Habitat type spatial classifications
Understanding the influence of environmental heterogeneity on large mammal movements is critical for conservation (Newak and Richart 2012). Dispersal and migration of river mammals such as dolphins, manatees, and otters implies, for these types of organisms, the challenge of facing constrasting environmental conditions along the river courses (Sawyer et al. 2009; Sawyer and Kauffman 2011) and converting the environmental heterogeneity into a crucial aspect for understanding the animals´ spatial ecology (Newak and Rickart 2012). The environmental heterogeneity of rivers, not only refers to the variability of river segments in terms of their physical and biological conditions, but also the fact that river landscapes can change drastically in time according to flood pulses. Plants and animals have adapted their behavior and reproductive cycles to the constant fluctuation of their niche. The instability of river habitats has selected for not only high vagility among riverine species (migrating patterns), but also it has selected a top predator, Amazon River Dolphins, for adaptations for the opportunistic use of a changing environment (adaptated to habitat heterogeneity). All Amazon River fish species show either lateral or longitudinal migratory patterns dictated by flood pulses (Zapata and Usma 2013; Barthem et al. 2017). More than 15 million years of evolution in Neotropical aquatic ecosystems (Geisler et al. 2011), have selected in I. geoffresis a set of adaptations for foraging along fluctuating and constrasting habitats. Compared with other cetateans, Amazon River dolphins have increased their maneuverability across different types of aquatic landscapes (unfused cervical vertebrae, large, broad, paddle-like flippers, da Silva and Martin 2014), and are the only cetacean exhibiting both, conical and molar-like teeth, expanding the spectrum of their fish prey size (25–90 cm, da Silva 1983; Best and da Silva 1989).
The intensity of use of different habitat types by I. geoffrensis is primarily influenced by (1) the abundance and availability of fish prey, (2) the accessibility to foraging locations, determined mainly by the flooding pulse and river geomorphology, and (3) the sexual and reproductive conditions of the individuals in a group (McGuire and Winemiller 1998; Martin and da Silva 1998; Trujillo 2000; Martin and da Silva 2004; McGuire and Henningsen 2007; Yamamoto et al. 2015; Mintzer et al. 2016; Mosquera-Guerra et al. 2021). Amazon River dolphins make up one of the smallest group sizes among odontocetes as a strategy to increase individual fitness, and reduce competition for prey during declines in fish abundance during the high water period (Gómez et al. 2011a). This species shows sexual segregation in habitat use (Mintzer et al. 2016), similar to other species of odontocete species (e.g., Beluga whale [Delphinapterus leucas, Loseto et al. 2006], northern bottlenose whales [Hyperoodon ampullatus, Gowans et al. 2001], and sperm whales [Physeter microcephalus, Whitehead and Weilgard 2000]).
Our results show strong sexual segregation for habitat use among satellite-monitored individuals in the Amazonas River. The documented differential behaviors in the intensity of habitat use between males and females coincide with observations made by Trujillo (2000) in the lakes of Tarapoto and the Colombian Amazonas and Martin and da Silva (1998; 2004) who analyzed data from 24 individuals monitored with radio telemetry in the Mamiraua Sustainable Development Reserve. They report that during the period of rising water adult females and calves use lagoons, floodplain and channels more intensively due to factors such as distribution patterns and abundance of small prey (gymnotids, cichlids, and catfish) associated with floating vegetation (Crampton 1999; Martin and da Silva 2004a). This allows them to meet the energetic requirements of the young as well as providing safety for mothers and young from male harassment (Martin and da Silva 2004a; Yamamoto et al. 2015). Lagoons and channels are characterized by slow river flow, necessitating lower energy expenditure in these habitats (Aliaga and Guizada 2020; Martin and da Silva 2004). Males use the main river and confluences due to the presence of large schools of characins that make breeding migrations or move between habitat types due to changes in dissolved oxygen concentration levels in the water at night (da Silva 1983; Martin and da Silva 1998; Crampton 1999; Martin and da Silva 2004a, Yamamoto et al. 2015; Mintzer et al. 2016).
In the case of individuals monitored in the five rivers of the Orinoco basin sexual segregation by habitat type was not as evident as that reported for groups analyzed in the Amazon basin. A not very evident habitat sexual segregation behavior in the Orinoco basin has been also reported by McGuire and Winemiller (1998) for the Cinaruco River in Venezuela, and McGuire and Aliaga-Rossel (2007). They compared reproductive patterns in the Orinoco, Amazonas, and Mamoré rivers and suggested that habitat selection by I. geoffrensis in the Orinoco basin is influenced by strong seasonality of this aquatic system and its effects on the fluctuations of local fish populations. Habitat availability and habitat extension related to flood plains and lagoons that are frequently associated with female parental care in I. geoffrensis at the Amazon are significantly reduced in the Orinoco basin due to geomorphological determinants (Mosquera-Guerra et al. 2021). Time of water retention is low and flow velocity is high in the Orinoco basin when compared with the Amazon basin; and this compromises the formation of meanders and lagoons that are ideal for female parental care (McGuire and Winemiller 1998) and explain the reduction of habitat segregation between males and females. Apparently, females in the Orinoco are adapted to the above-described situation using the main river channel, and tributaries for nursing. In addition, McGuire and Aliaga-Rossel (2007a) propose a geographical variation in reproductive seasonality influenced by the effects of latitude on the water level flow and prey abundance. Reinforcing these ideas, Mintzer et al. (2016) reported that males and non-lactating females more frequently use the main river, the confluences, and the channels, and uses the lakes with lower intensity in the Japurá River in the Brazilian Amazon basin.
