Predators don’t just kill ‘prey’ species; they also kill other predators whenever given the chance. Lions kill hyenas and cheetahs, tigers kill dholes, dholes kill tigers, wolves kill bears, otters kill mink… dinosaurs kill dinosaurs…
For various reasons my early plan to produce a new blog post every day has fallen by the wayside, as well it might given that this would cause me to spend what ‘computer time’ I have doing blog writing and nothing else. So in the interests of churning out new material, I have for a while been recycling old texts wherever possible. Several years ago, Dave Martill, Mike Barker and I collaborated on a large volume about the taphonomy of dinosaurs (taphonomy is the study of those events that occur after death). It was to be published by Johns Hopkins University Press no less. I put aside an entire year of my time as a phd student to write the book, and put loads of work into it. But for various reasons that I’d rather not discuss, the collaborative effort failed, and the book was never finished. Multiple chapters on dinosaur taphonomy were left half-finished and abandoned, and I had to give up and return to Eotyrannus and the other Wealden theropods. Most of the book’s contents can’t be salvaged without extensive addition and reworking, and I’m not about to do that. A few of the more interesting bits – the bits that really caught my imagination – are just about complete enough for some salvage to occur however, as the following chunk of text demonstrates. It’s on intraguild predation: the fascinating phenomenon whereby predators kill other predators.
While herbivorous species are usually regarded as the ones that fall prey to predators, it is increasingly recognized by field biologists and ecologists that predators are themselves often predated upon, a phenomenon that Polis et al. (1989) termed intraguild predation. Palomares & Caro (1999) reviewed the literature on intraguild predation in mammals and listed 27 predatory species reported as having killed other predatory mammals. Many of these cases involve adults of one species killing the babies of another. However, even with these discounted there are still many notable examples.
Among extant felids, wild Felis species fall prey to jackals, wolves and dogs, Lynx may be predated upon by wolves (Kitchener 1991), pumas kill bobcats, and Lynx predates upon smaller cats, genets, mongooses and foxes. In canids, larger species kill smaller-bodied contemporaries such that wolves (20-80 kg) kill coyotes (11-15 kg) and red foxes (5-6 kg), coyotes kill red foxes and kit foxes (1.8-3 kg) and red foxes kill Arctic foxes (3-4 kg) and kit foxes. Similarly, small wild felids often kill feral house cats (Felis catus) – even those larger in size than themselves – when they encounter them. Lions kill spotted hyenas and cheetahs, tigers kill dholes, dholes kill tigers, wolves kill bears, otters kill mink, and so on and on [in the adjacent photo, the jackal obviously isn't trying to kill the hyena, but it's a good image anyway. Borrowed from here].
Intraguild predation is less well studied among birds and other non-mammalian predators but peregrines, goshawks, sparrowhawks, buzzards, hen harriers, eagle owls and tawny owls are documented as occasional or significant predators of kestrels, peregrines have been observed killing short-toed eagles, sparrowhawks and other raptors (Hammond & Pearson 1993) and both golden and white-tailed eagles reportedly kill eagle owls. Some owl species routinely kill other, smaller owls. Eagle owls (Bubo bubo) are significant killers of other owls and raptors, occasionally killing species as large as buzzards, goshawks, peregrines, gyrfalons and snowy owls (for a previous post on some of the implications of this intraguild predation see Eagle owls take over Britain).
Predation by eagle owls and goshawks was regarded by Erritzoe & Fuller (1998) as the primary factor affecting the distribution in winter of migrating Long-eared owls Asio otus. In fact predation upon carnivores by carnivores can be so significant that it may represent a serious conservation problem for endangered, localized species; it may be the dominant cause of mortality in some predator populations; and it may mean that large species can totally eradicate smaller ones on isolated islands (Macdonald 1987) or habitat pockets (Erritzoe & Fuller 1998).
The possible importance of intraguild predation has not been missed in the dinosaur literature, and theropods have often been visualized as significant dangers to other, smaller theropod species (e.g., Paul 1988). While intraguild predation surely occurred among theropods, demonstrating its occurrence from taphonomic data is another matter. Theropod tooth marks on theropod bones have been documented and may suggest intraguild predation. Naish (1999) reported a tetanuran tibia from the Lower Cretaceous Hastings Beds Group bearing theropod tooth marks, though whether these resulted from intraspecific aggression, cannibalism or intraguild predation couldn’t be demonstrated.
