The deer's reproductive system is triggered by two things

1. Number of daylight hours.

2. Hormones released by the deer’s endocrine system (their system of glands in the body).

Both of these affect the males and females, but in very different ways.

However, before we get into the details, I thought you may like to know this:

The reproductive strategy used by roe deer is very different to that used other deer

Firstly, roe deer are monoestrous. Most researchers agree – but not all – that there is only one chance a year for a roe to become pregnant. 

However, most of the other deer species are polyoestrous, so red, fallow and sika have more than one opportunity in 12-month period.

Secondly, roebucks cast their antlers in autumn, and grow their new ones in the winter, when rich food sources are least abundant. Whereas, other deer shed their antlers in winter, and grow their new pair in spring and summer, when high density food supplies are plentiful.

Okay, going back to what triggers the reproductive cycle in roe deer, I will deal with the bucks first – because they are a little more straightforward:

What controls the bucks?

During the short, but lengthening, daylight hours in the latter part of winter, steroid hormones, released by the males’ endocrine system, trigger territorial behaviour and production of sperm in the testes.

Long daylight hours then continue to stimulate the bucks’ body (i.e. causing aggressive behaviour, increasing their muscle mass and producing sperm) to reach full sexual ‘readiness’ just before the peak of the rut, when the females will be receptive.

So, testosterone levels rise at the commencement of territoriality (in late winter / early spring) and they are elevated by the long daylight hours, until around mid-August when the days get short enough to cause blood testosterone concentration levels to fall dramatically.

After that, the bucks are far less aggressive and become elusive, as they rest and recoup their body condition, after their summer activities.

What happens in the does?

In the bucks, it is the increasing number of daylight hours that triggers and maintains their sexual activity. In the does though, it is the opposite way around; increasingly long daylight hours prevents sexual activity.

Why does this happen?

At the start of the year, the rising levels of luteinizing hormone (LH) in the blood increases the probability of ovulation. But a simultaneously high concentration of progesterone (which has been persistent since the previous rut), has a suppressing effect on ovulation.

After May, progesterone levels fall sharply, and in the presence of rising LH levels (which have been steadily rising since the turn of the year), ovulation would be expected.

But – and this is the important point – the increasing number of daylight hours as summer solstice approaches, stops the release of an egg.

However, as summer progresses, the females’ sexual ‘resistance’ to the long, daily photoperiod breaks down, and the decreasing number of daylight hours, after June 22nd, eventually stimulates their neuroendocrine cells to finally allow ovulation to take place.

And this is how the females start the rut

The above sequence of events induces the pre-oestrus and oestrus states, causing the sexually receptive does to release their pheromones, which then attracts and excites the bucks into courting and mating with the amorous does.

Delayed implantation in roe deer

A couple of days after the egg has been fertilised, further cell development virtually stops for about 5 months. This pause in the pregnancy is called embryonic diapause, or as it is most commonly known, delayed implantation.

This is what makes the roe unique among deer.

Why do roe deer use delayed implantation?

With the roe deer’s rut occurring in late July / early to mid-August, without delayed implantation their fawns would be born in the winter (instead of May or June).

And in the habitat in which they live, this is likely cause very high infant mortality for two reasons:

Firstly, the babies would be at risk of hypothermia.

Secondly – and most importantly of all – even if they did survive the low temperatures, there would not be enough high energy food available for the lactating mothers to provide sufficient milk to sustain their youngsters in such a cold environment.

In 1997, Mauget and Sempere reported in the Canadian Journal of Zoology, that a mother does’ energy expenditure went up by 27% in the first month, due to lactation. (Late spring / early summer is the only time of year when the females need to eat more than the males.)

With the European roe having a maximum body mass of about 25 kg (35 kg for the Siberian roe), another alternative would be for the roe to rut in the winter.

But that doesn’t happen either, because males do not produce sperm in the winter.

Plus, the bucks are not aggressive at this time of year, nor do they possess a territory; both are needed for the females to have the highest chance of mating with the most physically able bucks, which will carry the best genes.

Also, the rigours of rutting place a large energy demand on the animals. So, since roe deer carry a relatively small amount of body fat, and supplies of their preferred, nutrient dense foods will be low, they will not have the resources to make up the weight lost during the rut, or – for the does – to have enough energy to sustain their pregnancy.

So, when it comes to roe deer reproduction, this is what happens:

1. The egg is fertilised in the summer, but cell development virtually stops a couple of days after conception; enabling the doe to regain physical condition without having to support a rapidly growing embryo at the same time. Plus, she continues to nurse her young until the end of the summer, in addition to building up her energy reserves for the winter.

2. In December / January, the embryo sends a signal the mother, reactivating the pregnancy*, which then proceeds as normal. By that time, the female has the body condition to see herself through the rest of winter, as well as having enough energy to bring the pregnancy to full-term.

*In 2005, Roger Lambert, Paul Racey and Cheryl Ashworth from the University of Aberdeen and Scottish Agricultural College reported that the roe deer is the only animal where it is known that the reactivation of the pregnancy is initialled by the embryo sending a signal to the mother, rather than the other way around, which is what happens with the other animals that use delayed implantation, such as badgers, weasels and seals.

So, the roe deer is not only unique in the deer world by its use of delayed implantation, it is different to any other animal in the way that the embryonic diapause is ended by the developing offspring, and not the parent.