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Apex predators such as carnivorous sharks occupy the top trophic level in a community and tend to have strong top-down effects on population demography, structure, and ecosystem productivity (1, 2). Many sharks, particularly pelagic sharks, are opportunistic predators and dietary generalists, and scavenging on carrion likely plays a significant role in supplementing their diet (3–11). For example, estimates suggest that a large (~4 m) white shark that consumes 30 kg of blubber can be sustained for up to 1.5 months without additional food (12). Scavenging can also facilitate both bottom-up and top-down regulation of populations through different trophic levels and represents an important energy transfer pathway in marine ecosystems (13–15).

Normally, cetacean carcasses provide the most substantial source of energy for scavengers, and some deep-sea communities appear to subsist exclusively from whale falls (16). Unsurprisingly, and given their stochastic nature, carcasses often attract large numbers of highly mobile, typically solitary predators that are usually sparsely distributed (10, 11, 14, 15, 17). Previous studies show large lamnid and carcharhinid sharks to be the most common surface scavengers. This includes white [Carcharodon carcharhinus; (5–8, 10)], tiger [Galeocerdo cuvier; (17, 18)], and bull sharks [C. leucas; (17)], although reports vary on the level of aggressive, competitive, or peaceful interactions displayed during a scavenging event. Scavenging events, therefore, provide a unique opportunity to examine inter- and intra-specific behaviors between species, often top predators, not normally encountered together.

Photo identification is a common technique used for individual identification, movements, and population estimates of many species of elasmobranchs globally [e.g., (19–22)]. The method allows individuals with distinctive body features, such as natural markings and pigmentations, to be uniquely identified and has been used to successfully document population demographics for white sharks (23, 24), whale sharks (25–27), tiger sharks [TIG; (18, 28)], and, more recently, oceanic whitetip sharks [OCS; (29)].

In this study, we used the opportunistic finding of a heavily degraded carcass to document, to our knowledge, one of the first observations of OCS and TIG scavenging concurrently. OCS are typically a solitary species with low population densities (30, 31). OCS are classified as “oceanic pelagic” (32), meaning that they can potentially complete their entire life cycle in the open ocean (32). These characteristics make OCS notoriously difficult to study. Here, we (1) document the novel observations of a feeding aggregation of OCS scavenging on a carcass, (2) describe the inter- and intra-specific behaviors of OCS and TIG and (3) use photo identification to determine the demographics and number of individuals present at the feeding aggregation.

Although scavenging by sharks is relatively common, and likely an important component of their feeding ecology (9–11), documentation of these stochastic events is rare. Studies of scavenging by sharks are often limited to coastal species such as white sharks [C. carcharhinus; (5–8, 10)], TIG [G. cuvier; (17, 18)], and bull sharks [C. leucas; (17)] presumably because these events occur closer to the coast and provide easier access for observers. To our knowledge, this study is the first to scientifically report a scavenging event of OCS concurrently feeding with TIG. Although both species are opportunistic predators and scavengers, documentation of OCS scavenging events is rare as OCS are typically solitary, highly migratory, and spend most of their time in the open ocean (32). However, the Big Island, Hawai‘i, is known to aggregate OCS seasonally, usually in the spring and summer (29), and hold TIG year-round (36, 37). Boat operators (and co-authors) working in Kona waters daily report sightings of OCS and TIG in close proximity as extremely rare, with only one observation of a TIG and a group of OCS following an injured pilot whale in 4 years (pers. comm. Jim Ward and Dylan Currier). This is presumably because the two species occupy vastly different habitats around the Hawaiian archipelago, where OCS are strictly oceanic pelagic and TIG exhibit sex-related variation in habitat preferences, with mature females predominantly using coastal habitats 0–200 m (36, 38, 39) and mature males occupying more offshore, open-ocean habitats (39, 40).

The feeding aggregation in this study was dominated by male TIG (n = 4 male, n = 1 female) and female OCS (n = 7 female, n = 1 male, n = 1 unknown); although these numbers are likely to be an underestimation of all individuals in the aggregation, it highlights how the temporary availability of a single carcass can promote opportunistic scavenging and support high abundances of marine predators (14, 15, 17, 41). Dominance of male TIG confirms their preference for offshore environments (36) and likely reflects TIG ability to adapt patterns of movement to local resource distribution (36). While dominance of female OCS aligns with Scott et al. (29) that shows a heavily skewed sex ratio of approximately ~2:1 female: male OCS around the Island of Hawai‘i, suggesting the west-coast of Hawai‘i Island, may be an area of biological importance for OCS in the Pacific.