Cannibalism has been widely suspected in theropods and reported for coelophysids and tyrannosaurids, though the baby coelophysids supposedly eaten by adult members of the same species turned out not to be baby coelophysids at all (Nesbitt et al. 2006). Definitive evidence for cannibalism has been reported for multiple specimens of the Upper Cretaceous Madagascan abelisaurid Majungasaurus* where many bones of this theropod have been discovered bearing tooth marks made by other members of the same taxon (Rogers et al. 2003). Cannibalism can be regarded as intraspecific intraguild predation.
* I ordinarily refer to this taxon as Majungatholus, as the material associated with this name is diagnostic. Conversely, the specimen associated with the older name Majungasaurus is arguably not. However, a major in-press publication on this theropod uses the name Majungasaurus, so I’ll go with that.
The prevalence of intraguild predation got me wondering about those dinosaur faunas where the diversity of predatory theropods is apparently really high. I’m not talking about the Upper Cretaceous Mongolian faunas (such as those within the Djadokhta and Nemegt formations) where multiple oviraptorosaur and other taxa co-existed, as in these cases at least some of the theropods were omnivorous or herbivorous. Rather, the classic examplar is the Brazilian Santana Formation where, of the several dinosaurs known, all are theropods. In considering this unusual situation, I and colleagues (Naish et al. 2004*) wondered if these theropods might have been able to make a living by gleaning prey from the edge of the Santana lagoon (an option not available to herbivorous dinosaurs, obviously), if some of them might have been highly opportunistic and not strictly carnivorous as their morphology suggests, or if some of them might have practiced extensive intraguild predation. This is only a speculation unfortunately as we lack the relevant taphonomic data. Note that the possibility exists that theropods are over-represented in the Santana Formation due to taphonomic biases, and we discussed that possibility too.
* This is the famous paper that names the small coelurosaur Mirischia asymmetrica by the way.
The fact that intraguild predation is so prevalent among modern predators strongly indicates that this has been the case throughout history. It might be difficult to demonstrate its existence from fossils, but I think we can be very confident that theropods killed theropods, theropods killed sebecosuchians, sebecosuchians killed theropods… you get the idea.
I missed the BBC documentary on hypothetical post-Cretaceous dinosaur survival (broadcast 9pm this evening). I did catch a few bits, including Larry Witmer CT-scanning a troodontid skull. For reasons associated with personal research, I’ll say that the scans looked incredibly interesting. Other than that, did I miss anything good?
Refs – –
Erritzoe, J. & Fuller, R. 1998. Sex differences in winter distribution of Long-eared owls (Asio otus) in Denmark and neighbouring countries. Vogelwarte 40, 80-87.
Hammond, N. & Pearson, B. 1993. Birds of Prey. Hamlyn, London.
Kitchener, A. C. 1991. The Natural History of the Wild Cats. Christopher Helm (London)
Macdonald, D. W. 1987. Running With the Fox. Unwin Hyman, London.
Naish, D. 1999. Theropod dinosaur diversity and palaeobiology in the Wealden Group (Early Cretaceous) of England: evidence from a previously undescribed tibia. Geologie en Mijnbouw 78, 367-373.
– ., Martill, D. M. & Frey, E. 2004. Ecology, systematics and biogeographical relationships of dinosaurs, including a new theropod, from the Santana Formation (?Albian, Early Cretaceous) of Brazil. Historical Biology 16, 57-70.
Nesbitt, S. J., Turner, A. H., Erickson, G. M. & Norell, M. A. 2006. Prey choice and cannibalistic behaviour in the theropod Coelophysis. Biology Letters 2, 611-614
Palomares, F. & Caro, T. M. 1999. Interspecific killing among mammalian carnivores. The American Naturalist 153, 492-508.
Paul, G. S. 1988. Predatory Dinosaurs of the World. Simon & Schuster, New York.
Polis, G. A., Myers, C. A. & Holt, R. D. 1989. The ecology and evolution of intraguild predation: potential competitors that eat each other. Annual Review of Ecology and Systematics 20, 297-330.
Rogers, R. R., Krause, D. W. & Curry Rogers, K. 2003. Cannibalism in the Madagascan dinosaur Majungatholus atopus. Nature 422, 515-518.