Introduction
Animals adopt behavioral strategies to meet their biological needs and adjust their activity cycles. While some mammals confine their activities to specific times of day, others are cathemeral, exhibiting peaks of activity both during the day and night. This allows them to adapt efficiently to their habitats, optimizing foraging and minimizing predation risks (Charles-Dominique Reference Charles-Dominique, Luckett and Szalay1975, Donati and Borgognini-Tarli Reference Donati and Borgognini–Tarli2006, Donati et al. Reference Donati, Bollen, Borgognini-Tarli and Ganzhorn2007, Castro-Sa et al. Reference Castro-Sa, Dias-Silva and Barnett2021).
Cathemeral activity is observed in various mammals, including lemurs (Eulemur: Colquhoun Reference Colquhoun1998, Simons and Rumpler 1988, Hapalemur: Curtis et al. Reference Curtis, Zaramody and Martin1999, Donati et al. Reference Donati, Baldi, Morelli, Ganzhorn and Borgognini-Tarli2009, Varecia variegata: Wright Reference Wright1999), night monkeys (Aotus: Fernandez-Duque Reference Fernandez-Duque2003), orangutans (Pongo: Hoare Reference Hoare, Holmes, Hugh-Jones and Taylor2020), howler monkeys (Alouatta: Dahl and Hemingway Reference Dahl and Hemingway1988), sloths (Bradypus variegatus: Castro-Sá et al. Reference Castro-Sa, Dias-Silva and Barnett2021), African elephants (Loxodonta africana: Gunn et al. Reference Gunn, Hawkins, Barnes, Mofulu, Grant and Norton2013, Fison Reference Fison, Holmes, Hugh-Jones and Taylor2020), black rhinoceros (Diceros bicornis), giraffes (Giraffa: Fison Reference Fison, Holmes, Hugh-Jones and Taylor2020), and some carnivores, including fossa (Cryptoprocta ferox: Colquhoun Reference Colquhoun1998), crab-eating fox (Cerdocyon thous), jaguar (Panthera onca), puma (Puma concolor), ocelot (Leopardus pardalis), and cheetah (Acinonyx jubatus) (Arrais Reference Arrais2019, Fison Reference Fison, Holmes, Hugh-Jones and Taylor2020). This behavior has also been noted in rodents, for example capybara (Hydrochoerus hydrochaeris: Arrais Reference Arrais2019), and cetaceans, such as Amazon river dolphin (Inia geoffrensis: Erbs et al. Reference Erbs, Gaona, van der Schaar, Zaugg, Ramalho, Houser and André2023), and Ganges river dolphin (Platanista gangetica: Mann and Karniski Reference Mann and Karniski2017), and potentially in other manatee species, like West Indian manatee (Trichechus manatus), Amazonian manatee (Trichechus inunguis), and West African manatee (Trichechus senegalensis) (Holguin-Medina et al. Reference Holguin-Medina, Fontenele-Araujo, Alcaraz-Romero, Cortes and Muñoz-Delgado2015, Mukhametov et al. Reference Mukhametov, Lyamin, Chetyrbok, Vassilyev and Diaz1992, Powell Reference Powell2002).
Studies indicate that manatees may exhibit cathemeral behavior with sleep patterns similar to other aquatic mammals (Mukhametov et al. Reference Mukhametov, Lyamin, Chetyrbok, Vassilyev and Diaz1992, Holguin-Medina et al. Reference Holguin-Medina, Fontenele-Araujo, Alcaraz-Romero, Cortes and Muñoz-Delgado2015). Polysomnographic analysis of an Amazonian manatee confirmed a circadian sleep-wake rhythm (Mukhametov et al. Reference Mukhametov, Lyamin, Chetyrbok, Vassilyev and Diaz1992), though feeding activity was not studied. Further studies exploring photoperiod effects on manatee activity, rest, movement, and social interactions suggest variability influenced primarily by food availability (Hartman Reference Hartman1979, Best Reference Best1981, Bernier Reference Bernier1985, Reynolds and Odell Reference Reynolds and Odell1991, Morales-López et al. Reference Morales-López, Olivera-Gómez and Zenteno-Ruiz2012, Mukhametov et al. Reference Mukhametov, Lyamin, Chetyrbok, Vassilyev and Diaz1992). Amazonian manatees are known to exhibit flexible foraging behaviors, often influenced by seasonal changes in their habitat. Traditional knowledge and scientific studies have documented their feeding on various plant species, including those in the Polygonaceae family, such as maracarãna (Coccoloba densifrons), during nocturnal hours (Crema Reference Crema2017, Crema et al. Reference Crema, da Silva and Piedade2019b, Amaral et al. Reference Amaral, Marmontel, Souza, Carvalho, Valdevino, Guterres-Pazin, Mello, Lima, Chávez-Pérez and da Silva2023).
