Obligate vertebrate scavengers must be large soaring fliers

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Abstract

Among extant vertebrates, only the 23 species of vulture are obligate scavengers. We use an energetic modelling approach to explore the constraints imposed by an obligate scavenging lifestyle, and to ask whether obligate scavengers must always be avian and generally large-bodied users of soaring flight. Our model found that aerial scavengers always out-competed postulated terrestrial ones, mainly because flight allows area to be searched much more rapidly for carrion. Soaring was favoured over flapping flight because the reduction in flight speed (and so rate of area search) was more than compensated for by the decrease in the costs of transport. Large individual size is selected for if carrion is available in large packages, when obligate scavenger feed only infrequently, and so must be able to survive on body reserves in the periods between discovering food falls. In the absence of avian radiation, an obligate terrestrial scavenger seems energetically feasible, but we argue that such a beast is unlikely to have evolved. In birds, in order to become exclusive scavengers, vultures have needed to specialize for efficient soaring flight as a low energy form of travel, and as a consequence they have lost the agility needed to kill prey. In mammals, however, no comparable trade-off occurs. So for terrestrial carnivores there is probably no strong selection pressure towards being an exclusive scavenger. Indeed it will perhaps always be more advantageous to retain the flexibility of obtaining food by either predation or scavenging.

Introduction

Meat-eating animals can obtain a meal either by killing their own food, or scavenging it from a carcass. Many natural ecosystems have large numbers of animals dying from disease, malnutrition or accidents that provide an easy source of meat (Houston, 1979). Despite this, obligate scavengers are very rare. Among extant vertebrates, only vultures have a diet based almost exclusively on carrion (Houston, 2001), although many other predatory birds, reptiles and mammals are opportunist scavengers. Two quite separate lineages of birds (Cathartidae and Accipitridae) have independently evolved this obligate scavenging lifestyle. Here we ask whether we can identify factors that make flying animals more efficient obligate scavengers than terrestrial ones, and consider the selective pressures acting on them. The 23 species of extant vultures are generally characterized by a large body size in comparison to other groups of birds, and by a common dependence on soaring on air currents as an alternative to flapping flight.

It has frequently been argued that soaring should be attractive to an obligate scavenger because this provides a low cost means of travel (e.g. Bertram, 1979; Houston (1979), Houston (2001)). However, this argument does not consider the negative consequences of gliding being a relatively slow form of travel, restricting the rate at which area can be searched. In order to develop the argument more fully, verbal reasoning needs to be supported by quantitative calculation. Recently, we introduced a mathematical model of energy balance in an obligate scavenger, and used this to evaluate the feasibility of obligate scavenging among extinct dinosaurs (Ruxton and Houston, 2003). Here, we develop this model for evaluation of the energetic consequences of obligate scavenging among birds that fly by either flapping and soaring flight. This will allow us to quantify the energetics of obligate scavenging for these two broad categories of flight, and to explore the consequences of size of the bird for the feasibility of scavenging in a general context.

We will also extend the model to consider hypothetical mammalian and reptilian scavengers and compare these with avian ones. Although obligate scavenging in dinosaurs (and other extinct vertebrates) has often been postulated (see references in Ruxton and Houston, 2003), there has been little attempt to quantify the energetic feasibility of obligate scavenging for terrestrial animals. As with flapping and soaring birds, we would expect a trade-off between endothermy and ectothermy: endotherms have higher metabolic costs but this buys greater endurance (and so higher rates of searching). Hence, another aim of this project will be to quantify consequences of this trade-off and explore how it is affected by animal size. Finally, we will be able to compare the relative energetic feasibilities of scavenging in aerial and terrestrial taxa.

Section snippets

Methods

Our hypothesis is that the key constraint for scavengers is their ability to find food items. This can be contrasted with predators, where capturing rather than discovering prey is the main constraint, and herbivores, where processing consumed food is often the key restriction on energy gain rate. We assume that the scavenger spends a constant fraction (α) of its time searching for food items that are uniformly distributed at density (f). If, when active, the scavenger searches out an area at a

Results

Initially, we make the simplistic assumption that carrion is uniformly distributed in very small packages and Fig. 1 shows clearly that for all four life history strategies the minimum detection distance (dmin) needed for energy balance increases with body size. For any given mass (M), a soaring bird has a lower value of dmin than a flapping bird, which in turn has a lower value of dmin than the two terrestrial life history strategies. dmin is lower for reptiles than for mammals for M values

Discussion

Fig. 1 showed that for reptiles, birds and mammals dmin increased as the body mass increased, which means that it was becoming harder to balance the energy budget by obligate scavenging. This was due to increases in both travel costs and resting metabolic rate, both of which rose faster with body size than did the speed of movement. So larger scavengers can only exist if, with increasing mass, their ability to locate food increases faster than their increased food demands. However, Fig. 1 was

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