High moon brightness and low ambient temperatures affect sloth predation by harpy eagles

Abstract

Background Climate plays a key role in the life histories of tropical vertebrates. However, tropical forests are only weakly seasonal compared with temperate and boreal regions. For species with limited ability to control core body temperature, even mild climatic variation can determine major behavioural outcomes, such as foraging and predator avoidance. In tropical forests, sloths are the arboreal vertebrate attaining the greatest biomass density, but their capacity to regulate body temperature is limited, relying on behavioural adaptations to thermoregulate. Sloths are largely or strictly nocturnal, and depend on crypsis to avoid predation. The harpy eagle (Harpia harpyja) is a sloth-specialist and exerts strong top-down control over its prey species. Yet the role of environmental variables on the regulation of predator–prey interactions between sloths and harpy eagles are unknown. The harpy eagle is considered Near Threatened. This motivated a comprehensive effort to reintroduce this species into parts of Mesoamerica. This effort incidentally enabled us to understand the prey profile of harpy eagles over multiple seasons. Methods Our study was conducted between 2003 and 2009 at Soberanía National Park, Panamá. Telemetered harpy eagles were seen hunting and feeding on individual prey species. For each predation event, field assistants systematically recorded the species killed. We analysed the effects of climatic conditions and vegetation phenology on the prey species profile of harpy eagles using generalised linear mixed models. Results Here we show that sloth predation by harpy eagles was negatively affected by nocturnal ambient light (i.e. bright moonshine) and positively affected by seasonally cool temperatures. We suggest that the first ensured low detectability conditions for sloths foraging at night and the second posed a thermally unsuitable climate that forced sloths to forage under riskier daylight. We showed that even moderate seasonal variation in temperature can influence the relationship between a keystone tropical forest predator and a dominant prey item. Therefore, predator–prey ecology in the tropics can be modulated by subtle changes in environmental conditions. The seasonal effects shown here suggest important demographic consequences for sloths, which are under top-down regulation from harpy eagle predation, perhaps limiting their geographic distribution at higher latitudes.

Prey resources are equally important as climatic conditions for predicting the distribution of a broad‐ranged apex predator

“…A recent review summarizing harpy eagle diet across its range established a trend towards a semispecialized diet (Miranda, 2015), mainly comprised of arboreal mammals, including sloths, primates and tree porcupines. However, birds, reptiles and terrestrial mammals may also be taken, albeit less frequently (Aguiar‐Silva et al, 2015; Miranda, 2018; Miranda et al, 2020).…”

Temporal activity patterns of North China leopards and their prey in response to moonlight and habitat factors

“…On the contrary, the “visual acuity hypothesis” (Huck et al, 2017 ; Pratas‐Santiago et al, 2017 ) states that the brightness of a full moon provides “visually oriented” prey species heightened chance to forage and/or detect danger, with the result that they are expected to be more active during the full moon, showing “lunar philic” activity—in other words preferring brighter moon phases (Fernández Moya et al, 2021 ). For example, brighter nights seem to inhibit the activity of ocelots ( Leopardus pardalis ) (Leonard et al, 2020 ), two‐toed sloth ( Choloepus didactylus ) (De Miranda et al, 2020 ), and red muntjac ( Muntiacus muntjak ) (Rahman & Mardiastuti, 2021 ), while for other species, such as cheetah ( Acinonyx jubatus) (Broekhuis et al, 2014 ), bobcats ( Lynx rufus ) (Leonard et al, 2020 ), Bawean deer (Axis kuhlii ) (Rahman & Mardiastuti, 2021 ), Indian hare (Lepus nigricollis) (Bhatt et al, 2021 ), and some nocturnal birds (Pérez‐Granados & Schuchmann, 2021 ), brighter nights yield increased activity.…”

Prey resources are more important than climatic conditions for predicting the distribution of a broad-ranged apex predator

“…There were high positive responses between harpy eagle distribution and three-toed sloth occurrence and both sloth genera had the highest percent contributions to the Biotic model prediction. Indeed, in some parts of their range harpy eagles have narrow diets comprised of 80 to 95 % sloths (Miranda et al 2020), in central and eastern Amazonia (Galetti & de Carvalho 2000;Aguiar-Silva et al 2014) andnorth-east Ecuador (Muñiz-López 2008). However, the harpy eagle is not so specialized on a diet of sloths as to be absent from areas where sloths are not present.…”

“…However, birds, reptiles, and terrestrial mammals may also be taken, albeit less frequently (Aguiar-Silva et al . 2015; Miranda 2018; Miranda et al . 2020).…”