org).
2.
Sumner MD, Wotherspoon SJ, Hindell MA. 2009 Bayesian estimation of animal
movement from archival and satellite tags. PLoS ONE 4, e7324.
3.
Thiebot JB, Pinaud D. 2010 Quantitative method to estimate species habitat use from
light-based geolocation data. Endang. Species Res. 10: 341–343.
4.
Raya Rey A, Trathan P, Pütz K, Schiavini A. 2007 Effect of oceanographic conditions
on the winter movements of rockhopper penguins Eudyptes chrysocome chrysocome from
Staten Island, Argentina. Mar. Ecol. Prog. Ser. 330, 285–295.
S2: Methods – Stable isotope analyses
Blood samples (1 ml) were taken from the brachial vein within 3 min after capture
(heparinised syringe, 23-gauge needle). After centrifugation, plasma was removed and red
blood cell samples were frozen (-20°C) and later dried in a drying furnace (60°C). Dried red
blood cells were homogenised and ground to a fine powder. Stable isotope ratio of carbon
and nitrogen were measured on 0.8 mg aliquots, weighed into tin cups in continuous flow by
mass spectrometry (Isoprime 100, Isoprime, UK) coupled to an elemental analyser (Vario
Microcube, Elementar, Germany). Stable isotope ratios were expressed in δ notation as parts
per thousand (‰) deviation from the international standards Vienna Pee Dee Belemnite
(carbon) and AIR (nitrogen), according to,
δX= [(RSample-RStandard)/RStandard]*1000
where X is 15N or 13C and R is the corresponding ratio 15N/14N or 13C/12C. Pure gasses of CO2
and N2 were used and calibrated against certified reference materials, i.e. sucrose (IAEA-C6;
δ13C=−10.8±0.5‰), ammonium sulfate (IAEA-N2; δ15N=+20.3±0.2‰). The analysis'
performance was assessed by procedural blanks, replicated samples and in-house standard
(i.e. glycine). Analytical precision (±S.D.) on replicated samples equaled ± 0.3 and ± 0.5‰
for δ13C and δ15N, respectively.
S3: Results – Stable Isotope Bayesian Ellipses in R (SIBER)
Isotopic niche metrics of male and female southern rockhopper penguins from New Island
were calculated with the SIAR package in R 3.2.0 [1]. This Bayesian approach fits a standard
ellipse to bivariate data analytically using maximum likelihood estimators. The standard
ellipse area represents the core isotope niche width as described by [2].
Metric
Description
Males
Females
SEA
The area of the standard ellipse
1.22
0.63
SEAc
The area of the small sample size corrected
1.38
0.71
standard ellipse
overlap
The area of overlap between the two sample size
0.30
corrected standard ellipses
theta
The angle of the semi-major axis of the ellipse
1.15
1.08
with the x-axis
eccentricity
Eccentricity of the ellipse
0.992
0.996
a
The length of the semi-major axis a
1.78
1.54
b
The length of the semi-minor axis b
0.22
0.13
ac
The length of the semi-major axis a based on the
1.89
1.63
0.23
0.14
small sample size correction
bc
The length of the semi-minor axis b based on the
small sample size correction
References:
1.
R Development Core Team 2015 R: a language and environment for statistical
computing. Vienna, Austria: R Foundation for Statistical Computing (http://www.Rproject.org).
2.
Jackson AL, Inger R, Parnell AC, Bearhop S. 2011 Comparing isotopic niche widths
among and within communities: SIBER – stable isotope Bayesian ellipses in R. J. Anim. Ecol.
80, 595–602.