Consumption of carrion

When considering the decomposition of dead bodies, it is helpful to distinguish three categories of organisms that attack carcasses. As before, both decomposers and invertebrate detritivores have a role to play. For example, the tenebrionid beetles Argoporis apicalis and Cryptadius tarsalis are particularly abundant on islands in the Gulf of California where large colonies of seabirds nest; here they feed on bird carcasses, as well as fish debris associated with the bird colonies (Sanchez-Pinero & Polis, 2000). In the case of carrion feeding, however, scavenging vertebrates are often also of considerable importance. Many carcasses of a size to make a single meal for one of a few of these scavenging detritivores will be removed completely within a very short time of death, leaving nothing for bacteria, many carnivores are opportunistic carrion-feeders ...

fungi or invertebrates. This role is played, for example, by arctic foxes and skuas in polar regions, by crows, gluttons and badgers in temperate areas, and by a wide variety of birds and mammals, including kites, jackals and hyenas, in the tropics.

The chemical composition of the ... and vice versa diet of carrion-feeders is quite distinct from that of other detritivores, and this is reflected in their complement of enzymes. Carbohydrase activity is weak or absent, but protease and lipase activity is vigorous. Carrion-feeding detritivores possess basically the same enzymatic machinery as carnivores, reflecting the chemical identity of their food. In fact, many species of carnivore (such as lions, Panthera leo) are also opportunistic carrion-feeders (DeVault & Rhodes, 2002) whilst classic carrion-feeders such as hyenas (Crocuta crocuta) sometimes operate as carnivores.

Arctic foxes (Alopex lagopus) illustrate the arctic fox: a how the diet of facultative carrion-

facultative carrion- feeders can vary with food availability.

feeder Lemmings (Dicrostonyx and Lemmus spp.) are the live prey of foxes over much of their range and for much of the time (Elmhagen et al., 2000). However, lemming populations go through dramatic population cycles (see Chapter 14), forcing the foxes to switch to alternative foods such as migratory birds and their eggs (Samelius & Alisauskas, 2000). In winter, marine foods become available when foxes can move onto the sea ice and scavenge carcasses of seals killed by polar bears. Roth (2002) investigated the extent to which foxes switched to carrion feeding in winter by comparing the ratios of carbon isotopes (13C : 12C) of suspected food (marine organisms have characteristically higher ratios than terrestrial organisms) and of fox hair (since carbon isotope signatures of predator tissue reflect the ratios of the prey consumed). Figure 11.16 shows that in three of the 4 years of the study the isotope signature of fox hair samples was much increased in winter, as expected if seal carrion was a major component of the diet. In the winter of 1994, however, a marked shift was not evident and it is of interest that lemming density was high at this time. It seems that foxes switched to seal carrion when the formation of sea ice made this possible, but only when alternative prey were not available.

The relative roles played by decom-seasonal variation posers, invertebrates and vertebrates in invertebrate and are influenced by factors that affect microbial activity the speed with which carcasses are dis covered by scavengers in relation to the rate at which they disappear through microbial and invertebrate activity. This is illustrated for small rodent carcasses whose disappearance/decomposition was monitored in the Oxfordshire countryside in both the summer-fall and winter-spring periods (Figure 11.17). There are two points to note. First, the rate at which carcasses were removed was faster during the summer and fall, reflecting a greater scavenger activity at this time (presumably

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Figure 11.16 (a) Annual changes in lemming density in the summer, near Cape Churchill in Manitoba, Canada, and (b) carbon isotope ratios (mean ± SE) of fox hair in the winter (reflecting summer diet) and in the summer (reflecting winter diet). Numbers on the bars indicate sample sizes. (After Roth, 2002.)

because of higher scavenger population densities and/or higher feeding rates - these were not monitored in the study). Secondly, a greater percentage of the rodent bodies were removed in the winter-spring period, albeit over a longer timescale. At a time when microbial decay proceeds most slowly, all the carcasses persisted for long enough to be found by scavengers. During the summer and fall, decomposition was much more rapid and any carcass that was undiscovered for 7 or 8 days would have been largely decomposed and removed by bacteria, fungi and invertebrate detritivores.

