Supplementary Material
Journal: Behavioral Ecology and Sociobiology
Sex differences in the consequences of maternal loss in a long-lived
mammal, the red deer (Cervus elaphus)
Daniel Andres, Tim H. Clutton-Brock, Loeske E.B Kruuk, Josephine M. Pemberton,
Katie V. Stopher, Kathreen E. Ruckstuhl
Corresponding Author: Daniel Andres
Email Address: dandres@ucalgary.ca
Corresponding Author Affiliation:
Department of Biological Sciences, University of Calgary, Calgary, Alberta,
Canada
Comparison of capture weight of orphans and non-orphans: Confounding factors are
an obvious concern in analyzing the effect of orphaning on survival. For example,
orphaning may tend to occur because maternal condition is poor, and since maternal
condition likely affects birth weight and survival (Clutton-Brock et al. 1982), orphans
would be at an increased risk of death irrespective of changes in maternal care. One
option to control for this is to include capture weight as a covariate in the modeling of
survival; in our study, this would result in a loss of data, since not all individuals were
captured and weighed. The other alternative is to test for a difference in birth weight
between orphans and non-orphans: if no systematic bias is found, one could ignore the
effects of birth weight and accept a level of unexplained variation in survival time. The
analysis presented below indicates that failure to include capture weight in our survival
models would potentially bias results. Therefore, as outlined in the methods section of the
paper, we included capture weight in our survival models. The results here are presented
as a justification for the survival methods outlined in the main body of the paper.
In this analysis, we used a matched design to compare weights of orphans and
non-orphans. Observations were matched by year of birth, sex, age weighed and ‘event
age’ (see below). The matched set each observation belonged to was fitted as a random
effect. All animals were weighed within 13 days of birth. Non-orphan deer were matched
to individual orphans if their age at weighing was within plus or minus 12 hours of the
orphan’s age at weighing. Mortality risk declines with age and birth-weight (CluttonBrock et al. 1982), and as a consequence animals that survive long enough to lose their
mothers will tend to survive longer and be heavier than the starting crop of deer.
Therefore, we randomly matched non-orphans to orphans if non-orphan survival time
(months) was equal or greater than the age (months) orphans lost their mother; we refer to
this as ‘event age’ matching. Orphans with mothers dying from shooting were excluded
from this analysis.
Birth-weight was analyzed using general linear mixed models with orphan status
(orphaned or not) as a fixed effect and event age match ID as the random effect. In this
analysis, an orphan is defined as any individual losing its mother prior to an age of 36
months. Males and females were analyzed separately in these comparisons. The
denominator degrees of freedom was adjusted according to the Kenward Roger technique
assuming the correlation among responses follows compound symmetry; AIC was used
to select among compound symmetry, heterogeneous compound symmetry and
unstructured covariances). Under compound symmetry, the covariance between
treatments is assumed to be the same, as is the variance within treatments.
With the above models the data displayed slight deviations from normality and
this was confirmed by Kolmogorov-Smirnov tests (P<0.15); deviations were associated
with heavy tails and/or outliers, and this could not be remedied with transformations. The
departures from normality have the effect of reducing statistical power. In spite of this,
we found that orphan males were significantly heavier at birth compared to their nonorphan counterparts (0.37 kg heavier +/- 0.14 SE, F1 df, 333 ddf =2.62, P=0.009). Female
orphans were also significantly heavier than non-orphans (0.48 kg heavier +/-0.15 SE, F1
df, 317 ddf =
3.18, P=0.002). Therefore, failure to include capture weight as a covariate in
our survival models would lead to an under-estimate of the orphaning effect.
References
Clutton-Brock TH, Guinness FE, Albon SB (1982) Red deer: behavior and ecology of
two sexes. The University of Chicago Press, Chicago