Reports from riverside communities in Costa Rica and Brazil indicate crepuscular and nocturnal habits in West Indian manatee and Amazonian manatees, respectively (Jiménez-Pérez Reference Jiménez-Pérez1999, Crema et al. Reference Crema, da Silva, Botta, Trumbore and Piedade2019). Amazonian manatees are known for being active and feeding in shallow areas during the night (Kikuchi et al. Reference Kikuchi, Da Silva, Rosas and Miyazaki2010, Reference Kikuchi, Akamatsu, Gonzalez-Socoloske, de Souza, Olivera-Gomez and Da Silva2014, Crema et al. Reference Crema, da Silva and Piedade2019b, Arraut et al. Reference Arraut, Marmontel, Mantovani, Novo, Macdonald and Kenward2010) on members of the Polygonaceae family, such as erva-de-bicho (Persicaria spectabilis) (Colares and Colares Reference Colares and Colares2002), aruera (Symmeria paniculata) (Guterres-Pazin et al. Reference Guterres-Pazin, Marmontel, Rosas, Pazin and Venticinque2014), and maracarãna (Crema et al. Reference Crema, da Silva and Piedade2019b). Although these observations are interesting, the animals have never been recorded feeding during the night in the wild. Despite progress, nocturnal observations of manatees require further validation. Here, we report Amazonian manatee nocturnal activity using thermal cameras, contributing to the broader understanding of their ecological behavior.
Materials and methods
Observation area of the Amazonian manatee
Observations of the Amazonian manatee were conducted in the intensive use zone of Anavilhanas National Park (ANP), near Novo Airão, approximately 60 km from Manaus, Amazonas, Brazil. ANP spans approximately 647,610 hectares between latitudes 2°00’ to 3°02’S and longitudes 60°27’ to 61°07’W (ICMBio 2017). This area primarily comprises seasonal floodplain islands (igapó) and includes parts of the Northern Sector of the Rio Negro State Park and the Environmental Protection Area on the left bank of the Rio Negro in the Aturiá-Apuauzinho Sector. The archipelago consists of 191 islands, various lagoons, and lakes, extending 90 km in length and up to 12.5 km wide. The non-flooded terra firme forest covers the rest of the park. Observations took place on Papagaio Island (2°46’14”S, 60°34’46”W) (Fig. S1).
Observations were conducted during a period of tropical rainy climate (Af, Köppen scale). In September 2023, precipitation in Manaus was 44.1 mm, 55% of the average 79.0 mm (1991–2020) (INMET 2023). The average temperature was 27.2°C, with a peak of 36.9°C (extremes of 23.8°C and 39.3°C, the highest in 33 years) (INMET 2023). The Rio Negro’s water level was 22.22 meters, decreasing by 20–22 cm daily compared to 25.45 meters in 2022, with a decrease of 10–12 cm daily (INMET 2023).
Guided by experienced local residents from the Monte Sinai Community on the left bank of the Rio Negro (2°44’05”S, 60°34’19”W), this community-based tourism activity complied with the environmental conduct and protection norms set by the park Management Plan (ICMBio 2017). Since the activity was touristic and did not involve scientific data collection or direct contact with the animals, no special research permits were required, and the observations reported here fell within the legal and regulatory parameters established for ANP’s management (ICMBio 2017).
The habitat of the Amazonian manatee
The ecosystem of Anavilhanas National Park (ANP), an archipelago of blackwater rivers and dense seasonally flooded tropical forest known as igapó, is characterized by plant species adapted to specific conditions, extracting nutrients and growing during the terrestrial phase (Parolin et al. Reference Parolin, De Simone, Haase, Waldhof, Rottenberger, Kuhn and Kesselmeier2004, Piedade et al. Reference Piedade, Junk, Adis and Parolin2005). In the Rio Negro, water transparency varies between 0.9 and 1.3 metres, with temperatures from 28.3 to 31°C due to the dark water colour and heat retention. Surface dissolved oxygen averages 6.2–6.3 mg/L, decreasing in deeper layers, with pH from 3.2 to 4.7 and conductivity from 6.0 to 13.0 µS/cm. Suspended materials range from 0.9 to 14.8 mg/L. In ANP lakes, water clarity ranges from 1.5 to 3.5 metres, with surface temperatures of 28.1 to 31°C and bottom temperatures of 26.2 to 29.6°C, showing thermal stratification in some lakes.
