What is profitability? Describe it in your own words. How is it calculated?
Profitability is the measure of the net energy gain from eating a food source. Energy is expended in handling the foot item, and energy is gained by eating the food item. Handling time may waste valuable time searching for and consuming other items. Profitability is calculated as E/h, the energy gained by eating the item divided by the time handling the item.
You are standing in front of a McDenny's and it is dinner time. You consider ordering chicken nuggets, but you are very hungry and not quite sure they will satisfy you. There is a supermarket not too far away, it would probably take you 30 minutes to go to the supermarket and buy a steak. And while the steak has 500 calories, it also takes 20 minutes to cook. The line at McDenny's is not too long, and it will only take you about 7 minutes to get your food, but chicken nuggets only contain 220 calories. Should you stay and order the nuggets or go to the supermarket and get a steak?
The profitability, E/h of chicken nuggets is 220/7= 31.4. E/T for the steak is 500/(20+30)= 10. You should eat the nuggets; you know you want to.
You are walking in the woods, and are very hungry. Berries are very plentiful where you are, but the do not contain as many calories as apples, which are not quite as easy to find. As you are walking along, you come across an apple tree. Should you spend your time eating the apple, or spend your time eating berries, which you would be able to eat continuously?
You should eat the apple. You should always eat the more profitable food source if you find it. The contingency theory assumes you have the less profitable food item in front of you and are deciding whether to eat it or search for better food items. If you have found the more profitable food item, it always pays to eat it.
Describe in your own words why an animal should leave a food patch when the rate of consumption lowers beyond a certain point.
As an animal consumes the food in a given patch, the food in that patch becomes scarcer, and so the rate of consumption decreases, even though the total consumption is still increasing. After the optimal time to stay at any one patch, it would be more profitable to search for a fresh patch where the rate of consumption would be much higher.
The above graph shows you the rate of calorie consumption as a function of time spent at one food source. Food is running out, and you know there is another, completely fresh source about 70 minutes away. What is the total time you should spend at the source you are currently at before leaving for a new source?
We know the search time until we find the next source is 70 minutes. We draw a line from 70 minutes that is tangent to the curve, and we find that it touches the curve at a handling time of 40 minutes. This means that after we have been at our current food source for 40 minutes, we should leave for the new source.