A Message From Nature To Muskrats: “Don’t Chill
Out”
A comprehensive study finds that muskrats, unlike other
aquatic mammals,
receive no metabolic benefit from hypothermia
San Diego, CA -- Muskrats are mainly vegetarians
but will eat animals as well. These small animals consume about one-third
of their weight every day. Their digestive system is designed for green
vegetation. In the summer they eat the roots of aquatic plants. In the
winter, they swim under the surface ice to get to the plants. To the
distress of farmers, muskrats also eat agricultural crops.
Since muskrats forage underwater they should exhibit
traits that maximize their breath-hold capacity. It is generally
acknowledged that an animal’s dive time is limited by finite on-board oxygen
supply and that its aerobic dive limit (ADL) defines the maximum dive
duration that can be supported by an oxygen-based metabolism (accessible
body oxygen stores divided by the rate at which these stores are depleted
underwater). In order to maximize time spent underwater, animals must
acquire strategies that enhance their ADL, their capacity for recovery from
diving, or their ability to continue diving beyond the ADL (i.e.
their capacity for anaerobic metabolism).
One strategy that many assume the muskrat adopts for
increased diving time is related to a well-established finding that a drop
in temperature reduces rates of cellular reactions (“Q10 effect”)
and this phenomenon may account for the hypometabolic state of cooled
tissues. Essentially, this effect depresses metabolism of cooled body
tissues, thereby conserving energy. Cooling the body to increase diving
time is known as “adaptive hypothermia” hypothesis.
A depression in diving metabolic rate (DMR) could
partially explain the breath-hold capacity of some divers. Calculated DMR
based on known body oxygen stores as well as diving patterns and behaviour,
has been predicted to be equivalent to, or lower than resting metabolic
rates in several penguin species (gentoo, king, emperor), as well as
southern elephant seals. Since oxygen affinity of mammalian hemoglobin is
typically increased in cold, a reduced sensitivity to temperature may serve
to maintain the unloading of oxygen to cooled tissues during diving.
The semi-aquatic muskrat (Ondatra zibethicus) is an
excellent model for investigating the adaptive hypothermia hypothesis, since
it routinely cools during voluntary swimming and diving and apparently does
not engage thermogenic pathways underwater. Both heart rate and Tb
of muskrats decline during voluntary diving, and active rewarming via brown
adipose tissue (BAT) does not appear to occur until surfacing. Given these
observations, the researchers hypothesized that hypothermia in diving
muskrats may enhance their dive performance by lowering diving metabolic
rate.
The authors of “Does Natural Hypothermia Improve the
Dive Performance of Muskrats?” are Allyson G. Hindle, Robert W. Senkiw, and
Robert A. MacArthur, all from the Department of Zoology, University of
Manitoba, Winnipeg, Manitoba. They will present their findings at “The Power
of Comparative Physiology: Evolution, Integration and Application,” an
American Physiological Society (APS) meeting scheduled for August 24-28,
2002 at the Town & Country Hotel, San Diego, CA.
Based on the premise that DMR is depressed with
hypothermia, the research team predicted a parallel reduction in diving
heart rate. Beat-to-beat changes in heart rate during submergence were also
analyzed to determine if the pattern of bradycardia normally observed during
diving is affected by hypothermia. To validate metabolic inferences derived
from telemetered heart rate, concurrent measurements of (oxygen consumption)
and heart rate were gathered for muskrats engaged in varying intensities of
diving and surface activities.
Methodology
Muskrats were surgically implanted with either
temperature-sensitive radio transmitters, or with electrocardiogram (ECG)/Tb
transmitters. Rates of O2 consumption in the metabolic chamber
were determined using a negative-pressure, open-circuit respirometry setup.
Prior to dive trials, muskrats were placed in a “chilling tank” where they
were immersed to a depth of 15 cm in 6 ± 1°C
water for up to one hour in order to induce mild hypothermia.
