BLOG 6 – FORAGING BEHAVIOR AND THE NATURE AROUND ME

The foraging theory studies how animals forage or search for their food to be able to survive. It is a branch of behavioral ecology or ethoecology which is the study of the evolutionary and ecological basis for the animal’s behavior and how it adapts to its environment.

Foraging Behavior, as stated earlier, is the manner on how animals search for their food. There are two groups where an animal can belong according to how it forages, namely foraging specialists and the foraging generalists. As the name implies, foraging specialists are selective in the food they consume. Meaning, they only eat a certain species like most herbivores do, except those which consume a variety of plants. Some examples of food specialists are koalas which only eat eucalyptus leaves, lynx which eats snowshoe hares and some insect species that eat a single plants species. On the other hand, the other category is the opposite of the food specialists. This group eats a variety of food, from different kinds of plants to meat or any type of either plant or meat. Omnivores, those who eat both plants and animals, are usually generalists. Some examples of the animals belonging to this group are opossums which eat from insects and berries to garbage, humans who eat a variety of plants and animals, raccoons which eat berries, insects, eggs and small animals and many more.

The Optimal Foraging Theory

Natural selection favored the maximized consumption of energy through foraging in the shortest time possible. This is referred to as Optimal Foraging. The Optimal Foraging Theory (OFT) predicts the foraging behavior to determine the optimal forager’s behavior. An optimal forager is an organism that can efficiently maximize food intake. It was first proposed by Robert MacArthur together with Eric Pianka and another paper proposed by J. Meritt Emten. OFT is an idea in ecology based on the study of foraging behavior that states the organism’s forage in a manner of acquiring much calories or any nutrient needed in the least time possible. The theory also explains how animals decide what to eat and why they choose it.

Fig. 1. Model of Optimal Foraging Theory

Fig.2 An Example of Optimal Foraging Theory

One Youtube video taken by David Attenborough shows how a certain animal managed to cope with its environment. The video contained how the crow efficiently used its surrounding, a city, to be able to lessen the energy spent to acquire the crow’s needed nutrients. Carrying a nut, the crow dropped it on the road and waited for vehicles to pass by and eventually crack it open; doing so, the crow was successful in preventing loss of too much energy trying to crack the shell of the nut. Now the nest challenge, how can the crow safely consume the cracked nut? Well, our little crow had that in min, too. It made sure that the nut will be dropped in a pedestrian lane, so that when the green light flashes, it would go towards the crack nut and eats it. A very fascinating way on how the crow adapted to the city life of Japan.

Another video, also found in Youtube, is about the BBC’s Planet Earth─Unique Dolphin Hunting Technique. The dolphins were able to reach the shore where food is easier to find by riding the waves. Thus, saves them energy rather than swimming in the vast ocean in search of food. Tail-slapping is the technique dolphins use to stun their pray and consume it. But due to the shallow waters, it doesn’t seem to work. Instead it resorted to gaining some speed and hiding and waiting for its prey. The momentum of its speed enabled them to get close to its prey and eventually consumes it. 

  

Assumptions of OFT

Predators usually select the most fruitful prey; however it is not always abundant to the environment so they resort in eating other prey types that may be easier to find. Energy is not the only nutritional requirement after all. Variation allows a more sufficient diet for the predators themselves.

There are some assumptions offered by the OFT. First, due to Darwins’ theory of evolution, the individual’s contribution to the next generation is based on how it searches for food. Second, there should be inheritance of foraging behavior from parent to offspring. Lastly, the currency of fitness which is the relationships between foraging behavior and fitness.

Models associated with OFT

There are 3 main models based on the rate of maximization explained by the OFT. The first one is the Marginal Value Theory or Patch choice theory. Animals encounter patches of food when they travel. The longer the time the animal spends in the patch, the lesser energy or nutrients it gains. This results to the animal moving to a new patch when the rate of gain on a patch equals the maximum rate of gain. Thus, the more the animal moves from patch to patch the higher the nutrients it gains. When the profitability of patch equals the profitability of the average patch and time for search or travels to a new patch this is the time the animal should leave the patch and search for another. The formula for this is stated below:

dE (h) / dh = E (h) / (s+h)

wherein:

·         energy (E) à acquired net calories by consuming prey

• Handling time (h) time consumed form capturing to digesting
• Search time (s) encounters of items of same food type; ease of locating it.

The second model is the Contingency theory or Prey Choice Model which is the selective way of acquiring food. As stated earlier, a predator does not always eat what’s in front of it. For example, shorecrabs eat muscles that are small instead of trying to crack a large muscle with hard shells. Thus, saves energy and increase their net food intake. The variables used in this model are the same as the ones used in the patch use model although there is an added variable. That variable is T which corresponds to the total time foraging (sum of searching and handling times). The formula for the model is:

IF  E₂/h₂ > E₁/(s₁+h₁)

Then the animal should eat food type no. 2 (E2). When there is abundance in food, the animal can be choosier on what it eats. In nature, the model does not exactly apply but is close.

The last model is the Central Place Foraging which means that the food the animal finds is being brought to its storage site or to its offspring. This is commonly observed in birds which deliver worms to their young. Examples of animals that deliver it to the storage site are bees that store collected nectar in their hive.