Vegetation characteristics
ANP hosts diverse ecosystems including Dense Lowland Rainforest, Alluvial Dense Rainforest, White-sand scrubland (Campinarana), and river/lake-influenced vegetation. Igapó vegetation is characterized by low biomass and diversity. Common species in flooded areas include capim-de-arroz (Oryza glumaepatula), guamirim-da-várzea (Eugenia inundata), aruera, maracarãna, and macacarecuia (Eschweilera tenuifolia) (Piedade et al. Reference Piedade, Junk, Adis and Parolin2005). Tovomita sp., cipó-de-leite (Malouetia furfuracea), and maracarãna are adapted to extended inundation periods. Flowering and fruiting synchronize with floods to aid propagule dispersion (Piedade et al. Reference Piedade, Junk, Adis and Parolin2005). Approximately 86% of igapó tree species synchronize leaf production (Haugaasen and Peres Reference Haugaasen and Peres2005, Ferreira and Parolin Reference Ferreira and Parolin2007, Parolin and Wittmann Reference Parolin, Wittmann, Junk, Piedade, Wittmann, Schöngart and Parolin2010). Leaf shedding during floods affects 80% of igapó plant species (Revilla Reference Revilla1981, Haugaasen and Peres Reference Haugaasen and Peres2005, Ferreira and Parolin Reference Ferreira and Parolin2007, Parolin and Wittmann Reference Parolin, Wittmann, Junk, Piedade, Wittmann, Schöngart and Parolin2010), while 20% continue producing leaves year-round, with mature leaves having distinct nutritional compositions (Haugaasen and Peres Reference Haugaasen and Peres2005, Ferreira and Parolin Reference Ferreira and Parolin2007, Parolin and Wittmann Reference Parolin, Wittmann, Junk, Piedade, Wittmann, Schöngart and Parolin2010).
Observed species
The Amazonian manatee is the smallest manatee species (2.75 m; 420 kg), and the only freshwater sirenian (Attademo et al. Reference Attademo, Gomes, Silva, Modeira, Freire, Leitão and Luna2020). It is a docile species endemic to the Amazon region. In the Brazilian Amazon, they are widely distributed, inhabiting the Negro, Solimões, Japurá, Juruá, Branco, Tocantins, and Tapajós river basins (Amaral et al. Reference Amaral, Marmontel, Souza, Carvalho, Valdevino, Guterres-Pazin, Mello, Lima, Chávez-Pérez and da Silva2023). It has a greyish dorsum and light gray to pink ventral spots, and it lacks nails on its pectoral flippers. Its caudal fin is long, rounded, and dorsoventrally flattened, connected to the body by a robust and muscular caudal peduncle (Associação Amigos do Peixe-Boi 2025). Manatees communicate vocally, with vocalization rates varying by age and social behavior (Sousa-Lima et al. Reference Sousa-Lima, Paglia and Da Fonseca2002, Dantas Reference Dantas2009).
The Amazonian manatee is herbivorous with high digestive efficiency, consuming a variety of aquatic and semi-aquatic plants, submerged roots, leaves, fruits from riverside trees, and occasionally small invertebrates (Best Reference Best1981, Colares and Colares Reference Colares and Colares2002, Franzini et al. Reference Franzini, Castelblanco-Martinez, Rosas and da Silva2013, Guterres-Pazin et al. Reference Guterres-Pazin, Marmontel, Rosas, Pazin and Venticinque2014, Crema et al. Reference Crema, da Silva, Botta, Trumbore and Piedade2019, Tófoli Reference Tófoli2012, Carvalho et al. Reference Carvalho, Marmontel, Botta, Emin-Lima, Costa, Crema, da Silva, Piedade and Secchi2022, Crema Reference Crema2017). Their known diet includes over 60 plant species, mainly Poaceae and Azollaceae. At Anavilhanas, key dietary plants are batatarana (Ipomoea alba and Mikania micranta), cipó-icica (Tassadia berteriana), cipó-de-leite (Mesechites trifida), and maracarãna (Crema, da Silva and Piedade 2020). C4 plants are significant in the diet across age classes, especially in várzea forests, while C3 aquatic plants are primary food sources in flooded areas (Crema et al. Reference Crema, da Silva, Botta, Trumbore and Piedade2019a, Reference Crema, da Silva and Piedade2019b, Carvalho et al. Reference Carvalho, Marmontel, Botta, Emin-Lima, Costa, Crema, da Silva, Piedade and Secchi2022). Isotopic variations suggest different feeding preferences and habitat use, especially in igapó (Crema et al. Reference Crema, da Silva, Botta, Trumbore and Piedade2019a). However, no diet studies have been conducted explicitly during nighttime.