The experiments consisted of:
Experiment 1: Cost of Thermal
Recovery from Chilling: This experiment enabled the quantification of
post-immersion rewarming costs of hypothermic muskrats in the absence of
diving. Average was determined, as it reflects the combined costs of
recovery from chilling and post-immersion grooming and comfort movements
within the metabolic chamber.
Experiment 2: Free Diving during Continuous Immersion:
Pre-chilled and non-chilled muskrats were introduced into tanks where
submerged screens prevented diving muskrats from surfacing at any point
other than in a purpose-built metabolic chamber (14.6 L) situated at one end
of the covered tank. Several behavioural indices of dive performance were
measured during this aquatic phase, including dive frequency, average and
total dive time, and dive: surface ratio (ratio of surface recovery time to
duration of preceding dive).
Experiment 3: Dives of
Controlled Duration: Pre-chilled or non-chilled control muskrats
entered the water directly from the metabolic chamber and dive duration was
controlled by blocking the entrance to the metabolic chamber once the
muskrat had initiated a dive. Following each dive, muskrats were confined
to the metabolic chamber and post-dive excess was calculated for that
specific dive.
Experiment 4: Dive Trials on Juvenile Muskrats: Diving
metabolic rates of normothermic and hypothermic juveniles were calculated in
the same manner as for adults. However, due to time constraints associated
with working on young, rapidly growing animals, juveniles did not
participate in the same number of treatments as adults.
Experiment 5: Heart Rate During Diving and
Post-Immersion Recovery: All eight adults instrumented with ECG/Tb
transmitters participated in each “pre-chill” and “non-chill” trial. Three
replicate trials assigned in random order were conducted on each subject to
ensure the animal performed several long dives at each Tb.
Results
Pre-chilling elicited no overt behavioural changes to
diving in adult muskrats. Both diving and average rates of oxygen
consumption ( ) of adults were unaffected by hypothermia when animals were
tested in 30ºC water. However, significant interactions between water and
body temperatures (Tb) were observed for diving (P = 0.045) and
average (P = 0.040) , resulting in significantly higher (P = 0.045-0.017)
values for hypothermic adults diving from the water surface in 10°C
water. Hypothermia reduced diving heart rate only in dives < 25 s (P =
0.007), and did not appear to affect the onset or temporal pattern of diving
bradycardia. These study indicates that hypothermia does influence the dive
response in a way that could potentially extend dive time through oxygen
conservation.
However, the bradycardia response of hypothermic and
normothermic muskrats quickly becomes indistinguishable as dive length
increases. Consequently, the observed cardiac response to hypothermia was
not strong enough to yield a relationship between mean diving heart rate and
degree of pre-chilling (DTb).
As well, heart rates of muskrats resting at the surface did not differ
between hypothermic animals floating in cold (6ºC) water and normothermic
animals resting in warm (30ºC) water. Presumably hypothermic muskrats
experienced a greater vasoconstrictor response since abdominal Tb
was lower. If so, failure to detect an attendant reduction in heart rate
implies muskrats may readily tolerate hypertension during immersion.
Conclusions
This study documented few behavioural or metabolic
changes in dive performance related to hypothermia in adult muskrats. While
implying there is no energetic benefit to muskrats diving with a reduced Tb,
this finding also suggests an impressive resistance to the effects of
hypothermia in these animals. An elevation in average for hypothermic
individuals implies that adult muskrats may defer rewarming to surface
intervals.
The overriding conclusion to emerge from this study is
that hypothermia in muskrats exerts a negligible effect on dive performance,
as it influences neither submergence nor post-immersion metabolic costs.
-end-
The
American Physiological Society (APS) is one of the world’s most prestigious
organizations for physiological scientists. These researchers specialize in
understanding the processes and functions by which animals live, and thus
ultimately underlie human health and disease. Founded in 1887 the Bethesda,
MD-based Society has more than 10,000 members and publishes 3,800 articles
in its 14 peer-reviewed journals each year.
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EDITOR’S NOTE: For further
information or to schedule an interview, contact Donna Krupa at 703.967.2751
(cell), or by email at djkrupa1@aol.com.
