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Mating System
Fallow deer are highly dimorphic, polygynous breeders the breeding season or rut lasts approximate 135-days. In the Northern hemisphere the breeding season tends to occur in the second half of October, while it occurs in April in the south, some matings can still occur before and after. This mating behaviour within the rut most often occurs in leks, where males congregate in small groups on mating territories in which the females only purpose for visiting these territories is for copulation. However, there can be variation within fallow deer mating systems, other than the traditional behaviour of lekking different types of mating behaviours can include harems, dominance groups, stands, temporary stands, and multiple stands. Different populations, environmental variation, size, and even age can determine the type of variation within a fallow deer mating system. However, rutting behaviour is the most commonly found and studied in nature, variation can be explained by three characteristics (1) The optimal strategy under specific environmental or social conditions, (2) The strategy of an individual may be dependent on the strategies of other individual males within the same population, and (3) Individual males may be less capable at gaining access to females since they can be outcompeted by other males that are more capable. Lek mating systems are important to understanding, which leads to fewer studies focusing on the reproductive success of males away from the lek or other mating system types. The females are polyestrous, meaning that during their sexual cycle the females will “heat up” which means that they are ready to mate and willing to accept a male, however, if this does not occur the “heat” will come repeatedly throughout the breeding season. The heat is usually referred to as increased serum progesterone levels in the female fallow deer and is associated with the corpora lutea. The male rut behaviour includes licking and sniffing the anal area of the female, also the hair below the vulva, this helps the male to determine if the female is fertile. A high-pitched whine is used many times to initiate mating behaviour, after the male displays this a number of times the female will eventually allow the male to mount, copulation can last as long a 5 minutes.

Ecology and Mating System Characteristics
Many deer species including fallow deer have a social organization which can be tremendously plastic depending on their environment, meaning that group size and habitat type is closely linked to herd size. It is important to note that most of the detailed research on the ecological characteristics and behaviour of fallow deer occur in large blocks of woodland, which means there may be some bias present. Fallow deer can be found in a variety of habitats when can range from cool and wet to hot and dry habitats. Fallow deer seem to have a preference for more older forests with dispersed areas of grass, trees, and a variety of other vegetation. The largest herd occurs right before the rutting season, while the smallest groups are females with fawns. Throughout a large portion of the year the sexes remain separated and only congregate during the mating months, however, other patterns may be described; such as bachelor groups and even mixed groups. Ruts are characterized by males gaining the best territory possible to increase their odds for mating and is often characterized by the presence of females on stands. During this time males will stop feeding to defend their ruts from subordinate males, males defending this territory will often lose an average of 17% of their body weight, the liver will exhibit steatosis which is reversible. Throughout breeding seasons, the male may obtain the same rut, in some cases, ruts can be held by more than one individual, some possibilities for this include high population density and less rut space, or more suitable habitats which can be shared.

Parental Care
After the females are impregnated the gestation period lasts up to 245 days and usually birth one fawn as twins can be quite rare. The females can conceive when they are 16 months, whereas the males can successfully breed at 16 months, but most do not breed until they are 48 months. The females can become very cagy just before they give birth to their fawn and find secluded areas such as a bush or cave, sometimes females will give birth near the herd. As soon as the female gives birth, the female will then lick the fawn to clean it, this helps initiate the maternal bond between the two, females are the only sex that provides parental investment; males only provide the sperm for birth to occur. After the birth of the fawn occurs, the females do not return to the herd for at least 10 days and for most of the days the mother is separated from the fawn, only returning to feed the fawn. The nursing period lasts about 4 months and happens every 4 hours each day. Rumination is a critical part of development in the fawns life, and this develops about 2 to 3 weeks into the fawns life. Females initiate the weaning periods for the fawn which lasts about 20 days; 3 to 4 weeks later the fawn will start to follow their mothers and will finally rejoin the herd together. The mother frequently licks the fawns anal area to stimulate suckling, urination, and defecation which is a critical part of the development of the fawn. Weaning is completely cut off at around 7 months and at around 12 months the fawn is independent, after the 135-days of reproduction, the rut comes to an end which can be characterized by the changes in group size and behaviour.

Contests and Weaponry
Since fallow deer are polygynous species that congregate once every year, males must fight to obtain access to estrous females. The relationship between antler size and body condition can be treated as indicators to reflect body condition within a given year. These secondary sexual characteristics can have dual functions which include the attractiveness of males which females can ultimately choose, and fighting ability of the male. It was found that individuals with larger antlers had higher mating success, where asymmetrical antlers did not. When males develop their horns there are always trade-offs between reproduction and survival which can shape the decision in an individuals choice. Genetic variations exist within fallow deer populations with variable antler growth, males that exhibited faster-growing horns early in life are able to grow longer horns without any significant cost, this shows that there is, in fact, phenotypic variation among fallow deer populations. Aggressive behaviour is often observed when individuals are seeking out mates, scarce resources, and even territories. Species that compete using their weapons usually engage when there is a mutual agreement, however, if there are any noticeable asymmetries such as a broken or lost weapon this may alter the behaviour of an individual to engage in a fight. Likelihood and severity of antler damage were looked at in fallow deer, to test whether antler damage was associated with contest tactics and duration; and if there was an association with the tendency for individuals to engage in fighting. Individuals with undamaged antlers were more likely to attack using high-risk tactics which included jumping clashing, or backward pushing behaviour, this was exhibited by both contestants; dominant males were more likely to have damaged antlers. Dominance ranks exist within fallow deer populations which can be linked to aggression level and body size, when competing for a male, however how ranks are obtained is not studied extensively.

Endurance Rivalry
Male fallow deer is a highly competitive species during the rutting season, a main evolutionary advantage to successful mating’s our body size and dominance rank. Many factors can determine the seasonal reproductive success of an individual male fallow deer these factors include body size which can affect reproduction and survival. The amount of time spent in a lek can be an important factor in determining male reproductive success, energy can play an important role for the duration of competitive leks. Among ungulates, specifically fallow deer, exhibit one of the most outstanding examples of sexual dimorphism, as males are much larger than females. For sexual selection to lead to the evolution of sexual dimorphism, where males are bigger then females there must be advantages: (1) advantages during combat, (2) endurance rivalry advantage, (3) female preference for larger males, and (4) advantages during sperm competition. Sexual selection has chosen bigger males over an evolutionary time scale and conferred advantages during competition of mates by a variety of mechanisms which intrasexual competition, access to females, and resource accessibility which effects female attractiveness. Body size is important during male-male agonistic interactions and endurance rivalry, which females tend to have a preference for larger males. Dominance rank is a good indicator of body size and body mass, however, age was not an important factor. In a study done by McElligott et al. (2001) found that mating success was related to body size, pre-rut and rut rank. Similarly, in another study researchers found that age, weight, and display effort were all significant factors in determining mating success; in both studies, mating success was measured by the frequency of copulations. Meaning that a variety of factors in different fallow deer populations can affect the overall energy allocation which will ultimately affect mating success. Maternal investment early in life can be critical to the development of body size since it can be very variable at that stage depending on resources and habitat type. Mature male body size can be a better indicator of overall male quality rather than body mass since body mass depends on a variety of resources each year and is not a static trait, body mass can be a complex trait to measure.