The reproductive behaviours of deer are arguably the most discussed topic amongst deer-lovers. A person with even a casual interest in deer will be familiar the significant changes which occur in deer behaviour in response to breeding, making a typically inconspicuous animal (particularly mature males), anything but! It is inevitably going to be the reproductive behaviours which attract our attention and inspire interest in the animals. If you're a seeking the thrill of a close encounter with wild deer, your odds of success are dramatically increased in response to a mature male's overwhelming preoccupation with breeding, making a once wary animal significantly less cautious and hence, easier to locate and approach.
Before reviewing the specific information relating to the estrous cycle, let’s first look at what we’re generally more familiar with - ourselves. Humans have a menstrual cycle, which involves a series of hormonal changes occurring throughout a month-long cycle. When a pregnancy does not occur following ovulation, the lining of the uterus (the endometrium) is shed during menstruation. The cycle repeats until such time as a pregnancy occurs, or the female is no longer of reproductive age. The menstrual cycle is generally limited to primates, with the majority of placental mammals having an estrous cycle.
Animals with an estrous cycle have short durations where they are fertile, separated by longer periods where they are completely infertile and sexually unreceptive (VetSci, 2009). When a pregnancy does not occur following ovulation, the endometrium is reabsorbed, as opposed to being shed in the menstrual cycle. For deer, the estrous cycle spans a period of around three weeks depending on the species. The complete cycle is broken into phases differentiated by specific hormonal changes. Bubenik (2006) concisely describes this as a “cascade of events involving multiple hormones”, each triggering specific changes within the animal.
The diagram below illustrates the estrous cycle. The diagram maps the relative levels of hormones that can be detected in the deer’s blood as a function of time. Here, ‘P4’ refers to progesterone, and ‘E2’ represents estrogen. When progesterone levels are at a minimum, estrogen levels are at a maximum. This marks the ‘estrus’ phase (bright red), where the female will be sexually receptive for a brief window of time which precedes ovulation. This helps ensure sperm cells are already present such that fertilisation can occur when the egg is released. The timing of these events is a truly astonishing product of evolution.
Most temperate deer species are ‘seasonally polyestrous’, meaning they have a cluster of estrous cycles at a particular time of year. The majority of conceptions will occur during the first instance of estrous. In the event a given female was not mated or a viable pregnancy did not occur after her first estrus, the cycle will repeat, affording a subsequent opportunity for reproduction. As an example, Asher (1985) measured up to six consecutive estrous cycles in fallow deer in New Zealand, kept in isolation of reproductively capable males. Though unlikely to occur in the wild, this could theoretically allow a doe to be fertilised in as late as August, well beyond the traditional rutting period of April for Southern Hemisphere Fallow. The number of possible cycles varies between species, as does the timing and degree of seasonality.
The diagram (below) by Sengar and coworkers, illustrates four categories of animals with an estrous cycle. The typical pattern in temperate deer is depicted in the third data set labelled “seasonal polyestrous (short day)”, illustrating a cluster of peaks in estrogen corresponding to successive estrous cycles, which occurs during months where day length is short. Note that this graph applies to the Northern Hemisphere, and is therefore six-month out of phase from the Southern Hemisphere. This demonstrates that multiple opportunities for reproduction occurs within the breeding season for most deer species.
What Triggers the Estrous Cycle?
For temperate deer species, breeding is seasonal. In other words, the onset of their first estrous cycle correlates with a particular time of year. This ensures that the majority of births occur at the most climatically suitable time. Natural selection has determined that seasonal breeding is optimal for deer native to areas which experience significant climatic changes throughout the year. As a result, this is hard-wired into their genetics. The trigger for the onset of the estrous cycle in deer in photoperiod (day length), with light being detected and ‘measured’ by the pineal gland (Vasantha, 2016). The process is illustrated in the diagram below, where light is seen to enter through the eye and is detected by the brain. During the night, melatonin levels increase and drop sharply during the day (see second diagram below). As the length of the day shortens throughout Autumn, periods of darkness increase and therefore the amount of melatonin produced also increases. Bubenik (2006) describes the pineal gland as serving as both a “clock” and a “calendar”, affording the animal a subconscious ability to detect the time of year. When the quantity of melatonin increases beyond a threshold, other hormonal changes are triggered, leading to the onset of the breeding season. Day length is therefore attributed to being the primary trigger for the onset of the estrous cycle in temperate deer species.
