Human disturbances and the daytime activity of sympatric otters along equatorial Amazonian rivers

Abstract

Background Previous studies suggest coexistence between sympatric neotropical (Lontra longicaudis) and giant otters (Pteronura brasiliensis) maybe facilitated by temporal and spatial differences in activity. Yet, to date there has been no systematic evaluation of activity of these species in sympatry. Here we use extensive multi-year field data to compare temporal and spatial patterns in the diurnal activity of sympatric giant and neotropical otters to answer three questions: Do temporal patterns in daytime river use change in relation to seasonal river levels (low, rising, high and declining river levels), do they change due to human disturbances (boats and fishing nets) and do patterns in neotropical otter activity change due to the presence of the larger sized giant otter? Methods Direct observations of both species were recorded using standardized boat surveys along 218 km of rivers over 53 months during nine years (2011–2013 and 2015–2020). Complementary techniques (Generalized Additive Models, Kernel density estimates and non-parametric tests,) were used to compare diurnal activity patterns along rivers subdivided into 41 river reaches. Results The presence of giant otters decreased threefold from 67% of the least disturbed reaches (few boats no fishing nets) to 18% of the most disturbed reaches with many boats and fishing nets. In contrast neotropical otter presence nearly doubled from 44% of the least disturbed to 73% of the most disturbed reaches with fewest giant otter detections. Both species were observed across all daytime hours but were observed rarely on the same day. There was no evidence to suggest simultaneous use of the same reach. When species were detected on the same day, they were separated spatially (median distance between species 12.5 km) and temporally (median time difference 3.0 hours). There was little change in activity of either species among seasons. Giant otters were less active in river reaches with fishing nets and boat use, whereas neotropical otter activity did not appear to be strongly affected by these activities. Conclusions Our findings support evidence that diurnal activity in both otter species is flexible, with daytime activity changing due to human disturbances in the case of giant otters.

Carnivore coexistence without competition: giant otters are more nocturnal around dens than sympatric neotropical otters

“…Overall, neotropical otters tended to leave later in the morning and enter earlier in the afternoon compared with giant otters. A previous study quantifying river based activity found no evidence of direct interactions between giant and neotropical otters ( Norris & Michalski, 2023 ). Indeed, Norris & Michalski (2023) found river use by giant and neotropical otters was separated temporally (median time difference 3.0 h) and spatially (median distance between species 12.5 km).…”

“…A previous study quantifying river based activity found no evidence of direct interactions between giant and neotropical otters ( Norris & Michalski, 2023 ). Indeed, Norris & Michalski (2023) found river use by giant and neotropical otters was separated temporally (median time difference 3.0 h) and spatially (median distance between species 12.5 km). However, results from this earlier study focused only on daytime activity and did not evaluate nocturnal activity of either species.…”

“…Avoiding nocturnal activity in areas where giant otters are active at night could therefore be a mechanism whereby neotropical otters reduce the chances of encounters when they would be less able to safely avoid or flee giant otters. Previous studies along the same rivers found diurnal river use by neotropical otters increased twofold in regions most intensively used by humans ( Norris & Michalski, 2023 ). This finding does not support the hypothesis that neotropical otters reduce diurnal activity in regions with high levels of anthropogenic use.…”

“…It is likely that neotropical otters can obtain food resources in our study area without extending activity into the night. Diurnal activity of neotropical otters in our study area also included times ( e.g ., midday) when giant otters are less active ( Duplaix, 1980 ; Leuchtenberger et al, 2014 ; Norris & Michalski, 2023 ). Thereby reducing temporal overlap with giant otters.…”

“…Commercial fishing is not permitted along this river, which is bordered by closed canopy rainforest and protected areas on both banks and has only six resident families. Both otter species occurred historically along this river and have been monitored since 2011 ( Michalski et al, 2012 , 2021 ; Norris & Michalski, 2023 ). Five dens were monitored simultaneously using camera-traps during the low river season from August to November 2021 ( Supplemental Information S1 , further details of the study area are presented in Michalski et al (2021) ).…”