Anthropology P380: Foraging Strategy

Foraging economics

Foraging economics

Issue: COSTS of foraging vs BENEFITS of foraging for different types of foods

What I want you to think about today is the economics of foraging -- how the costs of foraging can weigh against the nutritional (or social) benefits of foraging when people (or chimps!) are making decisions about what to eat.

What environmental variables influence a consumer's choice of foods?

  • Food Quality (e.g. avoid very lean meat)
  • Food availability
    • DENSITY
    • DIVERSITY ..... together = PRODUCTIVITY
    • PREDICTABILITY in space & time (e.g. MEAT vs PLANT)

as density, diversity and predictability diminish, Nutritional STRESS INCREASES (e.g. risk of foraging failure)

      • LATITUDE variation: from equator to arctic (temperature & seasonality change)
        • PLANT biomass: trunks/leaves & fruity/leafy env to seasonal shortage, to zippo in arctic
        • ANIMAL biomass: from small animals (lotsa insects!) to herds
      • RAINFALL ... look at resource diversity relative to biomass

How to INCREASE chances of acquiring food (reduce risk)??

  • STORE the foods you have = technology+ labor investment
  • increase BREADTH of your diet
    • acquire lower quality foods = digestive challenge!
    • or use technology to get access to more expensive foods

The costs and benefits of food can be broken down into:

searching for food (and travel)

handling the food once you have found it -- everything you need to do to get it into your mouth! (e.g. acquiring it and processing it to make it edible)

Issue: COSTS of foraging vs BENEFITS of foraging for different types of foods

The costs and benefits of food can be broken down into:

searching for food (and travel)

handling the food once you have found it -- everything you need to do to get it into your mouth! (e.g. acquiring it and processing it to make it edible)

basic economic principles should allow us to predict aspects of foraging strategy (e.g. which foods are chosen & why) on the assumption that over time animals that can optimize benefits / costs will have an adaptive advantage and evolutionary success.
Simple Optimal Diet Breadth Model

  • Foods can be RANKED economically:
  • BENEFITS vs COSTS (handling & processing)
  • common CURRENCY (e.g. energy ... or TIME )

Which foods should you prefer?
 food item  Twinkies  Tofu
 Benefits:  300 kcal/item  100 kcal/item
Handling Costs:  1 Twink / 10 minutes 1 Tofu / 10 minutes
Return Rate:

(ranking criterion)

 = 6 x300

= 1800 kcal/hour

  = 6 x100

= 600 kcal/hour

So, if their handling costs are similar, you should always prefer Twinkies!

But when should you eat TOFU? ... when its cheaper to include Tofu in diet than ignore it ... ... when Twinkies are RARE and the costs of looking for Twinkies gets too expensive

 Handling Costs

+ Equal Search Costs

 Twinkies only

6 twinks/hour = 1800kcal

 Tofu only

6 tofu/hour = 600 kcal

 Twinkies + Tofu

average=

(1800 + 600 )/2 kcal

= 1250 kcal /hour average return

 Different Search Costs:
 half the density

3 Twinks / hour
= 3 x300

= 900 kcal /hour

same density

6 Tofu / hr)

= 600 kcal /hour

 average 1500/2 = 750 kcal /hour ...

... stick to Twinkies !

 

How rare would Twinks have to be before you should eat Tofu too?

 
 2 Twinks / hr

= 2 x 300

= 600 kcal /hour

 same density

6 Tofu / hr)

= 600 kcal /hour

 average 1200/2 = = 600 kcal /hour

Forage for both!

Lets look at the mongongo nut: should the !Kung eat Mongongos?

 YES!
 NO!
 = abundant & predictable (store naturally)

= concentrated in patches:

you can "load up"!

  = patches are located far away from camps (water)

travel time = 6 hours

  = nutrient-rich (protein & fat)

flesh: 312 kcal / 100 g flesh

kernel: 650 kcal / 100 g nutmeat

Dry nuts (no flesh): 1 kg whole nuts = 1400 kcal

1 load = 12 kg whole nuts = 11,000 - 17,000 kcal

 = require lots of processing time

1 load = 1 hour roasting time + 10 hours of cracking time

= 1100-1700 kcal/hour (processing costs alone)

   17 hours of total work (travel + processing costs)

= 650-1000 kcal/hour

Include mongongos in diet ONLY when the net benefits are better than alternative foraging choices, e.g. whether or not you eat mongongos depends less on how nutritious the nuts are than how expensive they are to exploit, compared to other foods

... dependent on availability of other foods

Predictions of Diet Breadth Model:

  • Each food type will either always be ignored, or always exploited, depending upon whether it is in the optimal set
  • Food value of a resource type is NOT ENOUGH to decide whether a forager should pursue it ... foods are ranked by their profitability (net benefits/costs)
  • The decision to exploit a particular food type doesn't depend on its abundance, but on the abundance of other, preferred foods ... as more time is spent searching for preferred foods, the overall return rate will decrease and only then will less profitable foods be included in the diet

 


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Last updated: 1 October 1999
URL: http://www.indiana.edu/~origins/teach/P380syll.html
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Copyright 1998 Jeanne Sept