 Foraging tactics

            Organisms in nature have developed different foraging tactics to more efficiently gather and consume the nutrients they need for survival. There are three major tactics used by animals depending of the tropic level and food type. The different animals are grouped under croppers, active hunters and sit-and-wait hunters. In croppers, the most common foraging tactic makes use of the density of food supply; time and energy devoted to ingesting and digesting the food; less effort in search or capture; ensures high quantity of low quality foods and animals mostly under this group are herbivores and filter feeders. The second group are active hunters. Unlike croppers they use food in low density, patchy or difficult to catch; time and energy devoted to capture or search; less energy use in ingesting and digesting; low quantity of high quality and they mostly compose of mammalian carnivores and frugivorous primates. Last but not the least, the sit-and-wait hunter, which is the least common among the three, uses dense and mobile food; little energy to any component of foraging except capture and digestion; very low quantity of high quality food offset by very low costs as to croppers and active hunters and those organisms included in this group are some predatory fish, ant lions and web spiders.

The foraging tactics in video 1 are active hunters. (Note: Video 2-4 are not working).

Functional Responses

            Functional responses explain how the consumption rate of an individual changes with respect to resource density. Holling identified three types of functional response namely type 1, type 2 and type 3. Type I (linear) response in which the attack rate of the individual consumer increases linearly with prey density but then suddenly reaches a constant value when the consumer is satiated. Type II (cyrtoid) functional response in which the attack rate increases at a decreasing rate with prey density until it becomes constant at satiation. Cyrtoid behavioral responses are typical of predators that specialize on one or a few prey. For example, small mammals destroy most of gypsy moth pupae in sparse populations of gypsy moth. Type III (sigmoid) functional response in which the attack rate accelerates at first and then decelerates towards satiation. Sigmoid functional responses are typical of generalists natural enemies which readily switch from one food species to another and/or which concentrate their feeding in areas where certain resources are most abundant. For example, many predators respond to kairomones (chemicals emitted by prey) and increase their activity. Polyphagous vertebrate predators (e.g., birds) can switch to the most abundant prey species by learning to recognize it visually. 

                Holling (1959) suggested a model of functional response which remains most popular among ecologists. This model is often called "disc equation" because Holling used paper discs to simulate the area examined by predators. This model illustrates the principal of time budget in behavioral ecology. It assumes that a predator spends its time on 2 kinds of activities namely searching for prey and prey handling which includes chasing, killing, eating and digesting.

            Numerical response means that predators become more abundant as prey density increases. The most simple model of predator's numerical response is based on the assumption that reproduction rate of predators is proportional to the number of prey consumed. This is like conversion of prey into new predators. For example, as 10 prey species are consumed, a new predator is born. Aggregational response is better than "numerical response" because it is not ambiguous. Aggregational response was shown to be very important for several predator-prey systems. Predators selected for biological control of insect pests should have a strong aggregational response. Otherwise they would not be able to suppress prey populations. Also, aggregational response increases the stability of the spatially-distributed predator-prey (or host-parasite) system.

THE NATURE AROUND ME

For an organism to survive, it needs to acquire its lost nutrients through the process of food consumption. Nature offered us with fascinating ways on how various organisms search, capture and consume their prey. Because of man’s curiosity, we have developed different concepts regarding them. I remember once I was watching my son and playmates play with the spiders when about 3 years ago. I recall they the spiders on a stick and the rest was up to them. Of course, in a match there is always a winner or a loser, too bad for the loser because it had to be tied up and enclosed in its foe’s web. It’s a way how spiders adapt to the environment so that they won’t lose much of their energy in immobilizing their prey simply by wrapping them up. My son placed the winning spider and the losing one in the matchbox and he told me that the spider will drink up all that is inside the spider and only the skin will be left. After a few time passed, we checked it out and indeed the web and the skin only remained, though I did not bother to touch it.

Another usual experience I always observe is the case of a lizard waiting for something on the ceiling. Different kinds of insects, especially small ones, tend to gather round in any source of light such as light bulbs. The lizard patiently waits for the right moment to stick its tongue out and have itself a meal. Eventually, of course, it did because there are many insects along the area. I noticed that it decided to go to a place where it would acquire most of the nutrients it needed. Being cautious and making its best efforts not to alarm the insects so that they won’t fly away, it was able to take in nutrients with it expending not too much energy. Same goes when we take our lunch at around 1 pm during weekends that we just see the lizards coming out and looking at us while we eat. Being pitiful about it, we would try some unusual things to happen- pinch off a small part of our food and give it to the lizard. This was possible when the lizard fell down from the ceiling to catch for some food in our table. Trying to place the food very near the lizard caused them to crawl fast, so, we decided to throw it near them. Sometimes only one would eat, sometimes the lizard would bring the food to its companion and eat together. There is one though, which has become accustomed to living with humans. It can actually come near us or near our plates. Every time we see it trying to find the remains of our food in the plate, we will all wait and then laugh. Why? It is because the plate is really close to us, yet, the lizard conquers its fear. The lizard’s nature, as I see it, comes to a place where it can find food mostly in large amounts.

In both experiences, I’ve learned that organisms have different ways of foraging techniques that help them acquire the nutrients they need. They choose the best way possible to consume and catch their prey by expending the least energy possible, as explained by the optimal foraging theory.  Because of this, the science of behavioral ecology grows more and more as time passes by due to the adaptive mechanisms every organism develops to survive their environment.