Thursday, January 28, 2010

CLAM-DIGGING BROWN BEARS

A female brown bear nimbly uses her large claws to separate the valves of a razor clam and get to the juicy morsels within. See slide show below for more on this bear. Photo by Scott W. Michael.

When it comes to food, brown bears are very opportunistic. As discussed in past posts, they are also very intelligent. Consequently, they’re always looking for new food sources that they can exploit. Along the coast of Alaska, brown bears congregate in salt marshes in spring and early summer. They feed on sedges, grasses and forbs as they wait for migrating salmon to arrive in nearby estuaries. Some bears move out of the marshes and sedge flats and feed on animals that inhabit shore habitats. Along the Katmai coast, there are large tides that expose an expansive intertidal flat that is exposed for three to five hours a day. Certain bears utilize this habitat, hunting for mussels, isopods, barnacles, marine worms (polychaetes and peanut worms) and small intertidal fishes (blennies and sand lances). While it may seem that burrowing clams would be impervious to brown bear attack, there are some bruins that have learned how to locate, capture and handle these infaunal mollusks.

The intertidal zone along the Katmai coast is home to a handful of clam species. The four species that are known to be eaten by brown bears are the soft-shelled clam (Mya arenaria), the razor clam (Siliqua patula), Nuttall’s cockle (Clinodcardium nuttallii) and the Alaska surf clam (Spisula polynyma). These mollusks spend most of their lives below the sand surface, taking in fresh seawater and planktonic food through tube like siphons. The siphons extends from the clam up to the sand surface. (Those species with a shorter siphon remain near the sand surface in order to respire and feed, while those with a longer siphon can burrow to greater depths.) Many of the burrowing clams are adapted to living in the intertidal zone. When the tide goes out, they remain buried in their wet, sandy sanctuary and await the rising tide to bring back life giving oxygen and food.

video

The slide show above documents the clamming behavior of a female brown bear, with three spring cubs (the latter are not seen in the slideshow). Photos by Scott W. Michael

Clam-hunting Techniques

At low tide along the Katmai coast, you may see an individual brown bear, or a bruin family (sow a cubs), slowly meandering across the tidal flat with nose skimming the sand surface. Somehow the bears detect the buried clams (possibly by using their keen olfactory senses). When they do, the ursid will stop and begin digging at a leisurely pace. The mollusk-hunting bear will lean on one front paw and use the other as a shovel. It will methodically lift the substrate from the hole and push it back behind the excavation. If the clam is deeper in the sand, the bear might lean on its elbow, laying its head on the foreleg, in order to thrust the digging paw deeper into the substrate. When it reaches the bivalve, it will extract it by lifting it out with its claws/paw or by shoving its head into the hole and grasping the clam in its jaws. Troyer (2005) reports that like fishing, bears vary in their clam-digging prowess. Some bears are very effective “clamers,” while others are inept. If a bear’s digging activity never or rarely results in positive reinforcement, they give up this activity after a time. Troyer also notes that when it comes to clam-digging, some bears are exclusively “right-pawed,” while others only use their left paw.

In a study conducted on Katmai bears, it was found that the average harvest rate for clamming bears was 0.69 ± 0.46 clams per minute (Smith et al. 2004). The percentage of excavations that yielded clams was 63%. When it comes to the number of clams ingested, Troyer (2005) reported seeing individual brown bears dig up as many as 50 to 100 clams in one cycle of the ebb-tide. In one four hour period, I watched a female with cubs dig between 180 to 200 holes (it was impossible to determine how many clams the bear ate during this period).

Once they extract the mollusk from its subterranean refuge, the bear has to get to the meat that is housed within the clam’s calcareous armor. In the case of the soft-shelled and razor clams, the valves (which are the two parts that make-up the shell) are not that hard. A bear’s clam-handling technique can vary from one individual to the next. After pulling a clam from the substrate, some bears nimbly use the claws to pull the valves apart (see slide show below). Some bears place the bivalve on the beach and then stomp on it or roll it under their massive paw. This results in the valves breaking apart, which enables the bruin to extract the meat with their lips and tongue. In the case of soft-shell clams, the bear may take the entire bivalve into its mouth and masticate it. This resulted in their swallowing a considerable amount of shell material as well as the clam flesh (along the Katmai coast it is not unusual to see scat that contains clam shell fragments).

Clams Eaten

Katmai Brown bears do not feed on the various clams species in equal numbers. Smith et al. (2004) found that 77% of the time bears dug-up soft-shelled clams (Mya arenaria), 18% of the time they pulled out razor clams (Siliqua patula) and 2% of the time they excavated other clam species (namely Nuttall’s cockle [Clinodcardium nuttallii] and the Alaska surf clam [Spisula polynyma]). Clam species selection is thought to be more a function of species availability rather than preferences related to a species nutritional value. For example, the bears apparently feed more on the soft-shelled clams because they are found closer to shore and thus are available to foraging bears for longer period of time. There is not a difference in the bear’s harvest rate or success rate between soft-shelled and razor clams, even though the razor clam have what appeared to be a more effective anti-predation strategy – that is, when disturbed they pull themselves deeper into the substrate.

Who Digs Clams?

When it comes to a proclivity to dig clams, there are differences between the sexes. Smith et al. (2004) reported that of 233 clamming bouts observed, less than 1% of the clam-diggers were adult males, 44% were females with cubs and 55% were single bears of undetermined sex. How does the overall population of bears in this area compare with those observed clamming? The researchers found that of 16,738 bear (or bear family groups) encounters (over two years), 13% were large adult males, 8% were females with cubs-of-the-year (spring cubs), 17% were females with dependent young and 62% were single bears. It is obvious that females with cubs are overrepresented on the intertidal flats (44% of total population vs. 25% of bears observed clamming), while adult males are greatly underrepresented (13% vs. 1%). During focal sampling of bears working the intertidal zone (that is, where they followed individual bears and recorded their behavior), large adult males spent 0.1% of their time clamming, while females with dependent young invested 4.7% of their time digging mollusks (the single unsexed bears spent 1.4% of their time). The conclusion: females with young cubs most often feed on clams on the intertidal flats.

Benefits to Clam-digging

So what are the benefits for the bruins that dig clams? First of all, clams are a good source of digestible protein. Those brown bears that supplement their diets with these nutritional mollusks, are able to spend much less time munching on vegetation in order to meet their springtime energy requirements. Smith et al. (2004) predicted that a 160-kg brown bear that consumed razor clams for two hours a day could cut down total foraging time by 27% (compared to bears that only consumed vegetation). There is another advantage for clam-digging females that have cubs in tow. Because adult males are rarely seen on the intertidal flats, females with dependent young can forage in this habitat without exposing their offspring to potentially dangerous males. Less time spent feeding and less dangerous for young ones – a definite twofold advantage to hunting clams.

References

Smith, T.S. and S.T. Partridge. 2004. Dynamics of intertidal foraging by coastal brown bears in southwestern Alaska. Jour. Wildlife Manag. 68(20):233-240.

Troyer, W. Into Brown Bear Country. Univ. Alaska Press, Fairbanks, 130 Pp.
Copyright (2008) Scott W. Michael

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