Roe deer mating behaviour and rut, part 1
The rut is the deer’s breeding season.
Essentially, the mating (breeding) behaviour of the roe deer is as follows:
Females come into oestrus (often called, 'in season'), and over a period of 2 to 6 days, they become wide ranging in their movements, courting and mating with selected males during visits to their territories.
The rest of this post (part 1 and 2) will go into depth about the roe deer’s mating system, giving possible explanations for what happens, and briefly discussing how various aspects of it relate the character of the species as a whole.
A key part to understanding the mating system is to understand the crucial importance of the bucks’ territory.
The males’ territory
As mentioned in a previous post, a roe deer’s territory is an area selected by a buck where he will aggressively deter the presence of other non-fawn males by overt threat displays and fighting, as well as scent marking and scraping the ground, warning others of his presence.
(Females are not territorial in the strictest sense of the word, although they are known to behave aggressively towards other does that encroach in her core area, especially at fawning time.)
Male territoriality occurs for all roe communities, from the deer living in rich pasturelands of Europe, through to the boreal forests found in Siberia.
However, they are not territorial throughout the whole year, as we shall see shortly.
Territorial behaviour is also age dependent: male fawns never defend an area (since they are guided by their mother’s behaviour) and although some well-developed yearlings show certain territorial behavioural traits, they don’t become fully territorial, like the adult bucks do.
A roe deer is said to become an adult when they reach 3 years of age, and nearly all successfully breeding adults are territorial.
So, what happens to the sub-adult males – roebucks that are 2-years-old?
Like so many things with roe deer, that depends on what else is happening within their range.
If a sub-adult finds himself being the oldest buck in the range, he will claim the best territory in the area, and will become fully territorial in his behaviour i.e. anointing vegetation with his scent, aggressively scraping at the ground in certain spots and, most telling of all, engaging in full-on fights with all other bucks that directly challenge him.
I have known bucks of this age group to take control of an area and become the master buck, and father fawns in their first year of being the dominant beast. However, this is not the normal chain of events.
Roe do have the habit of confounding expectations though. For example, I have also seen yearling bucks mating with adult does, despite there being a large dominant beast nearby.
Territorial behaviour starts in spring
The precise timing is not the same everywhere, though.
Generally speaking, the milder the climate, the earlier the onset of territorial behaviour. Typically, one can normally expect to see males becoming increasingly aggressive from late February onwards, and by the middle of April, all dominant bucks will have staked out their domain for the year.
The territorial season will last until the end of the rut, which takes place over 2 to 3 weeks, from late July, through to mid-August.
So, here’s a key question…
If nearly all the roe’s mating activity takes place over the 2 to 3 weeks mentioned above, why do the males hold their territories for up to 6 months?
Claiming and retaining a good territory* is costly for the deer and it’s not without significant risk.
A large amount of energy is expended in patrolling his domain, marking prominent vegetation and chasing away challengers to his kingdom. And, of course, the biggest cost of all arises from the risk resulting from fighting, including the possibility of paying the ultimate price, death.
*A good territory is an area with a prolific supply of rich, nutrient dense food, woodland coverage from about 18 to 54% (Source: Lovari, Serrao and Mori 2017, Department of Life Sciences, University of Siena), spots offering shelter from inclement weather and lack of disturbance.
Also, roe deer are unique in the deer family in that they are the only species that uses the winter period as the main season to grow their elaborate head adornments.
This means that the time of year when there is the lowest abundance of rich, nutrient dense food available, they use a substantial amount of their energy and mineral intake to grow their antlers, which are non-essential for survival, but are nonetheless important implements to help them establish a territory.
There is not a complete consensus as to what is the correct explanation as to why roe deer hold their territory for several months before the actual breeding takes place. But it is highly likely that it is related to increasing the probability of mating success.
Second to survival, the urge pass on his genes to the next generation is a major driving force in a roebuck’s life.
Females in oestrus select mates based on a male’s physical attributes and the quality of their territory.
So, to repeat the original question:
If nearly all the roe’s mating activity takes place over the 2 to 3 weeks, why do males hold their territories for up to 6 months, even though it uses valuable energy resources and carries with it significant risk?
I believe the answer lies along the lines of the following:
The quality of a given territory is an integral part of the selection process used by an in season doe.
Bucks are genetically hard wired to know this. So, at the onset of aggressive behaviour, which is triggered by a sharp rise in testosterone levels during late winter / early spring, territories are established. The buck that starts behaving aggressively first (invariably the master), will take possession of his territory, and he will naturally select the best domain.
He will then have to mark and protect his domain until the peak of the rut (late July / early August), otherwise, other bucks will take possession for themselves.
The buck that is able to claim and defend his domain first, gets an advantage during the rut, since he will be ensconced in the best location when the fertile does come into oestrus and make their mate selection.
