| |
Abstract |
Hormones are a link between environmental stimuli and the behavioural and/or
physiological responses of organisms. The release of corticosterone (hereafter CORT) has
major effects on both energy mobilization and its allocation among the various requirements
of an individual. However, specific effects of CORT on behaviours related to energy
acquisition and, consequently, its allocation with regards to survival and reproduction, are
complex and remain to be deciphered at a fine scale. We examined here the effects of elevated
baseline CORT levels on this trade-off through a detailed study of foraging behaviour in the
Adélie penguin (Pygoscelis adeliae) during the chick-rearing stage. We monitored the at-sea
behaviour of CORT-implanted and control male birds using time-depth recorders, and
monitored the effects of CORT treatment on their body conditions, as well as their chicks’
body masses and survival. Bio-logged data were examined via both traditional measures of
diving behaviour and fractal analysis as an index of behavioural complexity. At the level of
the foraging trip, CORT administration led to a transient decrease in overall foraging effort
(i.e. reductions in the duration of at-sea trips, the time spent diving and the number of dives
performed) as well as the complexity with which treated birds foraged. In contrast, per-dive
performance indices suggest an increase in both efficiency and prey pursuit rates. These
contrasting results suggest the importance of assessing diving behaviour at scales
encompassing both entire foraging trips, as well as individual dives. Ultimately, however, we
observed no short-term effects of treatment on either adult body condition or on chick body
mass and survival. We conclude that under conditions of energy constraint, as mimicked by
our experimental paradigm, sequences of behaviour may become more structured and
periodic as observed in CORT-treated birds. The increased dive-scale effort observed in
CORT-birds could then be a strategy allowing a reduction in trip-scale effort. Indeed, the
increased CORT levels may have forced treated penguins to revert to mechanisms preserving
their survival by engaging in less costly foraging behaviours. This study highlights the utility
of using both traditional and fractal analyses to better understand the responses of free-living
animals to energetic challenges and various environmental constraints. |
|
