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 KFI Less reproduction by yearling females Male 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 lobes Acorns mature in one season Acorns germinate in fall Lower tannic acid <2%tannic acid Less fatty acids 5-10% fat Yields estimated for current year

Oaks Subgenera Red/black oaks -- black, n. red, s. red, blackjack, etc. Bristle-tipped lobes Acorns mature in two growing seasons Two age classes of acorns on the tree at the same side Germinate in spring Higher tannic acid 6-10% Higher in fats 18-25% fat Yields estimated for current year and the following year

R/B with 2 age- classes of acorns

Coevolution of Oaks and Squirrels Squirrels 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? pleistocene How do oaks spread up hill? Buried acorns more likely to survive

Coevolution of Oaks and Squirrels Oak 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 germination Therefore some years are masting yearsmasting 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 acorns But abundance of acorns matters. too White oaks “counter” by germinating quickly and become less palatable Downside 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

But there’s more!! Weevils tend to counteract the protein binding action of tannins. So, by eating acorns w/ weevil, squirrels increase protein availability.

Summary Table