Mintzer et al. (2016) report that habitat use mediated by sexual segregation may influence the levels of risk to different types of human threats for river dolphins. Males and non-lactating females that intensively use the main river and channels are more likely to be exposed to collision by boats and negative interactions with fisheries, while females with calves in lakes and bays are more likely to suffer entanglements in monofilament nets used by fisherman in this type of habitat. In addition, these researchers highlight the intense use of the Amazon River dolphins of different habitat types within the Mamiraua Sustainable Development Reserve demonstrating the importance of the protected areas for I. geoffrensis populations in the Amazon basin.
Home range and hotspot activity
Home range size is the result of dynamic processes strongly influenced by both, large scale factors (hydroclimatic cycles and geomorphology) and fine-scale determinants (spatio-temporal distribution of important resources). In addition, besides changes in habitat availability the condition of the individuals (sex and state of development) determines habitat preferences, affecting home range size (Börger et al. 2008; Van Beest et al. 2011; Viana et al. 2018). Seasonal variation of home range in dolphins is determined by changes in habitat selection (Van Beest et al. 2011; Viana et al. 2018) associated with mating and rutting seasons (e.g., Dahle and Swenson 2003; Viana et al. 2018; Mosquera-Guerra et al. 2021).
Our mean home range value for monitored I. geoffrensis individuals in the Amazon River was higher than previously reported values and lower than home range values for individuals monitored in the Orinoco River in the same study (Mosquera-Guerra et al. 2021). Generalized additive models for location, scale, and shape (GAMLSS) have been used to evaluate the best ecological predictors that explain the home range size of Amazon River dolphins: (1) body mass and length (e.g., Buskirk 2004; Tucker et al. 2014; Kelt and Van Vuren 2001); (2) longest distance between locations (e.g., Trujillo 1994; 2000; Martin and da Silva 1998; 2004; 2004a; McGuire and Henningsen 2007; Denkinger 2010; Mintzer et al. 2016; Mosquera-Guerra et al. 2021), and (3) sex and reproductive status (e.g., Martin and da Silva 2006; da Silva and Martin 2014; Martin and da Silva 2018).
Ecological hotspot definitions are typically related to location abundance and require a threshold to be applied to differentiate hot and non-hot locations (Stahl et al. 2001; Nelson and Boots 2008). Hotspots are spatially explicit and can be detected at particular geographic locations (Chulick et al. 2002; Nelson and Boots 2008). Hotspot activity reported in this study for the individuals of I. geoffrensis monitored in the Amazonas River are associated with the following habitat types: (1) main river, (2) confluences, (3) tributaries, and (4) lagoons. The intense use of these habitat types by the Amazon River dolphins coincides with previously reported fidelity sites for the Amazonian rivers as follow: (1) Cuyabeno and Lagartococha in Ecuador (e.g., Denkinger 2010), (2) Marañon in Peru (e.g., McGuire and Henningsen 2007), (3) Tijamuchi and Mamoré in Bolivia (e.g., Aliaga-Rossel 2002; Aliaga-Rossel and Guizada-Durán 2020), (4) Amazonas in Colombia (e.g., Trujillo 2000; Gómez-Salazar et al. 2011a), and (5) Japurá and Solimoes in Brazil (e.g., Martin & Da Silva 2004; 2004a; Yamamoto et al. 2015; Mintzer et al. 2016). In the rivers of the Orinoco basin, hotspot activity was associated with the following: (1) main river, (2) confluences, (3) tributaries, (4) channels, (5) bays, and (6) lagoons. They coincide with those reported for the rivers: (1) Cinaruco in Venezuela (e.g., McGuire and Winemiller 1998), and (2) the Orinoco in Colombia (e.g., Trujillo 2000; Gómez-Salazar et al. 2011a).
Hot spot activity size may be explained by the fact, that some individuals may remain residents in relatively small areas (Bjørge 2002). Although these individuals generally have well-defined home ranges, their distribution and the use of space within these home ranges refers to individual movements rather than to an entire population (Morales et al. 2004; 2010). This is particularly true in fission-fusion societies, such as those seen in many cetacean species including Amazon River dolphins (Gómez et al. 2011; Gravena et al. 2019), where individuals within the same population may have significantly different ranging patterns (Defran et al. 1999) and individuals may alternate between local site-fidelity and longer ventures away from the site of their first identification (Sprogis et al. 2015; Rako-Gospić et al. 2017; Mosquera-Guerra et al. 2021).
Hotspot activity of I. geoffrensis in the Amazonas River is located within the wetlands of international importance or Ramsar sites: Tarapoto Lakes Complex and in the Orinoco basin, hotspots identified in the Guayabero River in the aquatic ecosystems of the National Natural Parks Serranía de La Macarena and Tinigua, and the area of ecological importance for the species designated by the environmental corporation Cormacarena at the confluence between the Yarumales stream and the Guayabero River, the Guaviare River in the Estrella Fluvial de Inírida Ramsar site, the Orinoco basin in the Bita River Ramsar site, and the El Tuparro Biosphere Reserve. Our results demonstrate the importance of spatial ecology analysis of endangered wildlife by the definition of protected areas, quantifying the use of ecosystems within these areas, and the location of priority areas for the implementation of management measures for these species and habitats (Reyers et al. 2009; Zhang and Fu 2014).