The biology of roe deer
The European roe deer (Capreolus capreolus) can be found throughout Europe, from the shores of the Mediterranean to the cool, boreal forests of Scandinavia.
What we will look at here are the physiological characteristics that allow this remarkable deer to flourish in such a diverse range of habitats – particular consideration will be given to the challenges they experience and the strategies they employ to enable them live in the northern regions of the continent.
Deer have quite long legs to enable them to move through the brushy areas found inside woody areas, etc., but because roe deer are not a large species of deer (no more than about 75cm high to the shoulder), their brisket height (the distance between their underbelly and the ground) is quite small — depending the size of the individual, somewhere in the range of 50 to 60cm.
This poses a problem for them when it comes to living it an environment where the snow gets much deeper than this, since their belly will be scraping along the snow’s surface, which will increase the energy needed to move (because of the drag exerted by the snow on the deer) and make it more likely they will get too cold (because the ice will conduct heat away from their warm body).
Therefore, when snow cover gets too deep, roe deer will move away from their normally preferred areas, to a place with less, or ideally, no snow at all.
There are three other reasons why roe deer vacate an area when it gets too cold:
A small body size means that their volume to surface area ratio is quite small. This means that heat will leave their body comparatively quickly, and therefore increase the amount of energy they need to consume over a set period of time.
Their base level of daily nutrimental requirement remains relatively high, even though their body size is modest. So, regular consumption of food is required, even though amount of highly nutritious food will be a low level in the winter.
The capacity of the gut is proportionally smaller than that of a larger deer, like for example, the moose. This means that roe deer are not able to eat and store a large volume of food inside their stomachs and hence increase its digestibility (which will increase the amount of energy extracted from it), which the larger bodied deer are able to do and utilise.
Roe deer are able to decrease their metabolic rate to partially compensate for these constraints, but not enough to completely mitigate the problems listed above, hence their movement to less extensively snow-covered areas at certain times of the year.
In order to stop themselves from overheating the summer and getting too cold in the winter, the roe deer change their pelage (body hair) twice a year.
During the late spring through to the early September, the deers’ coat consists of thin, russet coloured hairs of about an inch in length. But towards the middle of the month, much longer, thicker, dark brown / grey hairs push through the summer coat, giving the deer a well-insulated winter blanket.
The initial increase in hair length during this coat change is quite rapid, but the pelage will slowly increase in thickness, until it reaches a certain maximum value later in the winter.
Compared to the summer coat, the winter pelage is likely to be twice as effective as a thermal insulator.
In the spring, the oppose coat change occurs: the thick, winter hairs are jettisoned and are replaced with thinner, shorter, lighter brown coloured fur again.
But this time, instead of the rather seamless, smooth transition, which takes place in the autumn, this change of coat is a full-on moult. Hairs often fall out in clumps, leaving bare patches on the deer, where small areas of their skin can be seen. Inclement weather at from late March through to early May, make the deer especially vulnerable to be being chilled by snow, rain and being exposed to significant wind chill effects.
This is one reason why this is the time of year when the sick and old are most likely to die. And the young have it especially hard, since it is they who moult first; the oldest change last (which is the same age order of coat change that occurs in the autumn).
Body fat deposits
All species of cervids (deer) take in far more nutritional energy during the late spring through to the end of summer, than they do through the winter, sometimes as much as three times more. There are a number of reasons for this, and one of them is the need to store fat to sustain themselves through the lean times of winter.
Rich sources of food are widely available during the peak growing season (ie there lots of young, fresh plants are plentiful), which not only provide the energy needed to support lactating mothers feeding their young, but also supply enough energy for them to rut (the roe deers’ breeding season (i.e. the rut) occurs during late July and the first half of August) and, as mentioned above, to increase fat reserves for winter survival.
However, as a species, roe deer do not carry much in the way of body fat. As a rough guide, the percentage of body weight for bucks, does and fawns throughout the year are as follows:
Summer 4 to 5%
Late autumn 8 to 12%
Winter 3 to 5%
Very early spring / late winter 2%
Summer 2.5 to 4.5%
Late autumn 8 to 10%
Winter 3 to 4.25%
Very early spring / late winter 2%
Late autumn 4 to 6%
Winter 2 to 3%
Very early spring / late winter 1.6 to 2%
(Reference: Holand, O. Body Reserves and winter survival in European roe deer. Proceedings of the International Union of Game Biologists Congress 19, 187 – 191, 1990.)
Fawns have a higher metabolic rate than the adults and expend relatively more energy in moving around, which partially accounts for their lower body fat percentage.
Once the body the fat percent drops to around 1.5%, the deer is as risk of starvation. Looking at the above figures, you can see that fawns are the closest to the critical level of body fat, and, therefore, are at greater risk of starvation at the end of winter than the adults.
However, these numbers are only an approximate: roe deer in some geographical locations typically store more or less body fat than the same species in another area. For example, it has been found that deer in Sweden carry more fat reserves that roe found in the United Kingdom.
As a result of their low body fat reserves, a good supply of food must be available throughout the year, since their internal store of energy is not enough to sustain them through the winter.
Fat reserves can explain early mating and delayed embryonic plantation
The roe deer’s breeding season takes place during the mid-summer, whereas most cervids rut in the latter part of autumn.
So, why do roe deer rut much earlier than many of the other species?
Since the amount of body fat carried by roe is relatively low, they use the end of the summer and the whole of autumn to build up their fat reserves, instead of using a large amount of nutrition what ingest during August, September and October to supply the energy needed to rut.