Introduction

Apex predators play a keystone role in the ecosystems they inhabit (Estes et al. 2011) and, therefore, there is increasing interest in research on their relationships with other species, including interspecific lethal interactions with other predators (Ritchie & Johnson 2009). As they occupy the top of the ecosystem food web, apex predators are often considered free from natural interspecific predation themselves once they reach adult size (Sergio et al. 2014). However, predation of apex predators by other predator species (i.e., superpredation, sensu Lourenço et al. 2014), involving the killing and consumption of the victim, does not seem uncommon, especially within the same taxonomic order (Donadio & Buskirk 2006) and when the victim of the larger species is a young or subadult individual (Palomares & Caro 1999). These lethal interactions are usually asymmetric, implying that larger species prey on smaller ones and gregarious species prey on solitary ones more frequently than the other way around (Palomares & Caro 1999). In fact, examples of smaller predators preying on larger predators, from different taxonomic groups, and with a low degree of dietary overlap, are scarce.

Wolves (Canis lupus) and golden eagles (Aquila chrysaetos) are the two terrestrial apex predators with the widest distribution in the world, sharing large parts of their ranges from Alaska and Canada, in North America, to the whole Palearctic in Eurasia (Fig. 1); golden eagles share with wolves the 80% of their range and wolves share with golden eagles the 62% of their range (i.e., calculated according to IUCN distribution maps; BirdLife International 2023; Boitani et al. 2018). Wolves may be killed by a few species, e.g., by brown bears and pumas (Jimenez et al. 2008), or tigers (Miquelle et al. 1996), while their consumption appears to be exceptional (Naves et al. 2006), except perhaps by own congeners (Newsome et al. 2016). They also kill golden eagles in some infrequent circumstances (Ballard et al. 2003). On the other hand, golden eagles also kill and consume other apex raptors, such as eagle owls (Ellis & Gombobaatar 2020) or Bonelli’s eagles (Bosch et al. 2007), while their predation on large carnivores has been reported infrequently, i.e., a successful predation of a brown bear cub in Norway and a few records of golden eagles attacking brown bear cubs (Sørensen et al. 2008). However, we have not found a single case of adult or young wolves predated by free-ranging golden eagles and only a case of consumption of a wolf pup (Bergmanis 2000), despite the extensive literature describing the eagles’ diet (Dementiev et al. 1966; Watson 2010).

Fig. 1
figure 1

Locations of the four observations of golden eagles attacking and consuming wolf pups in Spain. The main map shows the distribution of the wolf (light grey), the golden eagle (dark grey), and overlapping distribution areas (black), according to the Spanish biodiversity inventory at 10 × 10 km scale (https://www.mapama.gob.es). Inset maps show the distribution of the two species at global and European scales following the same color scheme, according to IUCN distribution maps (https://www.iucnredlist.org)

Full size image

In this natural history note, we present four opportunistic field observations, one documenting the consumption of a wolf pup by golden eagles and three describing independent events of eagles attacking and stalking wolf pups in Spain. These observations represent rare evidence of predatory interaction between the two species.

Study area and field observations

The four field observations took place in the north of the Iberian Peninsula (Spain), including the Cantabrian Mountains and Meseta Norte highlands, located in the Eurosiberian and Mediterranean biogeographic regions, respectively (Fig. 1).

Observation # 1

In December 16th, 1995, at Fuentes del Narcea, Degaña e Ibias Natural Park, Cantabrian Mountains, Asturias (43° 05′ 00″ N, 6° 38′ 17″ W), we came across a golden eagle plucking spot near an eagle nest. The plucking spot was located on a small crag near a ravine with a creek. At the base of the crag, we found the remains of an adult red fox (Vulpes vulpes), including the skull, parts of a common buzzard (Buteo buteo), and a hedgehog (Erinaceus europaeus), and the remains of a wolf pup, as well as some golden eagle feathers. The wolf remains consisted of part of a hindleg and a complete tail. Based on our experience, and given the size and degree of development of the parts found, we estimated the age of the wolf at the time of consumption to be around 3–4 months old.

