Ja. Fargallo et al., Nest-maintenance effort and health status in chinstrap penguins, Pygoscelis antarctica: the functional significance of stone-provisioning behaviour, BEHAV ECO S, 50(2), 2001, pp. 141-150
Stone provisioning is a nest-maintenance activity accomplished by pygosceli
d penguins after reliefs during the incubation/brooding period. The functio
nal significance of this behaviour has been mainly explained as a parental
strategy preventing nest flooding under detrimental weather conditions. In
addition, and in the light of recent studies, this behaviour could also fit
into the sexual selection process. In this study, we tested the first idea
, that is, whether stone provisioning is a nest-maintenance behaviour to in
crease egg/nestling survival by lowering the risk of nest flooding, and can
thus be considered a form of parental care. Additionally, we investigated
if the effort invested by parents in nest maintenance is constrained by phy
siologically limiting resources. The effort of stone collection and the per
ceived risk of nest flooding were experimentally manipulated during the inc
ubation and early brooding phases in a chinstrap penguin, Pygoscelis antarc
tica, colony. Three groups of nests were established. After weighing, contr
ol nests were left unmanipulated. In a second group of nests (reduced group
), only one-half of the initial weight of stones was returned to the nests.
In a third group of nests (snow-added group), we both reduced nest weight
by a half and added snow outside the nest bowl over 6 consecutive days. Ten
days after manipulation, the difference in nest weight between initial and
final conditions was significantly related to treatment: penguins increase
d stone provisioning in the reduced group (44% of half-reduced nests), but
drastically more in the reduced and snow-added group (123% of half-reduced
nests), while the weight of control nests was unchanged compared to premani
pulation conditions. The intensity of stone provisioning was affected by ne
st date, peaking about hatching time and shortly after, and declining with
advancing chick age. These results suggest that stone provisioning is a mec
hanism that has evolved to prevent egg or chick mortality by nest flooding.
The haematocrit, but not leukocyte numbers as expressed by the buffy coat
layer, varied with the experimental conditions. Penguins investing more tim
e in nest maintenance had a lower haematocrit, suggesting a physiological t
rade-off probably mediated by competition between the time devoted to nest
maintenance versus foraging activities. The amount of stones collected and
the haematocrit were positively related to the number of neighbour nests, s
o those individuals surrounded by more nests seemed to obtain benefits in t
he availability of nest material and energy savings. This study indicates t
hat stone-provisioning behaviour is a nest-maintenance activity evolved to
improve thermal nest characteristics potentially increasing offspring survi
val, and competing in time and energy with other reproductive activities. S
tone provisioning in penguins should therefore be regarded as a form of par
ental care and an important part of individual reproductive effort in speci
es breeding in harsh environments. Furthermore, nest size and nest-maintena
nce effort should be considered reproductive traits indicative of parental
quality and thus could also be involved in the post-mating sexual selection
process.