Social Position Is Important to Female White-tail Deer
Female deer society is organized into groups in which the matriarch, or oldest female, establishes a home range where offspring often settle and remain throughout life. Social bonding has evolved as a means to better compete for resources, such as food and birth sites, and to better avoid the risk of predation. Some female deer are dominant over the other deer while others are subordinate, sometimes within groups or even between groups. Dominance is a function of age and body size in female deer, so females must survive long enough to achieve a high position in the deer population. The social position of the matriarch determines the longevity of each group, with the experience of dominant females increasing offspring survival and reproductive success.
Our studies in Illinois showed that dominant females live significantly longer than subordinate females (8 years vs. 5 years) and produce more fawns that live to adulthood. More fawns of both sexes remain close to the dominant mother and do not disperse away compared to the offspring of subordinate mothers. Offspring that do not move large distances to uncertain landscapes survive to an older age. And staying with the group provides the stock for perpetuating the social group if the dominant matriarch is killed, since the oldest female offspring often becomes the group leader. Dominant females also establish home ranges that are large in size and occupy the best foraging sites. These tend to remain stable over time, further benefitting dominant social groups.
In contrast, subordinate mothers tend to settle on home ranges where the landscape is more dangerous to survival. While agricultural crops provide food, the preferred habitat are forests. Landscapes in the Midwest are usually dominated by agricultural crops featuring a boom and bust cycle of food availability as crops grow, then disappear when harvested. Often these areas can be close to a busy highway, posing a danger from vehicles. These new sites also may be unsuitable for long term survival because they are hunted heavily, a feature not evident when the female settles, usually in late spring or early summer. Since both sexes move each year onto these sites from refuge areas and larger forests, they can counter to some extent the losses to hunting. Thus, the decisions on where to settle can seriously affect deer survival and therefore, deer populations. Higher death rates results in a rapid turnover in offspring as they are killed or move away from their natal range.
An example of the consequences of social status was noted while capturing deer during a research study. While dominants allowed subordinates to access to bait sites, subordinates were often subjected to harassment before being allowed to eat. Dominant deer often had an advantage by having first access to more and better quality food.
How do female deer end up subordinate within a social group? These females may have been born to a subordinate mother, orphaned while a fawn, crippled by an accident, or born to a migrating or dispersing mother and so matured without additional female kin to assist with daily living. Females produced by these mothers often also disperse away, moving an average of 30 to 40 km (18 to 25 mi) before settling in a new area.
Fortunately for subordinate females, Illinois has a relatively mild climate, and therefore, the boom and bust food cycle does not greatly impact their survival over winter as it might in other parts of their range. In winter and early spring, large groups, comprising both males and females, may be seen feeding together. These groups, of 30 to 40 deer or more, are usually led by a dominant female who determines the feeding and bedding locations of the group. Next time you view one of these groups, see if you can determine which deer is the dominant one.
Charlie Nixon is a wildlife ecologist retired from the Illinois Natural History Survey. Phil Mankin is a wildlife ecologist retired from the University of Illinois.