Food preferences

It must not be imagined that poly-phagous and oligophagous species are indiscriminate in what they choose from their acceptable range. On the contrary, some degree of preference is almost always apparent. An animal is said to exhibit a preference for a particular type of food when the proportion of that type in the animal's diet is higher than its proportion in the animal's environment. To measure food preference in nature, therefore, it is necessary not only to examine the animal's diet (usually by the analysis of gut contents) but also to assess the 'availability' of different food types. Ideally, this should be done not through the eyes of the observer (i.e. not by simply sampling the environment), but through the eyes of the animal itself.

A food preference can be expressed in two rather different contexts. There can be a preference for items that are the most valuable amongst those available or for items that provide an integral part of a mixed and balanced diet. These will be referred to as ranked and balanced preferences, respectively. In the terms of Chapter 3 (Section 3.8), where resources were classified, individuals exhibit ranked preferences in discriminating between resource types that are 'perfectly substitutable' and exhibit balanced preferences between resource types that are 'complementary'.

Ranked preferences are usually seen most clearly amongst carnivores. For instance, Figure 9.14 shows two examples in which carnivores actively selected prey items that were the most profitable in terms of energy intake per unit time spent dealing with (or 'handling') prey. Results such as these reflect the fact that a carnivore's food often varies little in composition (see Section 3.7.1), but may vary in size or accessibility. This allows a single measure (like 'energy gained per unit handling time') to be used to characterize food items, and it therefore allows food items to be ranked. In other words, Figure 9.14 shows consumers exhibiting an active preference for food of a high rank.

preference is defined by comparing diet with 'availability'

ranked preferences predominate when food items can be classified on a single scale...

For many consumers, however, especially herbivores and omnivores, no simple ranking is appropriate, since none of the available food items matches the nutritional requirements of the consumer. These requirements can therefore only be satisfied either by eating large quantities of food, and eliminating much of it in order to get a sufficient quantity of the nutrient in most limited supply (for example aphids and scale insects excrete vast amounts of carbon in honeydew to get sufficient nitrogen from plant sap), or by eating a combination of food items that between them match the consumer's requirements. In fact, many animals exhibit both sorts of response. They select food that is of generally high quality (so the proportion eliminated is minimized), but they also select items to meet specific requirements. For instance, sheep and cattle show a preference for high-quality food, selecting leaves in preference to stems, green matter in preference to dry or old material, and generally selecting material that is higher in nitrogen, phosphorus, sugars and gross energy, and lower in fiber, than what is generally available. In fact, all generalist herbivores appear to show rankings in the rate at which they eat different food plants when given a free choice in experimental tests (Crawley, 1983).

On the other hand, a balanced preference is also quite common. For instance, the plate limpet, Acmaea scutum, selects a diet of two species of encrusting microalgae that contains 60% of one species and 40% of the other, almost irrespective of the proportions in which they are available (Kitting, 1980). Whilst caribou, which survive on lichen through the winter, develop a sodium deficiency by the spring that they overcome by drinking seawater, eating urine-contaminated snow and gnawing shed antlers (Staaland et al., 1980). We have only to look at ourselves to see an example in which 'performance' is far better on a mixed diet than on a pure diet of even the 'best' food.

There are two other important reasons why a mixed diet may be favored. First, consumers may accept low-quality items simply because, having encountered them, they have more to gain by eating them (poor as they are) than by ignoring them and continuing to search. This is discussed in detail in Section 9.5.3. Second, consumers may benefit from a mixed diet because each food type may contain a different undesirable toxic chemical. A mixed diet would then keep the concentrations of all of these chemicals within acceptable limits. It is certainly the case that toxins can play an important role in food preference. For instance, dry matter intake by Australian ringtail possums (Pseudocheirus peregrinus) feeding on Eucalyptus tree leaves was strongly negatively correlated with the concentration of sideroxylonal, a toxin found in Eucalyptus leaves, but was not related to nutritional characteristics such as nitrogen or cellulose (Lawler et al., 2000).

Overall, however, it would be quite wrong to give the impression that all preferences have been clearly linked with one explanation or another. For example, Thompson (1988) reviewed the relationship between the oviposition preferences of phytophagous insects and the performance of their offspring on the selected food plants in terms of growth, survival and reproduction. A number of studies have shown a good association (i.e. females preferentially oviposit on plants where their offspring perform best), but in many others the association is poor. In such cases there is generally no shortage of explanations for the apparently unsuitable behavior, but these explanations are, as yet, often just untested hypotheses.

Energy —

- /

1 \ ~

1

1 1 1

10 20 30 Length of mussel (mm)

10 20 30 Length of mussel (mm)

Energy value j_L

Energy value j_L

6 7 8 9 Prey length (mm)

LL 10

Flies selected Flies available

Flies selected Flies available

LL 10

5 6 7 8 9 10 Prey length (mm)

Figure 9.14 Predators eating 'profitable' prey, i.e. predators showing a preponderance in their diet for those prey items that provide them with the most energy. (a) When crabs (Carcinus maenas) were presented with equal quantities of six size classes of mussels (Mytilus edulis), they tended to show a preference for those providing the greatest energy gain (energy per unit handling time). (After Elner & Hughes, 1978.) (b) Pied wagtails (Motacilla alba yarrellii) tended to select, from scatophagid flies available, those providing the greatest energy gain per unit handling time. (After Davies, 1977; Krebs, 1978.)

