Behaviors observed at latrines indicated that river otters are indeed using these areas to exchange information. Otters were more likely to come to the latrine and engage in a behavior, although they did, on occasion, travel through the latrine without stopping. When river otters did stop in the latrine, they were most often observed sniffing or standing, supporting the idea that they may be collecting olfactory information about other animals carried through the air (Eisenberg and Kleiman 1972). Wild-caught, captive otters displayed sniffing behavior when investigating experimentally placed feces (Rostain et al. 2004). Patterns in the sniffing behavior indicated that otters could discriminate species, sex, and social familiarity on the basis of feces (Rostain et al. 2004). The sniffing behavior included a description of a slight bobbing of the head and flaring of the nostrils if the animal was facing the camera (Rostain et al. 2004), behavior which we also observed in the latrines.
By definition, latrine sites are associated with feces and other secretions (e.g., urine, anal jelly) used for chemical communication (Eisenberg and Kleiman 1972; Irwin et al. 2004). On the basis of latrine use by other species, it is reasonable that river otters stop at latrines to deposit scat that would, in turn, communicate information to others. Surprisingly, few defecation events were observed during our study. At times it was difficult to discern what specific scent-marking event (defecation, urination, jelly deposition) was occurring on a video because of distance from the camera, the position of the animal, or the position of vegetation that obscured the view. For the same reasons, video was not a reliable source for noting the position of scat once deposited, precluding our ability to determine whether any animals over-marked existing scat.
Previous studies have used ethograms to experimentally assess behavior of river otters captured in the wild and held in captivity (Rostain et al. 2004; Hansen et al. 2009). Although the animals were held in a captive facility rather than in a natural habit, there was some similarity in the behaviors observed in these studies. Rostain et al. (2004) observed sniffing, grooming, wrestling, and scent marking, and Hansen et al. (2009) observed grooming, wrestling, and mounting. The description of defecation provided by Rostain et al. (2004) suggests defecation often is associated with a “raise and flicking of the tail”. We also noted river otters raised or moved their tail during defecation. However, otters in our study typically stomped their hind feet during defecation and the stomping may have caused the observed tail movement. In our study, use of instantaneous scan sampling resulted in separate recording of defecation and stomping events. When ad libitum sampling was used, we found that stomping and defecation were often associated, although once we observed defecation or stomping behavior alone. It is also important to note that it is possible that stomping was actually defecation that was indiscernible on video. Combining stomping and defecation as one behavior group increases the percentage of defecation observations to approximately 5 %. Although this is still a small proportion of overall behaviors observed, it does not rule out latrines as communication stations.
Scent-marking by river otters is known to be facilitated by the deposition of scat, urine, and anal jelly (Mason and Macdonald 1987; Melquist and Hornocker 1983; Oldham and Black 2009; Rostain et al. 2004). However, the otters may also scent mark by foot-stomping and body rubbing relying on glands located in the pads of their feet and ventral region. Eurasian river otters (Lutra lutra Brünnich 1771) scrape sand and vegetation into small piles, probably scent marking them with inter-digital scent glands (Kruuk 2006). Male North American river otters have pedal glands which are believed to function in sexual communication given the sexual dimorphism of the presence of the gland (Kruuk 2006). North American river otters in our study typically combined stomping and defecation but we were unable to determine the sex of the individuals and could not, therefore, assess whether both males and females stomp their hind feed when defecating.