Data collection
During a touristic visit with friends from the Monte Sinai community (15 days), we ventured out to observe animals at the edge of the forest and on the islands immersed in the dark waters of the Rio Negro near the community. We aimed to test a thermal imaging camera and see which animals we could spot at night. Target species included caiman, such as the spectacled caiman (Caiman crocodilus), smooth-fronted caiman (Paleosuchus trigonatus), and black caiman (Melanosuchus niger), night monkeys (Aotus spp.), bushy-tailed rats (Makalata spp.), margay (Leopardus wiedii), ocelot, and Amazon river dolphin, among others. With the exception of the Amazonian manatee, these animals are traditionally difficult to observe even during the day. We did not expect to see nocturnally-active manatees.
For active searching, we traveled to Papagaio Island, a site near Praia da Torre where residents had previously reported seeing some of these species. To approach the shores and the islands quietly and conduct the searches, we used small paddled wooden canoes. We identified species presence via feces, feeding marks, or vocalizations. Visual contact locations were recorded using GPS.
We conducted active searches between 19 and 23 h, using the Pulsar Helion 2 XP50 Pro thermal camera (High-Definition Thermal Sensor 640 × 480 pixel @ 17 µm), which has high thermal sensitivity, to locate the animals. If no animal was detected, we changed the area; if contact was made, we began observations that continued until the group or individual moved or disappeared.
To count manatees, we adapted a traditional technique used for counting pirarucú (Arapaima gigas) in Amazonian lakes, which relies on observing surface breaks as the animals engage in aerial respiration (da Silva et al. Reference da Silva, Gonçalves and Marinho2018). The count was conducted from the moment the first animal was sighted and lasted for 20 minutes.
Results
We identified two individual Amazonian manatees (one adult and one calf) in the waters around Papagaio Island. At night, using the thermal camera, we captured images of the animals foraging and feeding (Figure 1). Amazonian manatees were sighted between 22 and 23 h in a calm body of water with few surface waves. The animals spent approximately 45 minutes feeding on the leaves and new shoots of the semi-submerged igapó shrub maracarãna (Figure 2c–e), which, growing on a submerged sandbank, formed a dense patch of greenery in the middle of the river (Figure 2a–b). The recording started at 22:20 h and lasted until 22:52 h, captured with the thermal camera. On the night of the survey, there was a waning gibbous moon (77% illumination), which rose at 22:17 h (Weatherspark 2023).
Record of an Amazonian manatee (Trichechus inunguis) taken between 22 and 23 h, feeding on the surface of the macarãna plant (Coccoloba densifrons).
Location of the encounter with Amazonian manatee (Trichechus inunguis) (a–b), feeding on the surface of the riverside plant maracarãna (Coccoloba densifrons) and details of the consumed species (c–e).
Discussion
As we explore the world at night under the full moon’s illumination (INMET 2023), even the most iconic animals like the Amazonian manatees reveal themselves in a new light. The first nocturnal feeding record of Amazonian manatees validates traditional knowledge and suggests they exhibit cathemerality, adjusting their behavior in response to environmental pressures such as food availability and predation risks. This behavioral flexibility, known as cathemerality, allows them to optimize foraging and avoid predators (Hartman Reference Hartman1971, Best Reference Best1981, Bernier Reference Bernier1985).
Amazonian manatees feed on the riparian plant maracarãna, which is accessible only during the flood season. As the water level decreases, this plant becomes inaccessible in terrestrial areas (Dias da Silva, pers. obs. Reference Dias-Silva2023). Similar phenomena likely occur with other high-salience plant species in ANP, such as batatarana, cipó-icica, and cipó-de-leite (Crema et al. Reference Crema, da Silva and Piedade2019b). Traditional knowledge from former manatee fishermen indicates that manatees consume riparian plants such as maracarãna and vines like feijãoarana (Vigna lasiocarpa) and feijão-graúdo (Cymbosema roseum) exclusively at night. These fishermen have never observed manatees feeding on these plants during the day, suggesting that this behavior may be related to temperature or predation risks. Furthermore, prolonged exposure to the sun can dry out the manatee’s skin, emphasizing the importance of nocturnal activity to maintain skin moisture (Luna et al. Reference Luna, Miranda, Sousa, Torres-Florez, Fruet and Attademo2021). This aligns with findings that during high-water periods, manatees feed in shallow places at night, likely due to better food accessibility and cooler temperatures (Crema Reference Crema2017).