... but many consumers show a combination of ranked and balanced preferences

mixed diets can be favored for a variety of reasons

Figure 9.15 Switching. (a) A lack of switching: snails exhibit a consistent preference amongst the mussels Mytilus edulis and M. californianus, irrespective of their relative abundance (means plus standard errors). (After Murdoch & Stewart-Oaten, 1975.) (b) Switching by guppies fed on tubificids and fruit-flies: they take a disproportionate amount of whichever prey type is the more available (means and total ranges). (After Murdoch et al., 1975.) (c) Preferences shown by the individual guppies in (b) when offered equal amounts of the two prey types: individuals were mostly specialists on one or other type. (d) Switching by sticklebacks fed mixtures of Gammarus and Artemia: overall they take a disproportionate amount of whichever is more available. However, in the first series of trials, with Gammarus availability decreasing (closed symbols), first-day trialists (■) tended to take more Gammarus than third-day trialists (•), whereas with Gammarus availability increasing, firsts (□) tended to take less Gammarus than thirds (o). The effects of learning are apparent. (After Hughes & Croy, 1993.)

The preferences of many consumers are fixed; in other words, they are maintained irrespective of the relative availabilities of alternative food types. But many others switch their preference, such that food items are eaten disproportionately often when they are common and are disproportionately ignored when they are rare. The two types of preference are contrasted in Figure 9.15. Figure 9.15a shows the fixed preference exhibited by predatory shore snails when they were presented with two species of mussel prey at a range of proportions. The line in Figure 9.15a has been drawn on the assumption that they exhibited the same preference at all proportions. This assumption is clearly justified: irrespective of availability, the predatory snails showed the same marked preference for the thin-shelled, less protected Mytilus edulis, which they could exploit more effectively. By contrast,

Figure 9.15b shows what happened when guppies (fish) were offered a choice between fruit-flies and tubificid worms as prey. The guppies clearly switched their preference, and consumed a disproportionate number of the more abundant prey type.

There are a number of situations in which switching can arise. Probably the most common is where different types of prey are found in different microhabitats, and the consumers concentrate on the most profitable microhabitat. This was the case for the guppies in Figure 9.15b: the fruit-flies floated at the water surface whilst the tubificids were found at the bottom. Switching can also occur (Bergelson, 1985) in the following situations:

1 When there is an increased probability of orientating toward a common prey type, i.e. consumers develop a 'search image' for abundant food (Tinbergen, 1960) and concentrate on their 'image' prey to the relative exclusion of nonimage prey.

9.5.2 Switching switching involves a preference for food types that are common

r 80

Expected if no preference

Expected if no preference

0 0.2 0.4 0.6 0.Î Proportion of tubificids available

Expected if no preference

0 0.2 0.4 0.6 0.Î Proportion of tubificids available

0 0.2 0.4 0.6 0.8 1.0 Proportion of tubificids in diet

Proportion of Gammarus available

when might switching arise?

2 When there is an increased probability of pursuing a common prey type.

3 When there is an increased probability of capturing a common prey type.

4 When there is an increased efficiency in handling a common prey type.

In each case, increasingly common prey generate increased interest and/or success on the part of the predator, and hence an increased rate of consumption. For instance, switching occurred in the 15-spined stickleback, Spinachia spinachia, feeding on the crustaceans Gammarus and Artemia as alternative prey (Figure 9.15d) as a result of learned improvements in capturing and handling efficiencies, especially of Gammarus. Fish were fed Gammarus for 7 days, which was then replaced in the diet, in 10% steps, with Artemia until the diet was 100% Artemia. This diet was then maintained for a further 7 days, when the process was reversed back down to 100% Gammarus. Each 'step' itself lasted 3 days, on each of which the fish were tested. The learning process is apparent in Figure 9.15d in the tendency for first-day trialists to be more influenced than third-day trialists by the previous dietary mix.

Interestingly, switching in a population often seems to be a consequence not of individual consumers gradually changing their preference, but of the proportion of specialists changing. Figure 9.15c shows this for the guppies. When the prey types were equally abundant, individual guppies were not generalists - rather, there were approximately equal numbers of fruit-fly and tubificid specialists.

It may come as a surprise that a a plant that 'switches' plant may show behavior akin to switching. The northern pitcher plant Sarracenia purpurea lives in nutrient-poor bogs and fens, circumstances that are thought to favor carnivory in plants. Carnivorous plants such as pitcher plants invest an excess of carbon (captured in photosynthesis) in specialist organs for capturing invertebrate prey (effectively nitrogen-capturing structures). Figure 9.16 shows how relative size of the pitcher keel responded to nitrogen addition to plots in Molly Bog in Vermont, USA. The more nitrogen that was applied, the larger the relative keel size - this corresponds to an increase in size of the noncarnivorous keel of the pitcher and a decrease in size of the prey-catching tube. Thus, with increasing nitrogen levels, the capacity for carnivory decreased while maximum photosynthesis rates increased. In effect, the plants switched effort from nitrogen to carbon capture when more nitrogen was available in their environment.

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