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Hibernation

This Year’s Black Bear Cubs Growing Up Fast

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It’s hard to believe that four months ago when it was born this Black Bear cub weighed less than a pound and measured about eight inches in length! Most Black Bears mate in June, but because of delayed implantation their fertilized eggs don’t implant in the uterine wall and the embryos don’t begin developing until the fall (if the mother has had a sufficiently nutritional diet), just as the mother is entering hibernation.

The cubs are born in January, after only a few months inside their mother. They are just a fraction of one percent of the mother bear’s weight, compared to an average human baby that is about seven percent of its mother’s weight. The cubs nurse constantly for the next four months (during which time their mother is not eating or drinking).  The fat content of Black Bear milk can be as high as 20-25 percent. Human milk is comparable to cows’ milk, generally ranging between three and five percent fat. (A biologist who had the opportunity to sample Black Bear milk reported that it was similar in taste to sweetened condensed milk.)

In April, when the cubs emerge from their den, they weigh about six pounds.  Milk production and intake now increases four-fold. Peak lactation (45 ounces of milk per day per cub) occurs in June and July. As a result, the cubs have a huge growth spurt their first summer and will weigh between 40 and 60 pounds by the end of it.

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Mourning Cloaks Surviving On Sap

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Who doesn’t celebrate at the sight of a Mourning Cloak butterfly gliding through the woods on soft, spring breezes? Because the adults spend the winter hibernating behind loose bark, Mourning Cloaks are among the first butterflies to take flight in the spring. Most butterflies overwinter as eggs or pupae inside chrysalises, and have to complete metamorphosis before they can take to the air.

Surviving in March and April, when there is little, if any, nectar to be found, is challenging. Mourning Cloaks sustain themselves with the sap that exudes from broken tree branches or wounds in tree trunks. Oaks are their preferred source of sap. When they find some, they walk down the trunk to the sap and feed head downward (see photo).

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Boxelder Bugs Active

4-6-17 boxelder bugs 095You may have noticed ½-inch-long black insects with red markings emerging from cracks and crevices inside or outside your home with the recent arrival of warmer weather. These are adult Boxelder Bugs that have been hibernating all winter and have become active with the warming days. They may disappear on days such as today when the weather turns cold again, but they’ll emerge for good in late April or May, just about the time buds on Boxelder trees are beginning to open.

During the spring and early summer they seek out and feed on low vegetation and seeds on the ground. Starting in mid‑July, they move to female seed-bearing Boxelder trees (or occasionally other maple or ash trees) where they lay eggs on trunks, branches, and leaves.  Red nymphs hatch in roughly two weeks, and proceed, like their parents, to feed on Boxelder foliage and seeds by using their piercing-sucking mouthparts. Even if their numbers are large, there is no noticeable feeding injury to these trees.   Come fall both adults and nymphs congregate in large numbers on the south side of trees, buildings and rocks exposed to the sun (only adults survive the winter) before settling in a protected hibernaculum. Boxelder Bugs are most abundant during hot, dry summers followed by warm springs. They do not bite people and are essentially harmless to property.  (Photo: adult Boxelder Bugs in spring; insert – adults and nymphs in fall) Thanks to Jeannie Killam for photo op.

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Woodchucks Active

3-15-17 woodchuck burrow IMG_0618Our recent snowstorm will make it a bit more challenging for male Woodchucks intent on mating, for they must work their way up through a foot or more of snow upon wakening. In March and April they come out of hibernation having lost 20 – 40 percent of their weight over the winter. Even so, sex is the driving force, not food, which is fortunate, as there is little for them to eat this early in the spring. Males dig their way out of their burrows and head straight for the burrows of females.   After mating, the female goes back to sleep for several weeks and the male returns to his burrow and does the same. Snow makes these tunnels much more obvious and thus easier to find, as dirt is scattered around their entrances. Equally obvious are the muddy trails males leave when in search of females.

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Black Bears Emerging From Hibernation

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Time to take in bird feeders, at least at night. Black Bear sightings are being reported throughout northern New England, a bit earlier than usual probably due to the erratic weather we’ve been having. Adult males emerge from their dens first, females with cubs  last – the opposite order in which they entered in the fall.

Having survived the winter by living off the fat they accumulated last fall, Black Bears weigh considerably less upon waking. Males will typically drop between 15 and 30 percent of their body weight, while lactating mothers can lose up to 40 percent. Even so, for the first two or three weeks following hibernation, bears eat and drink less than they normally do, while their metabolism adjusts to the changes. This is referred to as “walking hibernation” by some biologists. Once normal activity resumes, and until herbaceous green shoots appear in wetlands, bird feeders  are a main attraction.

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Woodchuck Hibernation

2-2-17-woodchuck-280As fun as it would be to see a Woodchuck on Groundhog Day, it’s not likely to occur in New England, even taking into account climate change, at least not yet. Rather than migrate or remain active and adapt to winter conditions, Woodchucks lower their metabolism and hibernate through the winter.   Their heart rate, body temperature and breathing rate are decreased significantly in order to conserve energy.

One assumes that once a hibernating animal’s metabolism is lowered, it remains lowered for the duration of hibernation. However, it turns out that this is not the case. All of the species of hibernators that have been studied have woken up periodically throughout the winter and warmed themselves up. During these two-three day (on average) arousals, the Woodchuck’s body temperature (roughly 38°F. during hibernation) rises to 98.6°F., its normal temperature during the summer. During these arousal times Woodchucks do not eat. Rather, they rely on deposits of stored body fat, which results in their losing about 40 percent of their body mass by the time green plants are available in the spring. Woodchucks’ bouts of hibernation are initially short, then they lengthen to an average of eight days, and then shorten again as the season progresses.

Arousal consumes a lot of energy (a single arousal may consume as much energy as ten days of hibernation) so it must have a crucial function. Theories regarding this function include restoration of depleted nutrients in the blood, invigoration of immune system, elimination of toxic substances, dealing with potassium loss and facilitation of sperm production in males. As of today, however, the reason for this arousal phenomenon has not been determined.

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Leopard Frogs – Too Little, Too Late

leopard-frogs-in-puddle-by-kelly-maginnis-northampton-ma-12-10-16At this time of year one would expect to find Leopard Frogs lying on the bottom of a pond, partially but not completely covered with leaves or mud as they hibernate their way through winter. Because of the depth of a pond, and the fact that in winter the water temperature is around 39°F., ice isn’t an issue at the bottom of a pond, and the frogs and turtles that overwinter there don’t usually freeze.

However, sometimes ponds freeze over before amphibians or reptiles that overwinter in them arrive at their hibernacula. Apparently this is what happened to these Leopard Frogs, and they took refuge in the only open body of water they could find – a large but shallow puddle about 10’ wide by 20’ long in a dirt road. Shortly after they arrived temperatures dropped and the frogs were trapped under (and eventually will be encased in) the ice. Unlike Wood Frogs, Spring Peepers and Gray Treefrogs, Leopard Frogs are not freeze tolerant, so their demise is inevitable. (Thanks to Kelly Maginnis for photo, and Jim Andrews for his herpetological expertise.)

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