The physiology of reproduction in the deer (family: Cervidae) can help us understand their behavior. Deer are seasonal breeders, with males exhibiting “rut” behavior in the early fall during the breeding season. The timing of the breeding season also has implications in survival of young. Different species of deer respond differently to seasonal changes. However, deer are not the only animals that exhibit seasonal breeding behavior. There are several practical implications of this reproductive phenomenon; here, we will look at population control.

What Is Seasonality:
Seasonal changes in temperature, rainfall, and day length all contribute to the cause of the breeding season in deer. In climates where seasonal changes are more extreme, seasonal changes in day length are the main cue used to time the breeding season (Lincoln). Puberty occurs at approximately 16 months of age and after this, they exhibit seasonal polyestrous. Deer respond best to short-day lighting, which means that they are not usually cycling during the summer months, but begin to show estrous behavior in late September and October (see diagram below) (Gordon).

The estrous cycle in deer varies from 17 - 22 days, depending on the species, and this cyclical breeding activity may continue for as long as six months in animals which do not become pregnant (Gordon). The seasonal changes in fertility are controlled by the secretion of LHRH (luteinizing hormone releasing hormone) from the hypothalamus, which is influenced by melatonin from the pineal gland. LHRH influences the secretion of LH and FSH from the anterior pituitary (Lincoln).

Seasonality in the Male:
The season when deer breed is called the "rut". Rut usually occurs during October, but some bucks come into rut during December, these are usually younger or weaker bucks. It is possible to advance the onset of the breeding season in bucks by controlling their melatonin levels (Adam). Melatonin is a modified amino acid hormone released by the pineal gland that has been seen to control seasonality in ewes. Less daylight triggers an increase in a buck’s testosterone, which causes antler maturation/growth. Fraction, volume and pH of the ejacuate, as well as sperm concentration and sperm motility change gradually during the pre-mating, mating and post-mating seasons of red deer (Gisejewski). The period of greatest libido (from the end of September until the end October) has the highest semen quality.

Seasonality in the Female:
Female deer are short day breeders, so they generally come into estrus in the fall, from October to December (Dewey). This is triggered mainly by a decrease in photoperiod. A hormone called melatonin is produced by the pineal gland in response to the onset of darkness. When it reaches a certain level in the blood plasma it induces estrus (Webster). However, it is unclear how exactly this occurs. It is thought to be very similar to the pathway in sheep, but this has yet to be proven (Adam). In this pathway, there are high amounts of progesterone present in the deer during the anestrous season; this is true for both pregnant and non-pregnant individuals, though it is higher in pregnant ones (Plotka). The high amounts of progesterone cause estrogen to have negative feedback, limiting the amount of GnRH and subsequently LH, that are produced to levels that do not support estrus. The presence of enough melatonin in the blood plasma somehow triggers progesterone levels to decrease while increasing the responsiveness of estrogen receptors. When the progesterone reaches significantly low levels, estrogen begins having positive feedback effects on GnRH production, resulting in the first LH surge (Parrish). Deer generally cycle only a few times, until they are bred and become pregnant, when the increased progesterone again causes negative feedback of GnRH by estrogen. Deer can, however, continue cycling through March, if they fail to be bred (Webster).
Seasonality of breeding is important in deer because it allows the offspring the maximum chance at survival since they are born in the spring to early summer when food is plentiful and it is not as cold out

Seasonality in Different Species of Deer:
White Tail Deer
White tail deer can be found in the U.S and even in South America. This type of deer is polyestrous and is a short day breeder. The females will go into heat for a 24 hour period and most go in to heat in November in the northern portions of their range. The entire mating season lasts from October to December. In the southern parts of their ranges, whitetail deer will not mate until January or February. If a doe is not mated, a second estrus occurs approximately 28 days later (Desert USA). The first fawns will be born in the later portion of spring after a gestation period of around 200 days. The females with give birth to 1-3 fawns at a time (National Geographic). Near mating season the deer will release secretions from sweat glands on their legs for intra-species communication  (Desert USA). The Bucks will also develop antlers and will fight with other males during the breeding season to win mating privileges (Desert USA).

Mule Deer
Mule deer are found throughout the entire western United States, including the four deserts of the American Southwest. These deer are also short day breeders and are polyestrous. The mating season for Mule Deer peaks in November and December. The Males will grow antlers prior to the breeding season and will often fight with other males for the right to mate with a female (Desert USA). Once the buck has found his doe, they will play chase games for several days before they will mate (Desert USA). They will then stay together for a few days after mating. Gestation is about 7 months in the mule deer. Females will give birth usually to a single fawn the first year she gives birth and will often produce twins in the following years (Desert USA).