Certain components of animal corpses are particularly resistant to attack and are the slowest to disappear. However, some consumer species possess the enzymes to deal with them. For example, the blowfly larvae of Lucilia species produce a collagenase that can digest the collagen specialist consumers of bone, hair and feathers

Figure 11.17 The rate of removal of small mammal corpses in the Oxfordshire (UK) countryside in two periods: summer-fall and winter-spring. (After Putman, 1983.)

and elastin present in tendons and soft bones. The chief constituent of hair and feathers, keratin, forms the basis of the diet of species characteristic of the later stages of carrion decomposition, in particular tineid moths and dermestid beetles. The midgut of these insects secretes strong reducing agents that break the resistant covalent links binding together peptide chains in the keratin. Hydrolytic enzymes then deal with the residues. Fungi in the family Onygenaceae are specialist consumers of horn and feathers. It is the corpses of larger animals that generally provide the widest variety of resources and thus attract the greatest diversity of carrion consumers (Doube, 1987). In contrast, the carrion community associated with dead snails and slugs consists of a relatively small number of sarcophagid and calliphorid flies (Kneidel, 1984).

One group of carrion-feeding invertebrates deserves special attention -the burying beetles (Nicrophorus spp.) (Scott, 1998). These species live exclusively on carrion on which they play out their extraordinary life history. Adult Nicrophorus, using their sensitive chemoreceptors, arrive at the carcass of a small mammal or bird within an hour or two of death. The beetle may tear flesh from the corpse and eat it or, if decomposition is sufficiently advanced, consume blowfly larvae instead. However, should a burying beetle arrive at a completely fresh corpse it sets about burying it where it lies, or may drag the body (many times its own weight) for several meters before starting to dig. It works beneath the corpse, painstakingly excavating and dragging the small mammal down little by little until it is completely underground (Figure 11.18). The various species of Nicrophorus vary in body size (and thus the size of corpse utilized), reproductive period (and remarkable burying beetles

Nicrophorus Breeding Milne
Figure 11.18 Burial of a mouse by a pair of Nicrophorus beetles. (After Milne & Milne, 1976.)

thus the season of activity), diel activity (some are diurnal, some crepuscular and some nocturnal) and the habitat they use (coniferous forest, hardwood forest, field, marsh or generalist) (Scott, 1998). Some species, such as N. vespilloides, only just cover the corpse, while others, including N. germanicus, may bury it to a depth of 20 cm. During the excavation, other burying beetles are likely to arrive. Competing individuals of the same or other species are fiercely repulsed, sometimes leading to the death of one combatant. A prospective mate, on the other hand, is accepted and the male and female work on together.

The buried corpse is much less susceptible to attack by other invertebrates than it was while on the surface. Additional protection is provided, under some circumstances, by virtue of a mutualistic relationship between the beetles and a species of mite, Poecilochirus necrophori, which invariably infests adult burying beetles, hitching a ride to a suitable carrion source. When the carcass is first buried the beetle systematically removes its hair and this clears it of virtually all the eggs of blowflies. However, if the carcass is buried only shallowly, flies will often lay more eggs and maggots will compete with the beetle larvae. It is now that the presence of mites has a beneficial effect. By piercing and consuming the fly eggs, the mites keep the carcass free of the beetle's competitors and dramatically improve beetle brood success (Wilson, 1986). Both adults, or sometimes just the female, remain in the chamber and provide parental care. A conical depression is prepared in the top of the meat-ball, into which droplets of partially digested meat are regurgitated. Older larvae are able to feed themselves but only when their offspring are ready to pupate do the adults force their way out through the soil and fly away.

We have already noted that in freshwater environments carrion lack a specialized fauna. However, specialist carrion-feeders are found on the sea bed in very deep parts of the oceans. As the detritus sinks through very deep water all but the largest particles of organic matter are completely decomposed before they reach the bottom. In contrast, the occasional body of a fish, mammal or large invertebrate does settle on the sea bed. A remarkable diversity of scavengers exist there, though at low density, and these possess several characteristics that match a way of life in which meals are well spread out in space and time. For example, Dahl (1979) described several genera of deep sea gammarid crustaceans which, unlike their relatives at shallower depths and in fresh water, possess dense bundles of exposed chemosensory hairs that sense food, and sharp mandibles that can take large bites from carrion. These animals also have the capacity to gorge themselves far beyond what is normal in amphipods. Thus Paralicella possesses a soft body wall that can be stretched when feeding on a large meal so that the animal swells to two or three times its normal size, and Hirondella has a midgut that expands to fill almost the entire abdominal cavity and in which it can store meat.

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