Coastal latrine sites as social information hubs and drivers of river otter fission–fusion dynamics

1. What they did

River otters on the coast of Alaska have a flexible social life. Males spend about half their time in groups of up to 18, which hunt schooling fish together, while females are mostly solitary. Otters regularly visit latrines on the shore, where they leave urine, scats and anal gland secretions. The researchers asked whether these latrines help otters decide when to join or leave a group (fission–fusion dynamics).

• Finding latrines. They surveyed the coastline for active latrines, judged by fresh scats, in Kenai Fjords National Park (2012) and Prince William Sound (2014).
• Camera traps (2014). In Prince William Sound they placed 36 cameras, mostly video, at latrines found active in a survey of 122 known sites. Each camera faced the main latrine entrance or a narrow passage on an otter trail, for 3 months. They counted visits, group size, social behaviour (grooming, touching noses, rubbing bodies, playing and wrestling) and signalling behaviour (sniffing air or ground intensively, displaying before scent marking, defecating and urinating).
• Describing the latrines. Each latrine was classed as a crossover site or a piazza-style site, and given a measure of how central it was in the coastal network.
• Proximity tags (2012 and 2014). They caught 21 adult otters (14 males and 7 females) with foot-hold traps at active latrines and glued a proximity tag to the fur on each otter’s back. Tags recorded when tagged otters were close to each other. Base units at 10 latrines in Kenai Fjords and 41 in Prince William Sound recorded tagged otters visiting.
• Analysis. They tested whether latrine type and centrality affected visits, group size and social behaviour, whether group splits and joins happened close in time to latrine visits (compared with randomly timed visits), and whether otters that shared more coastline also spent more time together.

2. What they found

Camera traps

• In 2354 camera days, they recorded 194 otter visits to latrines. Visits per latrine ranged from 0 to 23.
• Group size ranged from 1 to 18 (mean 3.2, median 2). 36% of visits were by single otters, and only 4% by groups of more than 10.
• In five visits, adults were accompanied by one to three pups.
• Otters signalled more often (53% of visits) than they socialized (26%).
• At crossover latrines, signalling was especially common (65% of visits, versus 48% at other latrines) and social behaviour less common (12%, versus 32% at piazza-style latrines).
• Otters interacting socially were on average 6.9 m apart at most; in a quarter of cases they interacted after moving 8 m or more apart.

What shaped visits and behaviour

• Crossover latrines and more central latrines received more visits.
• Groups were larger at crossover latrines. The authors report that group size declined over the summer, although in their best model this effect was not statistically significant.
• Social behaviour was more likely in larger groups, and largely at more central latrines (marginal support).
• There was no significant spatial autocorrelation in visits, group size or social behaviour.

Proximity tags

• Tags recorded 5370 detections among 11 males over 144 days, collapsed into 56 encounters lasting on average 8.5 hours, and 140 events of otters joining or leaving groups.
• None of the 7 tagged females was detected with any other otter, and females made only 10 of the 215 latrine visits recorded.
• Tagged males spent between 1% and 59% of their time with other tagged otters, so some were mostly solitary.
• Joining or leaving a group happened on average 10 hours from a latrine visit, significantly sooner than in the random model (16 hours).
• In Prince William Sound, males that shared more coastline spent more time together; in Kenai Fjords the relationship was not significant.

3. What they concluded

• The river otter social system is mostly male-driven. When social, otters usually gather in small to medium groups (2 to 8), and rarely in large ones.
• Latrines have several roles: general exchange of information on the presence of other otters and on food; fine-scale information on individuals, possibly guiding decisions to join or leave groups; meeting places for small social units (especially crossover latrines); and hubs of social activity (especially piazza-style latrines).
• Because otters signalled more than they socialized, and group changes came close in time to latrine visits, the authors suggest this flexible society is held together largely through scent communication at latrines.
• Space use and social life are interwoven, although cause and effect cannot be separated.
• Larger groups early in the summer may reflect behaviour after the breeding season, or the abundance of schooling fish in June and July.
• Limitations: few individuals (no tagged females associated with others); signals may have been lost when otters were under water; tags reliably detected each other only within 8 m, while video showed interactions up to 16 m apart; and not all otters in the population were tagged (about 22% in Prince William Sound and 40% in Kenai Fjords).

4. How tracking was or was not used

Tracks and signs were used to choose study sites, not as the main data. The researchers found active latrines by surveying the coastline for fresh scats, and chose trapping and base unit sites from scat surveys. Latrines were classed by the layout of the otters’ trails (trails crossing land between two bodies of water, or trails leading into one open space), and some cameras faced a narrow passage on an otter trail.

5. Tracker or observer certification

Not reported. The paper does not say who carried out the scat surveys or judged latrine activity.

6. Other technologies, skills or innovations

• An automated proximity tracking system (Encounternet): tags record other tags nearby, base units at latrines download the records, and a master node controls the system.
• Tags adapted to life in water: an outer layer of toughened epoxy, a middle layer of fibreglass and a light core of epoxy and glass microspheres, so the antenna would not de-tune. The tags weighed about 30 g, were glued to the fur and were expected to fall off within 2 to 3 months, during or before the autumn moult.
• Because the otters could not be recaptured, all encounters were read from the base units at latrines.
• Video camera traps mounted about 25 cm above the ground, facing latrine entrances or trails.
• A spatial network of the coastline, used to measure how central each latrine was.

7. Implications for conservation (from the paper)

The paper does not discuss conservation directly. The authors suggest that coastal river otters are a good system for exploring how chemical communication relates to social life, and that knowing how central a site is can help explain what drives animal social structure.