But, here’s 2 crucial points you need to understand, too:
Firstly, resident bucks have home advantage – meaning that most battles fought by males are won the territory holder.
So, it’s not a good tactic for a buck to delay taking a territory in favour of trying to evict a resident male near the peak in the mating season, because he’ll probably lose and waste valuable energy resources in doing so.
Secondly, researchers have found that bucks without a domain are far less likely to mate during the rut than a buck who holds a territory, because, as mentioned before, they’re missing a key part of the females’ selection criteria, a territory.
Therefore, territories are held for months before the peak of the rut because holding a territory is almost essential for mating success, and in the world of the roe deer, trying to take over a domain that is already occupied, is not a successful strategy.
Also, bucks that are displaced one season, but are able to regain a territory the next year, are far more successful in the next rut.
Soon after the breeding season has passed its peak, territories are largely abandoned and adult bucks start to become more wide-ranging in their movements for a short period of time.
After that, they become less active to recoup their body condition, as result of their seasonal rigours.
Where do roe deer make their territories?
Nearly all are established within 1km of where they resided during the winter – that’s for the majority of the European roe, living in an agricultural type of habitat.
For those deer living in the boreal forests, they may travel tens of kilometres between their winter range and the location where they become territorial.
Territory sizes are normally over 10 acres. In an environment with rich food supplies, high population of roe, and intense competition from other males, the territory size will be smaller.
The smallest domains are usually found in lowland, woody areas. In a relatively poor habitat, with a low population, such as cold, northern forests, over 350 acres is not uncommon.
One factor that can determine a buck’s territory size is visibility. This can explain why rich, woody areas, often lend themselves to producing small territory sizes.
Dense vegetation will negatively impact on the male’s ability to watch over his domain and hence hinder his ability to defend large areas of land.
Landmarks such as hedgerows, pathways, rivers, roads and the edges of a copse often form the territorial boundaries.
Territory positions don’t change year-on-year
Every spring, when their testosterone levels rise sharply, the adult bucks parcel their homeland into very similar territories to that of the year before. In addition, the same bucks will claim the same domain from one year to the next.
The longest I have known a buck to repeatedly go back to the same territory is four years. But in 1996, biologists Johansson and Liberg working in Ekenas, Sweden, reported that two bucks in their study area each occupied the same territories for six consecutive years.
Adult bucks have favourite areas and they like to stick to them
Even if territory becomes vacant due the death of its former occupant, the other nearby territorial males will not abandon their existing domain in favour of the other, even if the vacant territory is a better one.
Instead, if the range has a sizeable population of roe, the vacancy is likely to be filled by a non-territorial satellite or peripheral buck*. In the absence of other ‘available’ males, a dispersing, immigrant buck will fill the void.
*A satellite buck is subadult who has a home range partly, or completely, within the territory of an adult’s territory. A peripheral male is a subadult living as a non-territorial individual, between the adults’ domains.
In general, satellites are more successful at claiming a vacant territory than peripherals, most likely due to their proximity to the vacancy.
In 1994, Cederlund, investigating the effects of hunting on the spatial distribution of roebucks, reported to the International Union of Game Biologists XXI Conference, that in a 450 hectare estate, 8 radio-collared males did not change the spatial extent of their territory in any way that year and the next, and that’s after 22 other non-juvenile males were removed from the range during the peak of the rut. The unoccupied domains were claimed by other males from outside the range.
In the absence of predation, there are 3 main reasons why there will be a change in territory occupation.
Is killed by hunters.
Dies during a territorial battle.
Is evicted by a challenger, particularly if the former occupier is over 8 years of age.
In many areas, point 1 is most likely reason, especially if he has large antlers, which appears to be a strong draw for hunters.
Territories are but exclusive ranges
It’s easy to have the impression that a resident adult buck has exclusive use over his territory, however, this isn’t the case at all.
In fact, researchers have found that if we consider a territorial buck’s home range to be his territory, the amount of overlap between one male’s domain to his neighbour can be anywhere from 11%, in low density populations, up to nearly 50%, in areas with comparatively more adult males.
At this point, it’s worth considering the buck’s core area.
The core area is the area within his territory where he spends a particularly large amount of this time, and where most of the mating takes place, during the peak of the rut. When we take into account the males’ core areas, there is very little territorial overlap.
Territories are large areas of land containing abundant food supplies and woody cover, so apart from intrusions from other adult bucks, the territory holder will also have to contend with the presence of yearlings and subadults, as well as does, with attendant fawns.
How do males defend their territory?
When we talk about defending a territory, we’re considering everything that a buck will do to prevent other males from interfering with his life within his domain, since as written above, males cannot exclude all other bucks from his area.
And as we will see later, some other bucks are more likely to be tolerated that others.