Integrating scientific observations with traditional knowledge provides a comprehensive understanding of the Amazonian manatee’s adaptive behaviors. These nocturnal feeding habits underscore the importance of considering both environmental factors and physiological needs in future ecological studies and conservation strategies.
The Amazonian manatee has faced significant human predation pressure since pre-colonial times, leading to population declines and risk of extinction primarily due to exploitation for their fat, meat and hides (Domning Reference Domning1982, Best Reference Best1981, Antunes et al. Reference Antunes, Fewster, Venticinque, Peres, Levi, Rohe and Shepard2016, Marmontel et al. Reference Marmontel, Rosas, Kendall, Hines, Reynolds III, Aragones, Mignucci-Giannoni and Marmontel2016, Amaral et al. Reference Amaral, Marmontel, Souza, Carvalho, Valdevino, Guterres-Pazin, Mello, Lima, Chávez-Pérez and da Silva2023). Extreme environmental changes, such as severe droughts, further increase hunting vulnerability (Antunes et al. 2016). Despite reduced hunting, the species continues to face anthropogenic threats (Calvimontes and Marmontel Reference Calvimontes, Marmontel, Fuentes, Silva, Méndez, Azúa, Mejí and Santillan2010, da Silva et al. Reference da Silva, Rosas, Cantanhede, Machado, Drummond and Paglia2008, Reference da Silva, Shepard, do Carmo, Guillaume and Velden2017, Crema et al. Reference Crema, da Silva, Botta, Trumbore and Piedade2019). Consequently, manatees have become extremely wary of humans, complicating their study in natural environments.
The current observation confirms patterns of cathemeral activity in Amazonian manatees, consistent with previous research on manatee behavior (Hartman Reference Hartman1979, Best Reference Best1981, Bernier Reference Bernier1985, Reynolds and Odell Reference Reynolds and Odell1991, Mukhametov et al. Reference Mukhametov, Lyamin, Chetyrbok, Vassilyev and Diaz1992, Powell Reference Powell2002, Morales-López et al. Reference Morales-López, Olivera-Gómez and Zenteno-Ruiz2012, Castelblanco-Martínez et al. Reference Castelblanco-Martínez, Morales-Vela, Slone, Padilla-Saldivar, Reid and Hernandez-Arana2015, Holguin-Medina et al. Reference Holguin-Medina, Fontenele-Araujo, Alcaraz-Romero, Cortes and Muñoz-Delgado2015). Reports from riverside communities of crepuscular and nocturnal feeding and movement (Costa Rica: West Indian manatee [Jiménez-Pérez Reference Jiménez-Pérez1999]; Brazil: Amazonian manatees [Crema et al. Reference Crema, da Silva, Botta, Trumbore and Piedade2019]) align with these findings. The use of thermal imaging cameras provided new insights into nocturnal activities, revealing the cathemeral nature of manatees and offering future applications for species study.
Understanding the cathemeral behavior and feeding strategies of manatees is fundamental for their management and conservation. These findings can influence the designation of protected areas and the implementation of conservation measures, ensuring the preservation of these threatened mammals. This study provides solid evidence of the cathemeral behavior of Amazonian manatees, with significant implications for their conservation and management. Future research, especially using advanced technologies like thermal imaging, may uncover further insights into their nocturnal activities. Who knows what other secrets will be revealed in the river at night?
Supplementary material
The supplementary material for this article can be found at https://doi.org/10.1017/S0266467425000045
Acknowledgements
RHPDS was funded by a FAPEAM graduate grant (0082/2021). We would like to express our gratitude to the residents of the Monte Sinai Community in Anavilhanas National Park, Novo Airão, Amazonas, Brazil for their support and hospitality. Special thanks go to our field co-workers: Katherine Téllez Vera (friend), Sebastião Pereira Oliveira, “Sabázinho” (guide), Sebastião Moreira de Oliveira, “Sabá” (guide), and André Moreira de Oliveira (guide).
Author contributions
Conceptualization: RHPDS, MJCS, APB; Funding acquisition: CM; Investigation: RHPDS; Methodology: RHPDS, MJCS; Project administration: CM; Resources: CM; Software: RHPDS, JVAF; Supervision: RHPDS, CM, JVAF; Validation: RHPDS; Visualization: RHPDS, MJCS, MG, JVAF; Writing – original draft: RHPDS, MJCS, APB, JVAF; Writing – review & editing: RHPDS, APB, JVAF, CM.
Financial support
This research received no specific grant from any funding agency, commercial or not-for-profit sectors.
Competing interests
There are no competing interests to declare.
