David Beeson, January 2021
The weather changes in the UK from day to day and with the seasons. With the Earth at a moving orientation to the Sun throughout the year, the input of energy in a particular spot changes. In the UK winter, the constant energy output from our Sun is spread over a larger area than in the summer. Less energy per square kilometre means lower temperatures, in addition to a shorter daylength. Many organisms are content with that, yet many seek to avoid the colder, darker conditions. They will cut down their body chemistry (metabolism) or go into full dormancy – enter diapause. Hibernation.
In some parts of the world, it is not daylength or a reduction of input energy that causes diapause. It could be lack of water – drought. I have heard it suggested that deciduous trees lose their leaves due to drought rather than any other main reason, for roots need a temperature above four degrees Celsius to take up water. Also, if there is no soil water that will trigger leaf loss. Of course, there are other reasons as well for leaf loss; for example, to stop trees being upended in winter storms, or to allow easier wind pollination in the flowering season. Trees that retain their leaves, evergreens, may have a good climate strategy and sufficient water supply, or be blessed with water-retaining leaves with perhaps a waxy surface or few stomata.
Once an organism is adapted to its climate it will exhibit its survival strategies. If it is not sufficiently adapted it will lose the evolutionary race and become rare or extinct.
Spring-flowering bulbs (corms etc) will go into summer diapause. Bluebells and wild daffodils are good examples. They avoid the lack of light and, possibly, a summer drought in the soil’s surface. They store energy as carbohydrates and stop water loss by shedding their leaves and having a waterproof bulb surface. Many are highly toxic to discourage herbivores – you will have noticed that horses do not eat daffodil leaves.
Deciduous trees (and many animals) can measure daylength – the photoperiod, with light switching a leaf chemical between two forms. This, possibly in conjunction with environmental temperature, will trigger leaf fall. The leaf shedding will virtually stop water loss and may allow the removal of toxic metabolic waste.
Seeds would be wasted if they germinated in adverse conditions. Many have strategies to break their dormancy when conditions are optimal. Cactus seeds need a good dousing with water, other plants need a cold period before an equitable temperature and water content to the soil will trigger germination. [With birch seeds (Betula), they usually need a chill before they will grow, but may germinate with a long photoperiod and a suitable temperature. Either way, it will be spring conditions that allow growth, and the seeds will not be wasted.] Smoke can trigger some seeds to be released from fir cones and allow them to germinate (Yellowstone NP and Australian woodlands).
All these mechanisms are adaptations to ensure survival, or for life to recommence when environmental conditions are suitable.
For the UK, temperature is often the critical aspect controlling the need for dormancy. Enzymes are the catalysts that encourage chemical reactions to take place, and they are temperature dependent. Too cold and enzymes work too slowly to sustain energy release or maintain other life processes. Too hot and they are curdled, like cooking and killing an egg. So, temperature can be a key feature to trigger a shut down in an organism. But daylength is a predictor for a fall or rise in temperature, so it too can be a metabolic trigger.
So, why this topic? Ah, it is all about our dormice. The technical books about the species suggest that hazel dormice hibernate possibly as early as October, yet ours were active through at least two sharp frosts and until 22nd November (at least). So, what is the trigger? Temperature? Daylength? Food stores?
First, some background.
Average longevity in free-living edible dormice (Glis glis) can reach 9 years, which is extremely high for a small rodent. This remarkable life span has been related to a peculiar life history strategy and the rarity of reproductive bouts in these seed eaters. Most females (96%) reproduce only once or twice in their lifetime, predominantly during years of mast (high levels of) seeding of, e.g., beech nuts, but an entire population can skip reproduction in years of low seed availability. Surprisingly, in non-reproductive years, large fractions of populations apparently vanished and were never captured above ground. Therefore, the researchers studied the duration of above-ground activity, and body temperature profiles in dormice under semi-natural conditions in outdoor enclosures. They found that non-reproductive dormice returned to dormancy in underground burrows throughout summer after active seasons as short as <2 weeks. Thus, animals spent up to >10 months per year in dormancy. This exceeds dormancy duration of any other mammal under natural conditions. Summer dormancy was not caused by energy constraints, as it occurred in animals in good condition, fed ad libitum and without climatic stress. The researchers suggested that almost year-round torpor has evolved as a strategy to escape birds of prey, the major predators of this arboreal mammal in non-reproductive years. This unique predator-avoidance strategy clearly helps in explaining the unusually high longevity of dormice.
Reference: [Summer dormancy in edible dormice (Glis glis)without energetic constraints. Claudia Bieber & Thomas Ruf – research publication 2009.]
An amazing research finding!
Another publication, but, again, not the hazel dormouse, states: Prior to hibernation, juvenile hibernators have to sustain both growth and fattening to reach a sufficient body mass to survive the following winter season. This high demand for energy is especially challenging for juveniles born late in the season, since they might already experience reduced food availability and decreasing temperatures.
Yearly variations in the diet composition of the hazel dormouse (Muscardinus avellanarius) were studied in typical dormouse habitat in Lithuania over 5 years (2010–2014) with different feeding conditions. A high proportion of birch seeds in the dormouse diet in two out of 5 years was a very much unexpected result. Dormice consumed them from mid-June until late October even when the most preferable food—hazel nuts—was abundant. In autumn when accumulating fat reserves for hibernation, hazel dormice fed on four main food sources—fruits of buckthorn, oak acorns, hazel nuts and birch seeds. The consumption of these food sources was directly related to their availability. During the study period, only one, two or three of these food sources were abundant in any particular year, while others were absent or scarce. In total, the fruits of buckthorn and oak acorns accounted for the major portion of dormouse diet in autumn. Dormice living in habitat with irregular fruiting of the main food plants are adapted to feed on varying food sources and can switch from one food source to another in different years.
Now, let us introduce a hormone: Leptin.
Leptin is a hormone released from the fat cells (adipose tissue). It sends signals to the hypothalamus in the brain. This hormone helps regulate and alter long-term food intake and energy expenditure in humans. Not just from one meal to the next … the primary design of leptin is to help the human body maintain its weight.
Because it comes from fat cells, leptin amounts are directly connected to an individual’s amount of body fat. If the individual adds body fat, leptin levels will increase. If an individual lowers body fat percentages, the leptin will decrease as well.
So, Forest Edge’s late feeding dormice? Perhaps the books are correct, and most adult hazel dormice hibernate early as their fat levels, from feeding all year, are high … and in that species high leptin equals hibernation. Our young dormice were possibly late-born, and needed more food to build up fat and leptin levels. And, they were spending much of the night eating fat-rich walnuts – excellent prior to hibernation.
My guess the young dormice will come out of hibernation early … I’m expecting March 2021, even if the books say May!