It is unclear from the literature whether North American river otters also have ventral scent glands. Several species of mustelid, including the Eurasian river otter, are known to have both types of gland (Hutchings and White 2000; Kruuk 2006). Body rubbing is a well-established form of scent-marking among mustelids including stoats (Mustela ermine L. 1758), weasels (Mustela nivalis L. 1766), ferrets (Mustela furo), and honey badgers (Mellivora capensis) (Erlinge et al. 1982; Clapperton 1989; Begg et al. 2003). Erlinge et al. (1982) described body rubbing as animals rubbing the belly and front-lateral body regions against vegetation or bare ground. For some species, for example the African dwarf mongoose (Helogale undulata rufula), body rubbing extends to the side of the head to incorporate cheek glands (Rasa 1973). These glands, located between the eye and the ear, deposit a thin secretion on to the substrate (Rasa 1973). Eurasian otters also rub their cheeks on stones as a method of scent communication (Kruuk 2006). We observed river otters rubbing the head, neck, and body over 10 % of the time. Rubbing and rolling of North American river otters has also been described by Melquist and Hornocker (1983) although they attributed this behavior to grooming, to maintain fur insulation. However, animals rubbed and rolled on the grass or bare ground, often in locations associated with fecal marking (Melquist and Hornocker 1983). River otters in our study often combined body rubbing and rolling but often the otters would lower their body to the ground so that the ventral surface was in contact with the substrate. It is possible that this type of rubbing may function in scent marking. Combining body rubbing, defecation, and stomping as a group of olfactory scent marking behaviors makes scent marking the third most common behavior we observed (approximately 16 % of observations) after stand and sniff which composed 21 and 19 % of observations, respectively. Our observed distribution of behaviors supports the hypothesis of olfactory communication at latrine sites.
Chemical signals that persist long after an individual has left the area are important for information exchange among solitary animals. Interestingly, Eurasian river otters in Scotland, which tend to be solitary (Kruuk and Moorhouse 1991) had a preference for sprainting on river banks upwind from the river itself, indicating the importance of feces in intraspecific communication (Kruuk 2014). Although river otters are regarded as more social than most mustelids (Liers 1951; Melquist and Hornocker 1983), group size varies. Average mean group size in California estuaries was 1.6 (range 1.0–2.7) otters with maximum group sizes ranging from 1 to 7 animals (Brzeski et al. 2013) whereas otters in marine coastal Scotland tend to be solitary (Kruuk and Moorhouse 1991). Substantial variation is observed for North American river otters in Alaska, with reports of groups containing 5–18 individuals (Testa et al. 1994) and a mean minimum group size of 1 or 2 otters (Blundell et al. 2002). In Iowa, where habitat is similar to Illinois, adult and yearling river otter males are typically solitary as, also, are females (Melquist and Hornocker 1983). Females may be found in social family groups consisting of the female and her offspring, although occasionally a family group may be accompanied by an unrelated individual (Melquist and Hornocker 1983). In Illinois, visits by solitary animals occurred more often than visits by groups of animals. River otters spent more time in the latrine when visiting as a group. By using camera traps we were unable to reliably discern river otter age, so we cannot discount the possibility that group latrine visits were more likely to include juveniles engaging in social behaviors, such as play. Animals may spend less time in latrines as they age, leading to changing behavior preferences at latrine sites over the life of an individual.
Our finding that most visits were solitary agrees with previous studies in Pennsylvania and Maryland, where 59 % of visits were by single river otters (Stevens and Serfass 2008). However, a study in Idaho reported only 44 % of visits by single otters (Melquist and Hornocker 1983). It has been hypothesized that visits by solitary otters is a method of communicating fine scale resource availability (Kruuk 1992, 2006). Eurasian river otters use spraint to advertise whether they have fished in a particular site. By doing so, the second otter to attempt to use the same site gathers information on whether the area is likely to be depleted (Kruuk 1992, 2006).
Because most visits were by solitary animals, it was not surprising that solitary behaviors were observed more often than group behaviors. For example, self-grooming was observed more often than allogrooming, consistent with reports of free-ranging river otters in Idaho (Melquist and Hornocker 1983). Use of camera traps limited our ability to definitively differentiate between mock or play mating and true copulation. As a result, we combined these behaviors into a single mounting category. Observation frequency of mating behavior was low but consistent with low frequency of copulation observed among Idaho river otters (Melquist and Hornocker 1983). River otters copulate on land and in water (Liers 1951), but given the rarity of observations at the latrine site, if they copulate on land, they use terrestrial locations other than latrines.
