From The Australian Deer Magazine series, Game Management With Neal Finch
One way to describe and quantify a wild deer herd is productivity. Aldo Leopold defined deer productivity back in 1933 as ‘the rate at which breeding stock produces mature removable stock’. This definition includes several aspects of deer ecology including the birth rate and survival of young. A herd that is increasing in number is said to be productive whilst one that is decreasing is not. A herd may also be considered productive if there is an abundance of individuals recruited into each age category annually that can sustain harvesting to maintain the overall size of the herd. In recent decades the concept of a quality deer herd has expanded Aldo Leopold’s original definition of productivity to also include the number and size of mature males in the herd.
There are many influences on a wild deer herd’s productivity including the obvious things like food availability, seasons, predators, competitors and disease. Predator control is one of the oldest and most widely practiced forms of game management implemented around the world. Supplementary feeding has also been around a long time. Both these simple approaches can significantly increase the productivity of a wild deer herd in a few short years. However the long term consequences of these approaches can have detrimental consequences for habitats and ultimately deer when the carrying capacity of a landscape is exceeded.
Other more subtle effects on a wild deer herd’s productivity includes the density, age structure and sex ratio of the herd. As with all mammals deer behaviour and biology is controlled by their hormones. You may not realise it but your desire to eat, sleep, fight or reproduce are all triggered by your endocrine system. This is the term used to describe the many hormones, and the glands that produce them, we all have. Just what hormone is released, how much and when is influenced by many factors including our age, diet and who or what is around us. For instance when something frightens or excites us adrenaline is released into our blood system which gives us an increased heart rate and the ability to fight or run immediately if needed. Whether we choose to fight or run may depend on our level of testosterone in our body at the time. This kind of hormonal response is very obvious but most of the effects of our endocrine system are far more subtle.
In the 1970s wild deer managers in the United States started to look at the various effects decades of active herd management was having on the behaviour of wild deer. At this time most harvest seasons were limited to male only harvests leading to many skewed sex ratios in American wild deer herds. In many areas there were few large predators in the habitat (other than recreational hunters) and supplementary feeding was common. Densities of deer were high and dominated by young animals. Few males in these herds reached a mature age class. Whilst the deer being managed were native and wild the herds were not what we might describe as ‘natural’. Experiments were conducted where some herds were managed to actively transform these herd characteristics. Increased harvest rates included many females and males were left to mature. Some surprising results started to appear in just five years.
One of the most easily quantified changes was the length of the breeding season or rut. In a herd with a skewed sex ratio and young age classes the rut lasted up to 96 days. By comparison a herd with a balanced sex ratio and more mature animals the rut only lasted 46 days. These changes correlated with measurable differences in various hormones in the deer. Other changes were noticed as well and included dispersal patterns, the age at which animals reached sexual maturity and the physical development of animals. What may surprise many hunters is the antler size of males in each age category was measurably different. The presence of mature animals in the herd together with the overall density of deer was changing the hormonal activity in the herd’s individuals.
Since these early experiments there have been many research projects around the world that have explored various biological aspects of wild deer in relation to herd density and structure. It turns out that herd dynamics can have an effect on most physical attributes of individual animals either directly or indirectly. Many of the behaviours we see in wild deer are contributing to the hormonal activity in the other deer that share their world. Aggressive behaviour from other herd members as well as the nurturing behaviour of a mother will both affect a young animal. One of the interesting aspects of deer behaviour is they don’t have to be actually present to change the hormonal response in other deer. All of the sign posting that deer do throughout their territory contribute to communicating more than just their presence. Scapes, rubs, preaching stands and wallows are ways deer actively share information with others. Beds, trails, urine and faeces are also contributing to the hormonal activity of other deer who encounter these signs. Deer have many scent glands all leaving messages for other deer during their daily activities. When a deer encounters a message from others it will pick up far more than just the knowledge that another deer is present. It is likely that deer know the sex, age and reproductive status of all other deer that live in the same area.
The presence of older males can significantly change the development of younger males in many mammal species. Dominance behaviour in males takes several forms and physical aggression accounts for only a minor fraction of all the possible interactions that occur. An extreme example of this phenomenon occurred with African elephants as a result of culling activities in Kruger National Park back in the 1990s. At the time the culling programs occurring did not target juveniles which created orphans who matured without the normal range of elephant interactions during their life. Years later park rangers reported dozens of rhinos being killed by young male elephants. Whilst aggression between elephants and rhino had been reported before it was not a common occurrence. It is believed that the young male elephants responsible had hormonal imbalances caused by a lack of mature males present in their lives! The problem was resolved with the introduction of six mature male elephants into the same area.
All wild deer species present in Australia have breeding behaviour characterised by male dominance over potential rivals. Body size, fighting skill and antler size is only part of the story. All the ritualised breeding behaviours male deer exhibit are part of an elaborate game of telling other males who they are. Two potential rivals need never meet yet will know all they need to about the competition to pass on their genes to the next generation in their home range. All these behaviours are controlled by hormones, particularly testosterone. The males in each age category will respond differently to the hormonal cues left by a more mature or dominant male. In an immature male testosterone may be suppressed by the aggressive mating behaviour of a more mature animal. However a mature male may respond to the aggressive behaviour of a rival of equal standing with the release of more testosterone.
The sex ratio and age structure of a wild deer herd can drastically alter the herd productivity. Where there are more females than males the breeding seasons are likely to be prolonged. The effects of prolonged breeding can decrease the fitness of both males and females in the herd. It also increases the chances that less dominant males will have a chance to mate. The presence of mature males in a herd will delay the maturing of younger males which increases the likelihood that they will grow to their full genetic potential. In any herd where there are an equal number of males and females present the most dominant males will always pass on the majority of their genes to the next generation. Where there are significantly more females than males in a herd the less dominant males will be more likely to breed regardless of physical characteristics.
If as recreational deer hunters in Australia we acknowledge our role as active wildlife managers then we need to embrace our ability to alter wild deer herds. The choices we make in the field on what animals to harvest ultimately changes the herd structure. A commonly held belief among Australian deer hunters is that removing a cull stag will improve herd genetics. The theory seems sound enough. Males with poor quality antlers will pass on their poor genetics if allowed to breed therefore harvest them to prevent this happening. If the herd in question lives behind wire then yes it makes sense to keep the males with the most desirable characteristics to do all the breeding. However truly wild deer are responding to many variables in their environment particularly herd structure. Removing cull stags alters the sex ratio as does harvesting spikers (yearling males) for meat. Altering the sex ratio is likely to have more impact on the remaining deer than leaving a mature male in the herd.
McShea, W.J., Underwood H.B. and J.H. Rappole. 1997. The Science of Overabundance, Deer Ecology and Population Management. Smithsonian Books.
Leopold, A . 1933. Game Management. Charles Scribner's and Sons. New York.