Acorns as Food

Important food resource for many species Squirrel reproduction depends on acorn

production the previous fall In poor acorn crop years deer had

Lower KFILess reproduction by yearling femalesMale mass and antlers smaller

Black bear cub survival and growth dependent on fall acorn crop

Acorns

High in carbohydrates (25-40%), but low in protein

Masting phenomenon Production varies greatly between years and by

area Oaks a significant component in eastern

forests, but might be declining Oak decline Less fire

Oaks

Monoecious Acorns develop from fertilized female flowers

Subgenera White oaks --chestnut, white, post, etc.

Rounded lobesAcorns mature in one seasonAcorns germinate in fallLower tannic acid <2%Less fatty acids 5-10% fatYields estimated for current year

Oaks

Subgenera Red/black oaks -- black, n. red, s. red, blackjack,

etc.Bristle-tipped lobesAcorns mature in two growing seasons

Two age classes of acorns on the tree at the same sideGerminate in springHigher tannic acid 6-10%Higher in fats 18-25% fatYields estimated for current year

and the following year

R/B with 2 age-classes of acorns

Coevolution of Oaks and SquirrelsSquirrels need oaks for food

75% of winter and spring diet 10-fold decline after poor mast year

Oaks need squirrels for seed dispersal Fail to locate up to 70% of buried acorns How did oaks spread during the

pleistocene? How do oaks spread up hill? Buried acorns more likely to survive

Coevolution of Oaks and SquirrelsOak anti-predation strategies(1) Variable seed production

Doesn’t allow the squirrel population to build up to high levels

High squirrel populations can consume nearly all acorns produced in a given year

Oaks need some escapement for germinationTherefore some years are masting years

Predator saturation Hastings Reserve UC Berkeley

High deer populations sustained un-naturally can conteract this strategy

Coevolution of Oaks and Squirrels Oak anti-predation strategies(2) Palatability, tannic acid & germination

Squirrels tend not to eat acorns after they germinate

Tap root is less digestible More palatable white oaks germinate quickly to

“minimize” time they are susceptible to predation, but must over winter as seedlings

Less palatable red/black oaks germinate in spring and over winter as acorns.

Coevolution of Oaks and Squirrels But there’s more!!

Squirrels “prefer” white oak acorns over R/B acorns becauseof the lower tannic acid concentration

So, they tend to eat white oak acorns first

Tend to cache R/B acornsBut abundance of acorns matters. too

White oaks “counter” by germinating quickly and become less palatableDownside is that they must over winter as sprouts

But what do the squirrels do?

Coevolution of Oaks and Squirrels

Embryo

Coevolution of Oaks and Squirrels• So, what do squirrels do to white oak to counteract white oak acorn germination?

• They excise the embryo, so the acorn won’t germinate.

But, what do the trees do?

Tannic acid concentration is not uniformly distributed in the acorn

Back to the cross section!!

Coevolution of Oaks and Squirrels

Embryo

Tannic acid concentration low to high

Coevolution of Oaks and Squirrels• Acid concentration near the embryo “discourages” excising the embryo

• Squirrels also will eat only the top 1/2 of the acorn -- less tannins and the acorn will germinate

• But, there’s more!!

• Acorns are high in carbohydrates, but low on protein

• So, what do squirrels do to get more protein?

Coevolution Squirrels and Oaks

They “prefer” acorns infested with the acorn weevil Curculio spp

Coevolution Squirrels and Oaks

Coevolution Squirrels and Oaks

But there’s more!!

Weevils tend to counteract the protein binding action of tannins.

So, by eating acorns w/ weevil, squirrels increase protein availability.

Characteristics White Oaks Red/Black Oaks Leaves Rounded lobes Bristle tipped lobes Germinate Fall Spring Acorn age classes 1 2 Acorns mature 1 year 2 years Tannins <2% 6-10% Fat 5-10% 18-25% Protein Low Low Palatable More Less Use Consume Cache Squirrels Excise embryo Eat top , embryo survives

Summary Table