Domain defence basically comes in 2 forms: marking and physical confrontation.
Let’s considering marking first:
At various spots around his home range, the adult male will deposit scent from glands located between the hooves and on his forehead. By scraping at the soil, he anoints the area with his scent, as well as producing a visible scrape on the ground – a warning to all other antlered roe.
He will also rub his head along any suitably sturdy small branches or brambles, etc., rather like a cat does when marking out its territory. In addition, bucks will vigoursly rub their antlers up and down small, whippy looking stems, often stripping away some of the bark from the stem. This is known as fraying.
All marking activities are designed to produce visual and scent bases ‘calling cards’, warning all other antlered deer, the presence of another adult male.
More than 40 markings per acre can be expected in and around an occupied territory, and they are not restricted to the domain’s borders. Scrapes are slightly more common than scent deposits.
Now, let’s think about confrontation:
This is where the territory holder is physically close to an intruding male.
Contrary to what some people believe, this does not mean that the bucks engage in a full-on battle with ‘locked horns’, etc… (deer have antlers, not horns!).
Savage fights do, of course, occur, but it is certainly not the first behaviour that confronting bucks default to.
Before any head-on-head altercation occurs, both deer will perform a series of threat displays, intended to ‘psyche’ the other deer out, as well as to size up their opponent.
Such threat displays include leaping off the ground whilst approaching the intruder, lowering their head, flattening their ears, looking sideways at the other deer and then aggressively thrashing their antlers around in the vegetation and very aggressively pawing at the ground when in close proximity.
Normally, one of them will flee at this stage. If not, a savage fight will occur, however, these are comparatively rare.
Death is a possible consequence of combat, but again, it’s unusual. More often than not, both individuals will receive cuts and gouges to the head and neck area, and one will flee after a brief clash of antlers.
Most battles last less than a minute, but I have seen one fight last for over quarter of an hour. During that time, the two large adult bucks ran from field to field between clashes, covering well over 5 acres of each other’s territory. In this case, the injuries to both deer were superficial.
Most fights occur near territory borders, and interestingly, dominance in the battle swings from beast-to-beast as they pass from one domain to another.
Whether you consider the preliminary threat stage or a full-on battle, most confrontations are won by the territory holder.
Territorial defence, marking and physical confrontation, occurs throughout the time the bucks are territorial, but the number of incidences spike just before the peak of the rut.
What happens to the non-territorial males?
The yearling roebucks keep a low profile over their first summer of independence, staying away from the larger, more powerful, testosterone driven subadults and mature males. T
he younger deer do not develop the aggressive, territorial behaviour of the individuals of 3 years of age and older.
The subadults (2-year-olds) are noticeably bigger and stronger than the yearlings and show strong territorial tendencies.
With the occasional exception (for example, when there are no older bucks in the range), they fall into one of 2 groups: satellites and peripheral bucks.
As previously mentioned, a satellite buck is subadult who has a home range partly, or completely, within the territory of an adult’s territory.
A peripheral is a subadult living as a non-territorial individual, between the adults’ domains.
The yearlings’ wandering behaviour from one territory to another means that they will come into contact with many older males, but because of their relatively diminutive size, they avoid conflict by fleeing when they are confronted by an older beast (or a larger yearling).
Being a year older, the subadults possess a larger, more developed body and therefore experience more clashes with a dominant beast.
(The size of a buck’s antlers will determine how much harrying he receives from the master buck; big antlers are grown by large deer, so they are more of a threat to dominant beast.
Hence, well-developed non-territorial males are not tolerated by the controlling male, and many confrontations will occur.)
Satellites have more altercations then peripheral bucks for 2 main reasons:
Their proximity to the territory holder results in a relatively high detection rate by the dominant male.
They will also challenge any other roebuck intruding into ‘his’ patch, which may, or may not, be of benefit to the controlling male.
However, peripherals do not have an altogether peaceful life, even though their core area can be 400 metres away from a territory.
Confrontations still happen…
1. Peripheral bucks have the same urge to court and mate with the in season females, as the senior ranking individuals. So, they will periodically wander into the dominant beast’s domain, looking for an unattended in season female.
I have seen this scenario unfold many times, where the master was courting a rutting female at the opposite end of the range, leaving a receptive doe on her own in the prime territory.
In each case, a subadult male entered the dominant buck’s home and successfully courted and mated with the willing doe.
On one occasion, it was possible to photograph a 2-year-old male associating himself with one the oldest and most productive does in the range. An image of the two deer courting appears in the book, A Year in the Life of a Roe Deer.
2. In the absence of a nearby fertile female, the master himself will wander away from his stronghold, bringing himself into contact with the peripheral living hundreds of metres away from his territory. The underling will then be subjected to the wrath of the older male.