Although river otters have a reputation for being playful, play seems less prevalent among free-ranging otters than among captive otters (Melquist and Hornocker 1983). Wrestling has been regarded as play behavior, and even though play behavior typically occurs among juveniles, wrestling play has been observed among adult river otters also (Beckel 1991). Play behavior among Idaho river otters generally consisted of wrestling among juveniles and was only observed 6 % of the time. Although we could not determine age, wrestling comprised 3.4 % of all behavior events (based on frequency) at latrine sites, supporting the idea that free-ranging river otters may play less than captive otters. Furthermore, play is likely to occur throughout a river otter’s range, not exclusively at latrines. It is important to note that solitary animals do not wrestle, and if we limit the observations to social behaviors (i.e., allogroom, wrestle, mount), wrestling comprised 44.4 % of social observations. Wrestling has been observed among captive, wild-caught otters (Hansen et al. 2009); this behavior was specifically described as play by Rostain et al. (2004), given the lack of aggression. Allogrooming, social behavior also observed in latrines, was observed for the same otters (Rostain et al. 2004; Hansen et al. 2009).
Our study revealed seasonal and monthly variation in patterns of visitation rates similar, but not identical, to patterns observed across North America. River otters in Illinois most often visited latrines in the winter, specifically in December and January, more so than any other time of the year. In Idaho, river otters were most active in February with activity becoming progressively less in spring, summer, and fall (Melquist and Hornocker 1983). In Pennsylvania and Maryland, latrine visits peaked in February and March (Stevens and Serfass 2008), slightly later than in our study. Stevens and Serfass (2008) hypothesized a connection between visitation rate and breeding season. Although the exact breeding season of river otters in Stevens and Serfass’ (2008) study range was unknown, the breeding season is known to occur in March and April in Maryland (Mowbray et al. 1979) and New York (Hamilton and Eadie 1964). If the breeding season occurs in March and April, the peak in the visitation rate observed by Stevens and Serfass (2008) occurred just before the breeding season, during the time male river otters would be searching for mates. If visitation rates are a reliable indicator of breeding season, the results of our study are indicative of an earlier breeding season in Illinois than in other locations. Breeding season, specifically time of parturition, is likely to vary depending on geographic location (Harris 1968). However, further research on other latrines and years of observation are needed to determine the robustness of the visiting patterns observed to date.
In general, river otters tend to be more active at night or during crepuscular hours than during the day (Melquist and Hornocker 1983). Our data support this, with a distinct pattern of nocturnal activity in the latrines although the duration of visits was the same whether the animal visited the latrine at night or during the day. Several factors can affect daily activity patterns, including predator and prey activity (Stevens and Serfass 2008). It has also been suggested that humans may pose the biggest threat to river otters and that human presence may force them to be more active at night (Melquist and Hornocker 1983; Stevens and Serfass 2008). Our study site was located on private property where landowners were active, potentially pressuring the local otters to be more active at night.
In conclusion, our behavioral assessment supports the hypothesis that latrines are used for olfactory communication among river otters although the content of the messages is still unknown. The most common behaviors were standing and sniffing, which indicates that river otters gather information about conspecifics via olfactory cues during latrine visits. Although we observed low frequency of defecation events, body rubbing was common. It is possible that the otters use body rubbing to deposit scents at latrines in addition to depositing feces, urine, and anal jelly. However, the lack of defecation events raises concerns about the accuracy of the scat surveys used to assess river otter populations. Although latrines may be used to communicate information between individuals and groups, the fact that latrines were most often visited by solitary otters indicated that social activity at the latrine site is not likely to be a major attraction for river otter visits. We found river otters in Illinois to be more active at latrines at night than during the day, a pattern that might be explained by human activity. We also observed a peak in latrine visitation rate during the winter months of December and January, potentially indicating an earlier breeding season in Illinois. A great deal of information remains unknown regarding river otter latrine function and use. By studying additional latrine sites for additional years we may begin to understand the patterns of behavior of river otters in Illinois and provide the necessary baseline information for testing behavioral hypotheses pertaining to latrine function.