1 INTRODUCTION
Animal communication refers to the process by which an individual sends
signals to other individuals and the individual receiving the signals
responds to them. Chemical communication with characteristics such as
specificity and delay is considered to be one of the main modes of
communication in most mammals (Brown & Macdonald, 1985), as it is
independent of light, can propagate even in dark conditions, and can
still propagate when the signal releaser is away from the chemical
signal (Wyatt, 2014). Scent marking, the deposition of signals by
individuals on objects in the environment, is a common form of chemical
signal in mammals (Gosling & Roberts, 2001). The main vehicle for
transmitting information in mammalian chemical communication is the
chemical pheromone, which includes many complex information, such as
individual identity characteristics, sex, age, reproductive status,
social status and kinship (Brennan & Kendrick, 2006; Ferrero &
Liberles, 2010; Johansson & Jones, 2007; Jojola et al., 2012; Kean et
al., 2011).
”Signal detection theory” predicts that animals select scent-labeled
signal deposit substrates, resulting in a wider range and longer
retention of the released signal (Alberts, 1992). A growing number of
studies have demonstrated that this choice of signal deposit substrate
is more widespread in mammalian chemotaxis than we thought, such as the
spotted hyena (Crocuta crocuta ) (Gorman, 1990) and the African
wild dog (Lycaon pictus ) (Claase et al., 2022), which use urine
and feces, among other substances, for chemical communication. However,
we also need to be aware that both the chemical signal generation and
the marking process are very energy-consuming (Gosling et al., 2000);
and that after scent marking, the animal needs to be visited
periodically to observe and update the signal markers in order to
maintain the continued validity of the signal, a process that also
requires a significant investment of time and energy (Clapham et al.,
2014; Roberts & Gosling, 2001). The economic constraints associated
with travel and time costs of chemical signals deployment across an
animal’s entire home range preclude rangewide scent signal saturation,
forcing animals to be more strategic about selecting scent deposition
sites.
Giant pandas (Ailuropoda melanoleuca ) are typically solitary
mammals that rarely come into direct contact with other individuals
except for the rutting season when they form gatherings (Nie et al.,
2012a; Schaller, 1985), and coupled with their long-term life in dense
bamboo forests, which has led to a relative degradation of vision (Wei
et al., 2015), information exchange between individuals in the wild
relies mainly on olfaction and hearing (Hu et al., 1985). Auditory
communication refers to communication between individuals through
acoustic signals and generally occurs during the mating aggregation
period in the breeding season (Charlton et al., 2009) or during casual
contact between individuals in the non-breeding season. Scent
marker-based chemical communication thus becomes a major mode of
communication for wild giant pandas (Swaisgood et al., 1999; Wei et al.,
2015). Giant pandas communicate chemically mainly by tagging anogenital
gland secretion (AGS) and urine to transmit individual information,
using the chemicals stored therein for inter-individual signal exchange
(Hu et al., 1985).
However, the cost of generating chemical signals is too high for an
energetically marginal species like the giant panda, which, unlike other
bear species, uses urine and AGS as chemical communication signals
rather than feces (Pan et al., 2001). Both urine and feces are metabolic
by-products and therefore consume less energy, whereas AGS is produced
by specialized glands and has a high fat content (Hagey & Macdonald,
2003), further exacerbating energy expenditure.
Up to the present study, we found that giant pandas have a greater
number of scent markers at the ridge and will select the appropriate
substrate material depending on the marks (Nie et al., 2012a). However,
it is not rigorous to equate the number of markers with selection
preference. Because pandas mark more in a certain place may be because
they spend more time in that place, but not because they prefer to mark
there. We propose, for the first time, that there is a need to
demonstrate that giant pandas mark scent more frequently than expected
at a given location as an indication of selective preference. The
easiest way to do this is to collect feces during the sample line survey
and use fecal density to indicate the intensity of use of the site by
pandas; since pandas defecate nearly 50 times per day and do not use
feces for communication (Nie et al., 2012a), their choice of defecation
location is random, depending only on where they are located when they
want to defecate, so this method is rigorous.
We have innovated on previous studies by proposing for the first time to
use panda feces counts to laterally reflect the intensity of habitat use
as a way to assess the selection preferences of giant pandas when
scent-marking. This study allows us to investigate whether the scent
marking behavior of pandas is consistent with signal detection theory;
and whether pandas go out of their way to deposit markers in places
where they are most likely to be found. Based on the results of the
study, we can protect the habitats that pandas prefer to use with
precision and enhance communication among individual pandas and their
populations.