Observation # 2

On July 25, 2011, at Montes Torozos, Valladolid (41° 33′ 24″ N, 5° 01′ 53″ W) we observed an immature golden eagle attacking a lone wolf pup, about 2–3 months old, near a rendezvous site located in a high flat plateau. The eagle attacked three times and, on the second time, made contact with the wolf’s back, but failed to grab it as the pup turned its face in an attempt to bite the eagle. We missed the end of the third attempt because both went behind a ridge. We know that the eagle failed to get the pup because in subsequent days, we observed the same number of pups (four) as before the attack.

Observation # 3

On July 18th, 2013, at Montaña Palentina Natural Park (42° 52′ 17″ N, 4° 38′ 57″ W), Cantabrian Mountains, Palencia, we witnessed at 8:45 an immature golden eagle attacking a wolf pup in two attempts, the first apparently touching the pup, but the wolf managed to take cover in a broom patch of Genista spp. The eagle remained perched on a cliff for a few minutes until it left the area. We did not see other wolves that day, albeit the wolf pack used the spot as a rendezvous site, as we observed up to five pups and several adult wolves at subsequent days.

Observation # 4

On January 6th, 2019, at Las Ubiñas-La Mesa Natural Park (43° 11′ 36″ N, 6° 10′ 29″ W), Cantabrian Mountains, Asturias, we witnessed at 8:40 a subadult golden eagle closely following a traveling pack of six wolves, three adults and three pups (about 8 months old). The eagle flew at a height of around 20 m above the wolves in a stalking attitude for along 300 m, until it landed on a small crag 2 m high, about 50 m in front of the wolves. Just at that moment, several wolves took cover in a broom patch, while one of the adult wolves harassed the eagle, whereupon the bird left the scene. The wolves remained resting under cover at the site for at least the next 2 h. As we saw no signs of any carcass or kill site in the area, we interpret that the eagle’s behavior was not motivated by an attempt to obtain a share of a prey.

Discussion

The observations described herein are one of the few evidence documenting the consumption of a wolf pup and predatory behavior towards wolf pups by free-ranging golden eagles. Although consumption and stalking/attack events were independent and, therefore, we cannot confirm a complete predatory event (i.e., kill and consumption), considering these observations together, we suggest that the possibility of actual predation of wolf pups by eagles in this type of events is plausible. Indeed, captive golden eagles, trained by tribesmen of Central Asia, are capable of killing full-grown wolves, although in most cases, they require the help of the hunter (Watson 2010).

A few golden eagle–wolf interactions have been described in the scientific literature, mostly at ungulate carcasses where both compete for carrion. In these interactions, the wolves clearly dominate and may even kill the eagles on some occasions (Ballard et al. 2003). It was also suspected that wolves preyed on fledging eaglets in Mongolia (Ellis 2020). As for the opposite direction of interaction, Dekker (1985) and Watson (2010) stated that golden eagles employed a variation of their usual low-flying with slow descent attack against carnivores, such as foxes, coyotes, or even wolves, consisting on hanging a few meters above the prey, sometimes for several minutes, attacking only if the victim turned its head to another side. Also, Murie (1990) reported observations of golden eagles apparently confronting foxes and wolves, but the actual killing of wolves has not been confirmed in these aforementioned works. The only evidence of wolves consumed by golden eagles was reported by Bergmanis (2000), who found a fresh leg of a young wolf (about 2 months old when consumed, Bergmanis pers. comm) in an eyrie in Latvia.

Moreover, dogs (Canis familiaris) and coyotes (Canis latrans) have been cited as rare but unexceptional prey consumed by golden eagles (Collopy 1983; Watson 2010). On the other hand, foxes are cited as frequent prey in studies on golden eagles’ diet worldwide (Ellis & Gombobaatar 2020; Watson 2010), also in the Cantabrian Mountains (de Gabriel et al. 2024). Given that wolf pups are to some extent similar in size to adult foxes, it seems plausible that golden eagles see wolf pups as an actual food resource, at least in some areas and circumstances. Considering that mortality causes for wolves < 4 months old are practically unknown (Fuller et al. 2003), this fact could imply that predation by eagles may be an overlooked cause of mortality for wolves at this stage of their growth.

Among apex predators, golden eagles and wolves are the most extensively studied species in terms of their diets and with the largest overlapping ranges. This fact suggests that the intraguild apex predators’ interactions described in this note, in which a smaller raptor predator attacks or consumes a lager mammal and social predator, are rare events, but provide valuable information on potential trophic and behavioral relationships between keystone apex predators.