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.