Optimal Foraging

All animals face the problem of finding resources for growth, maintenance and reproduction. It is assumed that natural selection should tend to produce animals that are very efficient at propagating their genes, and hence at doing everything else, including finding food and mates. At some point in an animal’s life it may experience starvation, and prolonged starvation can lead to death. By natural selection, the animals that survive are able to pass their genes to the next generation, while the genes from animals that die are eliminated along with their unsuccessful foraging behaviour. Foraging (food seeking) in a patchy environment requires complex decisions, such as where to forage and for how long. To make these decisions animals should acquire information from the environment. The questions we may ask include: Should a predator eat only the most nutritious prey? What other factors should be taken into account in its choice of prey?

To optimise a diet

Different foods have different values. In order to optimise a diet, a forager must be able to distinguish between different food items and select the most profitable. This holds for all predators, insects, parasitoids, etc… Many studies have shown that foragers prefer the most profitable food, eg: the size of mussels preferred by crabs was calculated by being the most profitable. Mussels that are too small have a very low energy content while very large ones require a lot of effort (time and energy) to open. So, should the forager only eat the most profitable food items? To what extent should it include less profitable food in its diet? This depends on how long it takes to find the most profitable ones, i.e. how abundant they are both in general and in relation to other food items. The longer the forager has to spend looking for each food item, the more energy it uses, therefore the lower the net gain in energy. To calculate how many food types the forager should accept in its diet, travel time must be considered. Foods are ranked in order of profitability. The more food categories in the diet, travel time decreases, but so does the average profitability of food eaten. By combining the profitability and travel time, we can calculate the number of food types to include in the diet (the optimal diet breadth). In a poor habitat, where profitable foods are insufficient, there should be a wider diet breadth than in a good habitat, where profitable foods can be found with a shorter search time.

Factors affecting food choice

Ad mentioned above, energy determines diet intake. However, other factors may also be important, such as:
(1) the animal may also have requirements for specific nutrients or the animal may have an intolerance to a toxin. (Crabs prefer mussels with barnacles as they are easier to open, but don’t prefer mussels with hydrozoans, possibly because they are repelled by the hydrozoan stinging cells.)
(2) Foragers may concentrate on a particular food time if as a result of learning or a change in its digestive physiology, it got used to handle that particular food. (Bees learn how to get nectar from different flowers and may then concentrate on flowers they have become proficient at).

Choice of patches

Example:

Pick the third patch, eat until there are less than 3 foods left, because it can certainly do better somewhere else.

In many cases, food resources are patchily distributed, either in discrete units such as flowers on a bush, or just as a clumped distribution e.g. of insects in a field. The patches will differ in profitability. The animal should forage in the most profitable patches, and it should be possible to calculate the optimal patch-type breadth. The profitability of a patch declines with time as the forager feeds. Models predict that the optimal forager should stay in a patch until its rate of intake drops to a level equal to the average intake for that habitat. This is the Marginal Value Theorem. However, to do this, the predator has to know these values and follow some simple rules.

Leave after catching a certain number of prey.
Leave after a certain time in the patch.
Leave after a certain amount of time has elapsed since the last successful counter.

Factors affecting patch time in parasitoids

Trade offs and variable states

Foraging decisions depends on an animal’s internal state (hunger, age, egg load…). For ovipositing insects, age and egg load are important. Older insects and those carrying many eggs are the ones who accept low quality hosts. An ageing parasitoid with loads of eggs can afford to be less choosy. If it’s young and has small egg load it might be quiet choosy.
Female Parasitoids

Female parasitoids generally start searching for hosts shortly after emergence. However, most host species are not evenly dispersed over the habitat, but tend to cluster in groups of variable size (patches) and, thus, the parasitoid female has to divide her time between different patches to maximise the number of offspring that she can produce during her short adult life. Because of the direct link between oviposition and fitness, the behavioural mechanisms that determine the time a parasitoid spends on a patch are probably subject to strong selective pressures, and parasitoid patch-time allocation has therefore received much attention from behavioural ecologists. Researchers have shown that a wide variety of environmental stimuli affect patch-time allocation in parasitoids, but how the foraging animal integrates this information was unknown for a long time.

As newly born female parasitoids start to reproduce shortly after emergence, they lack such information and must acquire it independently while searching for suitable hosts. Female parasitoids use a wide variety of chemical stimuli to obtain information about their environment, and this enables them to restrict their foraging behaviour to that part of the habitat in which hosts are most likely to be found. Within these suitable areas, parasitoids use many types of cue as information for patch choice and patch-time decisions. This enables them to forage efficiently in the absence of information about the full distribution of hosts and host patches over the habitat. Some cues will increase the probability that the parasitoid will stay in the patch and are called incremental, whereas others decrease this probability and are called decremental.

Mechanisms for deciding when to leave a patch

Incremental mechanisms

This model considers the effect of several stimuli and integrated the influence of kairomones (i.e. substances produced by the host) that set the initial attractiveness of the patch, the progressive habituation of the parasitoid to this stimulus, and the effect of ovipositions. Each oviposition in an unparasitised host increases the motivation of the parasitoid to stay and to continue search. This mechanism results in the parasite spending more time in patches with more hosts. This model is based on only two of the stimuli affecting patch time; the kairomone and the encounters with unparasitised hosts.

Decremental mechanisms

In contrast with the predictions of this model, ovipositions decrease patch-residence time in many parasitoids, which use a decremental mechanism.

Which mechanism to choose?
Parasitoids foraging for hosts with a high variability in the number of host per patch, should use incremental mechanisms. Parasitoids foraging for a low variability in the number of hosts per patch, should use decremental mechanisms.

Some changes in the environment occur and parasitoids have evolved mechanisms to gather information. When the number of hosts in a patch is known (e.g. when the kairomone concentration is a good indicator of the number of hosts in the patch), each oviposition implies a decrease in host availability. The patch becomes less valuable compared to the rest of the environment and animals should use a decremental mechanism.

However, when information about host availability is unreliable (e.g. some patches are very rich, other extremely poor, but no specific cue is available), the timing of each additional oviposition reveals the quality of the patch and determines whether a parasitoid should continue to search it. In this case, an incremental mechanism would be expected.

When parasitoid density is high, kairomone concentration and host distribution no longer provide reliable information: host patches might have already been used by others and the hosts might already be parasitized. In this case, parasitoids should use an incremental mechanism. Parasitoids use cues other than kairomones to assess patch quality when the presence of competitors makes the kairomone concentration an unreliable predictor of patch quality.

Other cues affecting patch time
The females of several parasitoid species leave a marking substance on the substrate whilst searching, and, when a female visits a patch already searched, either by herself or by a different female, she responds to the marking substance by leaving sooner in comparison with unmarked patches. (decremental effect)

Encounters with parasitised hosts can also provide information about the current level of patch exploitation and hence about when to leave a patch. (decremental effect)

Encounters with other parasitoids. Compared with females that spent the previous day alone, females that had spent the previous day with competitors stayed longer on patches and superparasitised several hosts when foraging alone the following day. (incremental or decremental effects on patch time)

Patch leaving
The internal state of a parasitoid (e.g. egg load, and lipid and carbohydrate reserves) provides information about future life span and affect patch time.

Natural selection assumes that the mechanisms are under genetic control and that there is or has been genetic variability in patch-time allocation.