Objectives:

• To understand the basic concept of optimal foraging theory and its relevance in ecology.
• To analyze the behavior of a sit-and-wait (ambush) predator whose optimal foraging strategy is to maximize the energy that it gains.

Theory:

Behavioral ecology deals with the evolutionary mechanisms for animal behavior due to pressure from the ecological system. This investigates how the behavior of an organism is related to the ecosystem. In the past, ecologists took little importance to study the behavior of organism, but later the studies on survival and reproduction made the behavioral studies more important in ecology. An organism performs so many things such as food searching, defense mechanisms, mate location etc for their growth and survival. The habitat and the food on which the organism depends are critical factors to study the behavior of the organism. Most of the research on behavioral ecology looks at the problems of the organisms such as obtaining its food for survival, avoiding being eaten and reproductive mechanisms.  Foraging defines the searching of food and exploiting the food resources by the organism. Foraging theory is a branch of behavioral ecology that deals with the foraging behavior of the organisms with respect to the environment where the organism lives.  Optimal foraging is a field in which biologists have used optimization theory to make quantitative predictions about the feeding behavior which can be then be tested by observation and experiment.  The assumption of optimal foraging theory is that the individuals will be energy maximizers or time minimizers. Energy maximizers try to find most energy from the ecosystem while the time minimizers try to get energy in a least time. Since the energy is a limiting factor, such an approach is useful to study the organism’s behavior in the ecosystem. Optimal foraging theory was first proposed by Robert MacArthur, J M Emlen, and Eric Pianka in 1966. Optimal foraging illustrates the organisms forage in such a way as to maximize their net energy intake per unit time.  The first assumption of the optimal foraging theory is natural selection will only favor behavior that maximizes energy return. Understanding the rules that shape the foraging behavior of individuals in the ecosystem has been a central focus of behavioral ecology for the last decades (Pyke et al. 1977). 

Optimal Foraging

Optimal foraging theory defines the nature of the organisms forage in such a way to maximize the net energy intake of the organism per unit time. The organism behaves in such a way to consume food having most calories with in a least time period. Optimal foraging theory uses predators for the analysis. Predators are categories into two searching and sit-and-wait. A searching predator moves throughout its habitat and find its prey. That means actively foraging predators are characterized by their frequent wandering movements.  A sit-and-wait predator waits for its prey to near its point of observation. That is some predators attack their prey from ambush, whereas others usually hunt while on the move. A sit-and-wait strategy is mostly relying on moving preys or high prey mobility and the prey density must be relatively high. In order to favor the sit-and-wait tactic, predator’s energy requirements must be low. Whereas searching predators encounter and consume non-moving types of prey population. The success of foraging pattern of ‘searchers’ is influenced by the prey density and prey mobility along with the predator's energetic requirements. Generally it should be higher than those of sit-and-wait predators. However the searching abilities of the predator and the spatial distribution of its prey are paramount. The sit-and-wait foraging mode is less common during periods of prey scarcity than the widely-foraging pattern.  Some common examples of ambush predators include snakes, fish and other reptiles such as crocodiles as well as birds, some mammals and spiders.

 Case Studies on Sit and Wait Predator

Foraging of Kingfisher

Kingfisher is a sit-and-wait predator (Figure 1) whose optimal foraging strategy is to maximize the energy that it gains during each foraging course. It usually waits for prey to come within a striking distance. It waits in one place for long periods of time and makes decisions regarding when to hunt or when not to hunt the prey that it sees. A major part of this decision depends on how far the prey is from its predator. The Common Kingfisher hunts from a perch above the water, on a branch, beak pointing down as it seeks for prey. When food is detected, it dives steeply down to grab its prey. For example, consider a kingfisher waiting on a perch on a branch, looking down at a river and choosing which fish to go for. For the sake of understanding, let us assume that the pattern of the foraging area as a semicircle around it and the size and behavior of all the fishes are same. When a kingfisher takes decision to grab a particular fish, it dives from its perch, seizes the fish, and comeback to its perch.

Figure 1: Kingfisher,  a sit-and-wait predator

For getting more details about the mathematics of foraging of sit and wait predators, Refer Population Ecology II, Optimal Foraging: Sit-and-wait Predators that Maximize Energy

Optimal foraging in crows

Crows (Corvus caurinus) in coastal area of Canada feed on shellfish.  They hunt for whelks (Thais lamellosa) at low tide on the west coast. Having found a whelk, they fly with it in their beak to above a nearby rock. They stall and drop the whelk from the air hence to smash its shell on the rock thereby exposing the flesh inside it. A crow has to drop each whelk several times to break the whelk open. A crow has to expand its energy to fly upwards.  The crows might drop whelks from a height at which the crows would minimize the total upward vertical flight required per whelk eaten. If whelks are dropped from near the ground, many drops are required to break open the shell (Zach, 1979). Zach calculated the dropping height that minimizes the total upward vertical height is close to average of 5.2 m. Zach also suggested that greater the height from which a whelk is dropped, the more the chance that the whelk will fragment into small fragments, among those some are too small to retrieve. This may be the reason why the crows usually fly to a height of about 5 to 5.5 m rather than above 10 m.

Factors affecting the foraging behavior

1. The energy spent in waiting and pursuit of the prey depends on the size of the foraging area. Increasing the size of the foraging area decreases the time and energy spent in waiting for a fish (prey) to come into sight as there is a chance of choosing more fishes.
2. Increasing the size of the foraging area increases the average time and energy spent in capturing the prey, because the kingfisher has to fly longer distance to catch its prey.
3. The area of the foraging system determines the abundance of the prey in that ecosystem(More prey in the maximum foraging area) In case of Kingfisher, this means that increasing the size of the foraging area decreases the waiting time of the kingfisher for fish to appear. Waiting time is then proportional to the reciprocal of the total abundance.