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Brown Bear Diets - A tale of Feast and Famine Part 1:Grazing and Clamming

9/17/2012

Before I start this particular blog (or chapter may be more appropriate) I think it's important to point out that the life of a bear could be defined as a race. Each year is a race to put fat on before there is no food and the bear must hibernate. When they enter hibernation they have to count on the fat reserves they were able to store over summer. If there is not enough fat they could die during hibernation, or during the following spring before the summer growing period starts.
A female will only produce cubs if she has enough fat reserves when she goes into hibernate. Years of poor vegetation growth or salmon runs are followed by years of little to no cub production. A study of Katmai brown bear females found that they averaged losing 150 lbs when hibernating (approximately 30 to 40 % of their fall weight), most of which was fat. They need to get all of that back each year.
The questions below are one's I commonly receive so I put this blog in a question and answer format. I hope you enjoy reading it!

What do bears eat?

    Coastal brown bears have access to 4 different food groups: 1) vegetation, 2) clams and other intertidal invertebrates, 3) berries, and 4) the food item they are most associated with, salmon.  Vegetation is the most available, and green areas along the tidal zone appear first (as early as May 15).   These early green ups are critical for bears emerging from dens that have lost so much weight.

What type of vegetation do the bears eat?

    They won't graze on all the plants but brown bears have been recorded eating over 200 plant species so they are probably much more herbivorous than you thought. Goose tongue and arrow grass are some of their favorites in the Lake Clark National Park area.
The vegetation itself benefits from it's coastal location. First, the warming effects (just above freezing is warm for coastal Alaska in April-May) allow these zones to be the first ice free areas and the plants begin to grow. Second, each time the area is flooded with high tides it's essentially fertilized from nutrients in the water from the ocean. The slow retreat of the tide allows the vegetation to capture additional large particulates that adds important nutrients. A high tide essentially fertilizes the vegetation, and the bears take advantage of the extra growth and essential nutrient the plants provide.

Do bears digest vegetation as well as a cow?

     Even though they seem to graze like cattle, bears are not near as efficient as extracting calories and nutrients from vegetation as a cow. One research model predicted that a brown bear must eat for at least 12 hours and ingest 20 lbs to just maintain it's weight This large amount of forage is necessary partly because of their poor ability to digest vegetation and partly due to their large size. There have been numerous studies with conflicting results on how bears fare as herbivores, which is the most common part of their diet when salmon are not available. Interior bears may eat up to 90% vegetation in a season, but most coastal bears switch to more nutritious salmon when they begin to show up. Most studies found emerging coastal brown bears continue to lose weight after hibernation at least until the vegetation green up occurs in Mid May. Most scientists estimate they maintain the weight after the green up by grazing, with maybe some minor weight gain. It's obvious that coastal brown bears must have access to more nutritious sources to put on weight, and reach their large size.   As a fellow photographer I know the frustration of following grazing brown bears that seemingly never lift their heads. And Murphy always rules as their head seems glued down when you get that rare sweet light. I do ask however that you remain patient and not disturb the grazing bears. All that effort they are putting in for up to 12 hours just to maintain weight is a lot, and we don't want to negatively affect them by scaring them and reducing that valuable grazing time. They will pick that head up, just be ready!
    Berries can be important very early or late in their active season. The high sugar content can be crucial  to provide the extra fat, especially after the salmon run, that bears need as they near the end of their race to regain weight in late fall.  Key berries in Katmai and Lake Clark that brown bears consume include blue berries, salmon berries, elder berries, and nagoon berries.   Berry crops not eaten the fall before also may provide forage just after hibernation if not covered by snow and ice.

Are the bears really clamming out at low tides?
You bet! Along the coast, many bears will hunt for the clams uncovered during low tides. The majority of clams eaten on the Alaskan coast are Pacific razor clams. Studies of Alaska bears found they like razor clams, soft shell clams, and a few types of molluscs, and polychaete or tube worms they find in the sand. They probably use all of their senses to find them as they walk around with their nose close the ground. Smell, vision, and even feeling the clam move though it's paws make them so successful. A study where bears were observed clamming on 233 days over a 3 year period found that the average bear ate a clam almost each minute it foraged. Many ate from 50 to 100 in the short periods the tide was low enough. Researchers found some bears are right or left pawed, but I've seen the same bear use both. How they break and extract the clam from the shell can be different between bears as well. Clams are only available for a few hours and only about 15 days a month when the tide is low enough to expose them so not all bears use them. However, they provide another important pre salmon food source, which contains 3 times the amount of protein as a similar amount of vegetation. One study found that a brown bear that ate 50 - 100 clams in a couple hours was able to reduce it's necessary grazing time by 25%. As I mentioned above, this valuable time could allow her to add more fat, and be more successful in making sure she can make it through the winter.

Why don't more bears make use of the clams that are exposed during low tides?

That is an excellent question and not one easily answered. Observations of clamming bears throughout the coast discovered 2 key points, sows with cubs are seen clamming more than you would expect based on their numbers, and large boars are rarely seen on the beach. In fact, in the study above, 45% of the bears observed were females with cubs, and 55% were "unidentified gender and smaller (< 500 lbs) bears". Why large boars don't frequent beaches given the higher protein content is unknown. Some scientists hypothesized that because of their large size, the energy required to walk that far for a smaller amount of food may not be worth it for a large boar. Why females with cubs are found more seems fairly easy to explain. First, lactating females need more and richer foods, as do their rapidly growing cubs. Second, for a Mother bear to find an area to forage where the main cub predator, large boars, is absent makes life easier on them.

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Brown Bear Diets - A tale of Feast and Famine Part 1:Grazing and Clamming

Pictures from Katmai 2012 posted.

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