Acorns as Food

1. 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

2. 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

3. 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% • Less fatty acids 5-10% fat • Yields estimated for current year

4. 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

5. R/B with 2 age-classes of acorns

6. 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? • How do oaks spread up hill? • Buried acorns more likely to survive

7. 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 years • Predator saturation • Hastings Reserve UC Berkeley • High deer populations sustained un-naturally can conteract this strategy

8. 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.

9. 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?

10. Coevolution of Oaks and Squirrels Embryo

11. 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!!

12. Coevolution of Oaks and Squirrels Tannic acid concentration low to high Embryo

13. 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?

14. Coevolution Squirrels and Oaks They “prefer” acorns infested with the acorn weevil Curculio spp

15. Coevolution Squirrels and Oaks

16. 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.

17. Summary Table