WildLife Group
of the SAVA
hypothalamic-pituitary-adrenal axis that leads to the
release of glucocorticoids, and (2) the modulation
of the autonomic nervous system that involves a
decrease of the parasympathetic- and an increase of
the sympathetic tone (Boonstra, 2004).
To date, the primarily used indicator for stress in
wildlife and zoo animal research are glucocorticoid
levels. There are a number of sources from which
glucocorticoids can be measured such as blood, saliva,
faeces, urine, or integumentary structures; but each
of these sources carries with it a number of caveats
(Möstl and Palme, 2002; Sheriff et al., 2011; Dantzer
et al., 2014). For instance, individual and sex-specific
variation, storage conditions and uneven distribution
of metabolites affect final results when measuring
glucocorticoids from faeces (Hadinger et al., 2015).
In human medicine and farm and companion-animal
research, heart rate variability (HRV) has been used
increasingly over the past decades to evaluate
physical and psychological stress (Cardiology, 1996;
von Borell et al., 2007). Analysis of HRV investigates
the functioning of the autonomic nervous system of
an individual. In a healthy heart, both branches of the
autonomic nervous system are tonically active with a
dominance of the vagal tone (von Borell et al., 2007;
Porges, 2007; Pumprla et al., 2002.
This activity creates irregular time intervals between
consecutive heart beats. Measuring HRV quantifies
these time intervals and, thus provides information
about the balance between sympathetic and
parasympathetic activity (Cardiology, 1996; von Borell
et al., 2007) -- a high HRV indicates higher capacity of
an individual to cope with environmental demands,
whereas a low HRV is an indicator for stress (von
Borell et al.; 2007, Thayer and Sternberg, 2006). HRV
is a new approach in wildlife research to investigate
stress, and only few previous studies have applied it.
For example, HRV has been applied in brown bears to
identify the drivers of hibernation (Evans et al., 2016)
and to assess stress associated with human activity
(Støen et al., 2015). In European roe deer, HRV has
been applied to investigate seasonal changes (Theil et
al., 2004) and to analyse stress control during physical
24
capture (Mentaberre et al., 2010).
In a recent project, we applied HRV in free-ranging
Przewalski’s horses to assess seasonal stress. With a
self-constructed telemetry system consisting of an
implanted transmitter and a storage unit located on a
collar, we were able to obtain valuable data (including
cardiac activity and other physiological parameters)
over an extended period of time without disturbing
the horses. Heart rate and HRV varied throughout the
year, with lower values during the winter than during
the summer. In spring, heart rate peaked along with
a drop in HRV indicating increased allostatic load. In
summary, we found that HRV is a probate method to
study undisturbed reactions of wild animals to their
changing environment over the long term.
As current wildlife research focuses on the placement
of the whole organism in its natural setting, biologgers
are used more and more. Thus, when collecting
physiological data, we recommend to measure heart
rate on a beat-to-beat basis to allow the assessment
of HRV. The combined measurement of HRV as a
stress parameter and heart rate as an indicator
for energy expenditure enables the valuation of
allostatic load and facilitates the determination of an
individual’s regulatory range and adaptive capacity.
HRV represents a promising tool to understand how
animals integrate their life cycles in an ever-changing
environment and to identify the aspects of human
influence that cause stress.
References
Boonstra R.2004.Coping with changing northern
environments: the role of the stress axis in birds and
mammals. Int. Comp. Biol. 44: 95-108.
CARDIOLOGY, T.F.O.T.E.S.O. 1996. Heart rate variability
standards of measurement, physiological interpretation, and
clinical use. Eur. Heart J. 17: 354-381.
Dantzer B, Fletcher QE, Boonstra R, Sheriff MJ. 2014.
Measures of physiological stress: a transparent or opaque
window into the status, management and conservation of
species? Cons. Physiol. 2: cou023.
Evans A, Singh N, Friebe A, Arnemo JM, Laske T, Fröbert
O, Swenson JE, Blanc S. 2016. Drivers of hibernation in the