Effect of Latitude
The amount of daylight is influenced by latitude (i.e. distance from the equator), as shown in the diagram below. The ratio of dark/light in a 24hr period gradually changes with distance from the equator, with polar regions experiencing extreme variation in photoperiod throughout the year.
Given that photoperiod has a key role in relation to the estrous cycle in many deer species, would it not also make sense that the latitude would influence the timing of reproduction? The answer probably depends on the species, but in attempting to answer this question, I have been unable to find any specific examples where latitude did play a significant role in reproductive timing – this does not mean it isn’t a factor in any species, and probably warrants a deeper investigation of the individual species in question to be sure. For example, Fletcher (1974) compared calving dates of red deer in captive herds of widely varying latitude, to establish any differences in reproductive timing. It was found that aside from a six-month change seasonality between the Northern and Southern Hemisphere, no difference in timing of reproduction or length of gestation was measured in response to latitude. It is important to note that the lowest latitude herd studied was in Pretoria, South Africa (25°S), so on the basis of this study it remains unclear what would occur if red deer were placed in a near-equatorial area. Bronson (1988) reports that members of Odocoileus (whitetail/mule) below latitudes of 25° are non-seasonal in their reproduction, with very low latitude populations being continuous breeder. Unfortunately, it isn’t clear if these differences are in direct response to photoperiod, or differences in genetics or other climatic factors, given the populations studied were native subspecies and therefore natural inhabitants of the given location. I would speculate that if a seasonally-reproducing deer species was placed in an environment at low latitude, it would become aseasonal its reproduction, however, I have not been able to confirm this and therefore it remains an unanswered question, at least in my mind. Please make contact if you can shed some light on the topic!
Conclusion
In an attempt to make the content of this article applicable to deer in general, there remains many questions in relation to specific species and how they differ from one another. Consequently, a more specific review of individual species will need to be addressed in subsequent articles. In summary - photoperiod is a key factor for triggering reproductive cycles in non-tropical deer species, helping ensure the best survival outcome for the next generation through a clustered birth in optimal environmental conditions. Tropical deer species are not subject to the same seasonality as temperate species, and therefore lack a well defined seasonal reproductive period. Whilst a variety of other variables do exist which will influence when individual animals will experience first estrous, photoperiod is well established to be the main influencing factor on reproductive seasonality in deer species.
References
Asher, G. W. (1985). Oestrous cycle and breeding season of farmed fallow deer, Dama dama. Journal of Reproduction and Fertility, 75(2), 521–529.
Bronson, F. H. (1988). Mammalian reproductive strategies: genes, photoperiod and latitude. Reproduction, Nutrition, Development, 28(2B), 335–347.
Bubenik, G. A. (2006). Seasonal regulation of deer reproduction as related to the antler cycle-a review. Veterinarski Arhiv, 76, 275–285.
Burley, N. (1979). The Evolution of Concealed Ovulation. The American Naturalist, 114(6), 835–858.
Fletcher, T. J. (1974). The timing of reproduction in Red Deer (Cervus elaphus) in relation to latitude. Journal of Zoology, 172, 363–367.
Sengar, P. L. (2003). Pathways to pregnancy and parturition. Pullman, WA, Current Conceptions.
Vasantha, I. (2016). Physiology of Seasonal Breeding: A Review. Journal of Veterinary Science & Technology, 7(3), 1–4.
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