Overall and similar to previous studies, we observed a size-dependent hierarchy of the feeding aggregation (10, 42). Therefore TIG, which ranged in size from ~3–4 m, were the dominant species. Surprisingly, given the variation in sex and size of the scavenging individuals and the small size of the carcass, we did not observe any aggressive interactions. There were some instances of smaller sharks giving way to larger sharks, implying social precedence (18). But overall, we observed peaceful concurrent feeding, with no inter- or intra- specific aggression. The absence of agonistic or competitive behaviors has been previously reported between TIG and white sharks (43, 44), TIG and crocodiles (15), 40 individual white sharks (10), and TIG, bull, and tawny nurse sharks (17) scavenging concurrently on carcasses, although agonistic behaviors are also commonly reported for scavengers [e.g., (18, 45, 46)]. It is not clear why TIG exhibit such contrasting social behaviors in different locations, but it could be related to differences in food availability, with sharks being less competitive if resources are abundant (17) and/or variation among individuals and personalities, which may trigger inter- or intra-specific risk-taking or risk-averse behaviors (47, 48).

In fact, we believe that the larger TIG may have facilitated scavenging by the smaller OCS that were only observed ~5 m below the surface feeding from scraps and/or regurgitations from the TIG and never feeding directly from the carcass. Around Hawai‘i, OCS are often observed following pilot whales (Globicephala spp.), most likely to forage on their scraps as pilot whales are efficient foragers and have similar prey preference to OCS [i.e., cephalopods; (49–52)]. TIG have also been documented facilitating scavenging for bull sharks on a whale carcass by exposing softer tissues for the bull sharks to forage on (17). Surprisingly, no other coastal or pelagic shark species, such as silky (C. falciformis), galapagos (C. galapagensis), oceanic blacktip (C. limbatus), sandbar (C. plumbeus), or white (C. carcharhinus), were observed at the feeding aggregation. This may be due to; (a) the smaller species avoiding the carcass to reduce the risk of predation from larger sharks (53); (b) the carcass being in water depths >5,000 ft, which is outside the preferred habitat and depth range of coastal species [i.e., galapagos, sandbars; (38, 54)] and/or; (c) variation in personalities between individuals and species (47, 48). For example, silky sharks are considered to be “shier” and less bold than OCS and TIG and therefore less likely to approach the carcass (pers. obs. Olivia Miller). The absence of white sharks around the carcass is not surprising. Although white shark movements to Hawai‘i from California increase during springtime (55), the species is not commonly encountered in the coastal waters off west Hawai‘i.

Although OCS and TIG are considered opportunistic predators and scavengers, marine mammals make up very small proportions of their overall diet. In general, OCS diet is primarily composed of cephalopods (44%) and teleosts (43%), with a smaller proportion (13%) being a mix of birds, mollusks, crustaceans, and mammals (56, 57), whereas, for large TIGs (>3 m), elasmobranchs (42%) and teleosts (40%) tend to be the most common prey items, followed by crustaceans (35%), birds (25%), land mammals (19%), turtles (15%), cephalopods (10%), and marine mammals [7%; (58)]. During the feeding event, both species used feeding techniques previously documented for sharks. This included “saw-biting”, referring to a “side-to-side” twisting of the head and body resulting in swift cutting (4, 18). We observed OCS moved their head more rapidly than TIG when undertaking “saw-biting.” Additionally, both species were observed “rotary-biting,” in which they rolled and spun their entire body around the carcass (Supplementary Videos S8, S9). Rotary-biting is most likely more energetically cost-effective in removing mouth-sized pieces of flesh (18).

The high-resolution footage obtained in this study made photo identification of sharks to an individual level possible based on features of their dorsal fins and body markings. However, the number of individuals at the carcass was likely underestimated. Specifically for OCS, as we were only able to obtain right side imagery for three OCS, where left side images are needed for unique identification because left and right side dorsal patterns are different (29). It is also possible that other individuals from both species were less bold and remained out of view of the observers capturing the footage. Regardless, this study confirms that photo identification is an extremely valuable tool for identification and abundance estimates for a small number of individuals. For OCS, in particular, seven new individuals were added to the existing Hawaii Community Tagging Program catalog, and two individuals (both female) were confirmed as re-sights, one after 5 years, which is currently the longest re-sighting in the database and suggests repeated visits by OCS to the west side of Hawai‘i island. These data are extremely important for collecting crucial baseline information on population demographics of OCS, a threatened species, around the Hawaiian Islands, and will increase the ability for re-sightings in the future.

Finally, we acknowledge that although novel, this study represents a very small snapshot of the scavenging behaviors of OCS and TIG. Nevertheless, these stochastic events serve as an important reference for investigating trophic relationships and social interactions among aquatic apex predators that do not normally overlap in space and time. Understanding and documenting the scavenging process is vital for comprehending the biology, evolution, and behavioral ecology of top predators [e.g., (13, 14)].