Red Deer
Red deer are most often found in western Europe, northwest Africa, Asia and northwestern America. Males and females will live separate from each other except during breeding season, which occurs in October. Females will give birth in late spring and will have from 1-3 fawns (Charlton). One distinction that separates them from other types of deer is, the males do not use their antlers to attract mates. The males will roar to attract the females (Charlton). The roaring will affect the outcome of male to male interactions and can even advance female ovulation (Charlton). Females can distinguish the differences between the roars and they will often choose a male that has a lower roar (McComb). It is thought that they do so since males with lower roars tend to have a larger body size, which is a sign of strength and good health.

Reindeer are found in the Arctic and are seasonal breeders. Their breeding season begins in early September and lasts from 3 to 4 weeks. Some reindeer may start breeding as early as late August to accommodate the rough climate and to obtain better nutrition (Alberta Reindeer Association). The gestation period for reindeer is about seven months. Those that will breed early will start fawning in early April. The males will separate into smaller herds during the summer and will all join back up right before the breeding season. As the time for mating drwas near, several changes occur in the male: the testicles and epididymis increase considerably in size, teh velvet is lost form the antlers, the neck thickens, the stomach draws in, and they grow a mane (Alberta Reindeer Association). It is important that the males fatten up before the breeding season because they do not eat much during the season; they become thin and may lose up to 1/3 of their body weight (Alberta Reindeer Association).

Roe Deer
Roe Deer are located in Scotland, the UK and other portions of Europe. Their breeding season occurs starting in mid-July and continues through mid-August. However, even though mating season occurs in August, the fertilized egg won’t begin to develop until the end of December or early January (Gaillard). It is thought that this occurs to prevent the deer from giving birth during the winter when resources are scarce (Gaillard). Young will be born in May and June and females will give birth to 1-3 young. Twins are also very common in the Roe deer. To attract mates, males will become very aggressive and will defend their territories and will often fight over a female (Gaillard).

Fallow Deer
Fallow deer are found in Europe, Asia minor and in the United States-especially in Texas (Mammals of Texas). The deer will mate from September to November, with the main breeding time occurring in October. The gestation period is seven and a half months long and fawning begins in late May and lasts through June. Usually only one fawn is born, but twins are common. During the breeding season, males mark off a territory and no other males are allowed to enter (Mammals of Texas). Females will join a male in his territory and will remain there until she comes into heat and mating occurs (Mammals of Texas). After mating season, males will abandoned their territories and will join back with other groups of males.

Comparison to Domestic Animals:
Species Type of Estrus
Deer Seasonally Polyestrous- Short day breeder
Sheep Seasonally Polyestrous- Short day breeder
Elk Seasonally Polyestrous- Short day breeder
Goats Seasonally Polyestrous- Short day breeder
Horses Seasonally Polyestrous- Long day breeder
Cows Polyestrous
Cats Polyestrous
Pigs Polyestrous
Rodents Polyestrous
Dogs Monestrus
Wolves Monestrus
Foxes Monestrus
Bears Monestrus

Practial Implications:
Understanding reproduction physiology of deer can be helpful in a variety of situations, one of which being population control. It is necessary to control deer populations for many reasons. In 2008, the Wisconsin Department of Transportation reported nearly 16,000 crashes involving deer (Wisconsin Dept. of Transportation). The Wisconsin Department of Natural Resources reports that in 2008, 84.5% of the approximately $2.1 million in appraised losses from wildlife were from deer damage. Damage to corn and soybeans accounts for approximately two thirds of this (Wisconsin Department of Natural Resources). Some organizations also argue that deer populations have risen to a point where the habitat can no longer support the large numbers. Another concern with a large population within an area is the high risk of transmitting disease throughout the herd. Measures that have been taken to help control deer populations include restrictions on hunting, relocating individuals, and even birth control. Both chemical and mechanical methods have been used to attempt to prevent pregnancy in deer. Estrogens or progestins, fed orally during the breeding season, have not been reliable enough to apply to an entire population. Mechanical methods have included tubes containing either an estrogen or progestin implanted subcutaneously in females. The limitation with these has been that the biological life of the implants has not been determined beyond 150 days (Matschke).

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