Recently I read the publication ‘The red deer rut revisited’ (Stopher, K.V., 2011).
Some of the behaviours described in this study made me curious.
The publication was dedicated to red deer female behaviour during the mating period and specifically, roaming over rather long distances (up to 4 km) and joining of new harems that often resulted in the new male siring the offspring but that did not necessarily result in finding a male which was a better candidate (estimated by harem size, male age, relatedness).
What I found interesting was that the interherd movements were not necessarily ‘improvements’ for estrous females with regard to mate selection.
I was wondering if these movements (specifically, by females in estrus because females can also move during pre-estrous and post-estrous phases of the rutting period) could be related to other factors.
There are two causes to these movements that come to mind:
1. They could be related to some type of physical inconvenience that the female experiences during the time she is estrous and this state might be alleviated if the female is moving around rather than if she stays sedentary;
2. They could be related to biological benefits, i.e., if the female has had physical activities during estrus, she might be better able to conceive or otherwise improve her fertility (the health of the future offspring).
In the publication it was mentioned that –
‘The likelihood of females changing harem also varied with year (F29,6511.7 = 4.59, P < 0.001) and decreased as rainfall increased (Estimate = −0.009 ± 0.002, F1,15788.5 = 35.46).’
During years when rainfall increases, the foraging resources might be lusher (vegetation is more abundant or of greater quality) and therefore, the female might not feel pressured to to move (because either her physical condition is satisfactory enough to prevent her from experiencing any physical distress during estrus, or her physical condition has been satisfactory and she does not have the need to improve the conception chances).
Of course, during rainy years it is also possible that the females do not move as much because the weather prevents comfortable travel that is not too costly with respects to thermoregulation expenses.
Also –
‘Females moved shorter distances between observations during the peak of the rut but moved further when rainfall was higher and when the minimum daily temperature was warmer (see Table 1). ‘
It was not specified how rainfall and temperature affected females in estrus, in particular, only that estrous females still moved further than non-estrous females when accounting for weather factors.
This counteracts the former assumption that during rainy periods females are less prone to move because the statements refers to shorter temporal scale (intervals between observations rather than the entire year).
I imagine that perhaps rainfall and temperature might affect the capacity of the female to be active for the length of time necessary to travel certain distances.
For example, if estrus causes a certain physiological state which feels aggravating, it might be easier for the female to travel further (or to travel at all) on days when it is not too hot, nor too cold.
One of the findings concerned the consequential joining by the female of another herd which was usually larger.
While larger herds might be selected for because they demonstrate the viability of the male and they ensure lower harassment rates within the herd and protection from unwanted, young suitors from outside the herd, I was wondering that it might also be easier for the female to end up in a relatively large harem also for the reason that it could be statistically easier for the female to encounter a larger harem (rather than to encounter a small size group).
While estrous females, specifically, did not appear to opt for larger harem size (relative to the one they left), this might be a general consideration to contemplate regarding the average greater likelihood of all females (pre-estrous, estrous, post-estrous) to join larger herds.
I draw this idea from research regarding wolf-prey dynamics.
Wolves are cursory predators who tend to travel along routes where the likelihood to meet prey is high but still this prey-searching method is largely opportunistic.
It is more difficult for wolves to find solitary prey than to find prey that has formed larger groups.
But if the wolves encounter the solitary individual, the individual is very likely to be killed (because it is the only possible victim).
It is also comparatively difficult for wolves to find herds of the size 2 – 5 animals and these herds actually have the highest survival potential.
It is easier for wolves to encounter large herds (> 5 individuals) due to their general perceptibility (visual, olfactory, auditory) but in large herds the potential for a specific individual to be killed is rather low because the risk becomes ‘diluted’ (many individuals = many potential victims).
Thus, grouping in large herds does not benefit the group as such (because larger groups are easier detected by the wolves) but it can benefit individuals.
I cannot find which publication discussed this phenomenon (I believe it was a study in Yellowstone National Park, US) and if I come across it, I will reference it here.
Returning to the red deer rut matter – if the female red deer roamed, it might be equally easier for her to meet up with larger herds simply because larger herds are easier to detect.
A counterargument to this assumption is that the red deer harems are not necessarily distributed in a scattered manner on the landscape (but aggregated – one harem bordering another and some harems being more central than others, peripheral harems) and that in order to join a larger herd, the female would have to pass other herds that she selects not to join.
Additionally, larger herds might be better able to hold better quality grazing ranges so the female might join the larger herds for the quality of forage.
I am generally not convinced (but this is a personal opinion) that animals like red deer engage in many behaviours simply due to, e.g., competition or mate selection because I find that humans tend to prescribe certain states and motivations to animals that are less complex than the animal’s true perceptions of its environment and its internal aspirations.
For example, the females engage in this roaming behaviour despite apparent great benefits regarding to the mate selection hypothesis, and the costs that the red deer stags bear over their behaviour that has been considered purely competitive in its motivation are truly great only to later lose > 40% of females due to their own choice to wander off during the specific day of estrus.
The sheer rates of roaring and mate guarding by the red deer males appear somewhat exaggerated to me if competition is the only motivation and if the females are stolen or leave anyway.
It makes me wonder whether certain competitive displays by the males are not, as well, in fact, directed at improving the quality of their sperm (offspring) because such behaviours should affect the hormonal state of the animal and the competitiveness might only partly be related to the females themselves and to the agonistic males but also to ensuring that the physiological processes in the stag’s body are stimulated to produce the most viable sperm.
While I do not have biological data to substantiate this claim, it is my personal conviction that red deer stags do not perform all of their famed rutting displays with the purpose of impressing females and intimidating competitors but rather that they are true performances where, through certain acts, the stag aims to induce body states that might beneficially affect its fertility / viability as a father.
Therefore, my belief is that the red deer stags aim these displays not merely at other adult red deer but rather at their future offspring and that these behaviours are theatrical by nature in the sense that the red deer stag plays certain roles by executing certain activities (roaring, patrolling etc.) in order to maintain physiological rates needed in a future father to ensure fertilization and healthy offspring.
Notes by Ieva 