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Animals' Choices To Migrate Or Hibernate

Country Moonmigration

During the last week of January Bobbi Jo and Barbi, the girls at the State Farm Insurance office, told me they had seen quite a few robins. We in the northern states take this to be a sign that spring is here. Granted, we were having an unusually warm January and I did not doubt their word, but robins in January in Michigan? Yea, I was skeptical.

But then a few days later I saw a robin perched on the rail of my deck and, looking further, I could not believe that the whole backyard was covered in them. However, as I write this article, the temperature is 4 F and there is not a robin in sight. Bet they are all sorry now and each of them is wondering who among the flock had the bright idea to head back to Michigan in January anyhow!

Bird migration and animal hibernation have always been a mystery of nature. How do birds know when to fly south or bears know to crawl in their dens.  I was really curious how they survive all winter without eating (something I would definitely be interested in which would solve the weight issue!). So, I did a little digging.

Bird migration is the regular seasonal movement between north and south along a flyway between breeding and wintering grounds. The main reason for the yearly migration is pure survival. If no birds migrated, food supplies in breeding areas would be depleted, competition for nesting sites would be fierce and predators would be attracted to the easy meals of nestlings in the high concentration of breeding birds.

Knowing when to migrate is basically dependent on birds’ internal stimulus and weather conditions. Natural instinct tells them to go on a feeding binge to add body fat to survive the journey and also to form into flocks. Once the pre-migration flock is formed, feeding continues while the birds wait for suitable weather conditions. They rely on winds north in spring and south in winter to assist in flight. The flocks of many species fly high because of prevailing winds at higher altitudes and because the cold at these heights helps them disperse heat that is generated by flight muscles.

Most flights occur between 600 and 5000 feet above sea level. However if their paths involve scaling mountain tops, flights of 10,000 feet are not uncommon. The distances they fly are as impressive as the heights. Some arctic terns fly 11,000 miles each way, averaging distances of 200 to 400 miles per day. On the other hand, robins coming up the Gulf coast take a more leisurely flight, averaging only 13 miles per day.

Little is known as to how birds navigate. Although some use landmarks that they can distinguish from being up so high, experiments show that migratory birds have a built in sense of direction. Young crows, born and raised in Alberta, Canada, were caged. Yet, when released, they flew to the exact spot in Oklahoma where the rest of the flock was. It is believed that some also use wind direction and the sun to navigate.

Talk about a dating question. What happens when a male meets a female and they winter in different locations? If they aren’t of the species that mate for life, they break up at summer’s end. Talk about a summer romance! If they do mate for life, the male decides where to winter and the female decides where to fly back to for another year’s breeding.

Just as intriguing as animals that migrate, are animals that hibernate. The simple difference between the two is that larger animals cannot migrate far enough to escape the cold winter. Ectothermic species (reptiles and amphibians) hibernate because they cannot generate enough body heat to stay warm or to digest food even if food is plentiful. Endotherms like mammals hibernate so they can survive with very little food and water during winter.

Hibernation is more varied than one might think. Many animals hibernate in a den all winter and some in summer. If a lake dries up, certain species of fish hibernate in a waterproof mucus envelope. Some birds and bats enter a sort of daily hibernation called torpor. By definition, hibernation is a long-term state where body temperature is significantly decreased, metabolism slows down and animals enter a coma-like state.

So, how do they know when it’s time to go to sleep? Hibernation is regulated by a combination of temperature and the endocrine system, glands that alter the amounts of hormones. Essentially, large animals like bears eat a large amount of food before hibernation or store some food in the den and wake up briefly to eat.

Contrary to popular belief, hibernation is more than just a long nap because the body undergoes physiological changes that can be very drastic. Their minimum temperature is known as their “set point.” Like a thermostat, when a mammal’s body temperature reaches this point, it’s metabolism kicks in and burns some body fat. This generates energy which heats things back up above the set point.

Larger animals have a higher set point so if their body temperature goes too low it would require an enormous amount of energy to heat their body back up. If a sleeping bear were awakened from its hibernating state, it would be like killing it because it would use so much energy warming itself back up. When they do awake from hibernation bears actually show signs of sleep deprivation and need to sleep a lot over the next few days to recover.

Ever wonder what they do about bathroom facilities during hibernation? Bears extract enough water from their body fat to stay hydrated. Their bodies produce urea, a waste product of urine, but they don’t urinate since urea is broken down into amino acid. No fecal matter is produced because nothing is going through the digestive tract.

This whole hibernation process can be beneficial to humans. Researchers have used hydrogen sulfide to induce hibernation-like states in mice. If applied to humans, it could stabilize accident victims or slow the progression of disease. It could also make radiation therapy for cancer more effective by reducing healthy cells’ dependence on oxygen.

Nature does have her checks and balances. It is amazing to see how she uses processes like migration and hibernation to help the various species survive.