Introduction
Reproductive behaviour and mate selection of Red Deer (Cervus elaphus) has been studied by several researchers. Different aspect of those selection processes has been studied. Different factors that contribute to differential mate selection process among this species have been identified but there is no factor has been concluded to be the most important factor among those identified. Population density, body size, food availability, migration aspect, aggression, seasonal changes, groups, sound production etc. all have been found to play one role or other in mate selection process in red deer. Studying those mating process is somewhat demanding simply because of the nature of their feeding act, aggression and migrating factors which will not allow a study population to be monitored for appropriate period of time. The common terms in this research include: harem size (mean number of females seen per day with each male during the breading season), oestrous (a period of hormonal cycle), foraging (similar to a period of search for food), and population density (Number of Red deer within the study area). There is no specific factor(s) that has been assumed to be the prolific behaviour initiating mate selection among Red Deer. However, there has been several researches focusing on mate selection among that species of mammals and most of the results of those factors have supported different features of this group of mammals and the complexity nature of the breeding system. Three different articles will be reviewed and their findings as it relates to mate selection in red deer will be focused and discussed in the subsequent section of this paper.
Population density: a strong determinant of mate selection
An important determinant that has been found to contribute to the mate selection in this mammal is the 'high population density'. Clutton, Rose & Guinness, (1997) explored the sexual selection process of red deer and found out that population density plays an important role in the process. The data used were collected for a period of 18 years and consequences of changes in population densities were evaluated. Several factors were included as the prerequisite for each sex to be a member of the study population. Most of the females were found to remain in study area for the duration of study while about 12 per cent males remained. The others were found to have migrated to other regions. Those results were explored based on the changes in survival and the sex ratio which was found to be in favour of resident females. Changes in harem holding were found to be related to a form of biased sex ratio whereby harem held by young males could increase. Changes in the distribution of mating success were found to relate to decrease in numbers of males holding harems as sex ratio shift in favour of females. Mating success was found to reduce also based on the reduction in the proportion of the resident males or immigrants males. The collected data reveal the changes in survival and the sex ratio. The data shows the decline in the population of matured males (p < 0.001) with an appreciable decrease in survival of immature males (p <0.001) hence shifting the sex ratio in favour of females. The harem size does not change significantly as regards to those with mean (p = 0.0067) and median sizes (p =0.416). Adult sex ratio was found to shift towards female. The ratio shifted from 0.47 to 1.72 females per male. All these changes were noted by the researchers and some conclusion were made based on the complexity of the factors affecting the mate selection. Factors such as higher mortality rate among males when compared to females also contribute to the differential population density. The data presented by the researchers suggested that there is a high tendency that mate selection in a region with gender bias in favour of female is based on changes in intention for competition. The migration and competition for mate selection decreases in the cases of high population density with biased sex ratio.
Female-Female Competition for mate selection
When zoologist discusses the competition for mate selection, their focussed is mostly on the roles of the males in achieving dominance for mate selection. Researchers are changing that perspective and focusing more on the role of females in mate selection. Female to female competition is the focus of the Nicole & McElligott (2006) in their research which focused on the competition for mates in red deer. The research was based on the aggression of some female deer which were thought could be a sign of competition for mate selection. Different factors were considered by the authors as a factor that contributes for the need to compete for males among the females. The highlighted factors include; a densely populated zone which is female biased a situation whereby both female and males have similar parental roles, in need of food, and a case of avoiding predators or harassment.
The basis for their study was comparison between experience in harem as an oestrous female and when not oestrous. Female’s aggression rates where compared i.e. in foraging groups and when in female harem. Mode of checking for the aggression rates and group size was an important aspect of the study. Foraging was higher that harem in numbers. The findings were done with some mathematical calculations. Observation shows the p-value (<0.001) to less than 0.1 for the oestrous females in harems hence the aggression rates were consistent with other previous findings. That group of female shows highest aggression rates within the social context examined. The p value for other groups that were studies are 0.001 and 0.78 for the foraging group and oestrous group respectively. Overall aggression rate per hour from data presented shows the oestrus group to have the highest mean value of 1.19 when compared to other with 0.93 and 0.81 for foraging and harem groups respectively.
The author also indicated a factor that is dependent on aggression. It was indicated that the females in the foraging group were much easier to monitor or evaluate compared to those that are in other social context. The aggressive interactions the study was based on were: Ear threat, nose threat, displacement, kicking, biting and chasing. Aggressive interactions which the study was based on revealed that displacement is the most common form of aggression followed by nose threat and kicking.
The Bar chart showing the aggression rate per hour revealed that the group with highest aggression rate is the oestrus group followed by the foraging group. The data revealing highest aggression rate in oestrous group supports that there might be some level of female to female competition for mate selection. The authors suggested that since there has been lowest rate of the aggression in harem groups hence the increased rate within the oestrous females provide a strong indication for a need for mate. The aggression might contribute to a form of competition for mate need.
Acoustic Cues
The study conducted by Benjamin, David & Karen (2007) focused on the essence of acoustic cues produced by male deer as regards towards mate selection. The experiment explores the role of the sound in influencing the females mate choice. The acoustic cues were related to body size and they were able to find some level of relationship between the oestrous red deer and the quality and volume of sound that is being produced. The data presented by the authors from their study shows that the hind moves towards the simulator (speaker) playing large size variant of sounds compared to small size variant (p=0.046), the hind also took more steps towards speakers with abnormally large size variant than speakers with abnormally small size variant (p=0.006), time factor in response were found to be insignificant.
Their study gives them the basis for concluding that female’s deer mate choice context is dependent on acoustic cues for the mating response. The response was found to be sound dependent which suggests a presence of large sized male producing roar sound. Although the experimental data shows that it is the sound that actually matters most when compared to noting if the sound was actually produced by a large deer. They also concluded that such female preferences may have served to allow the differential selection process in support of male-male competition and size dependent mate selection. Their implicated the use of sound as a determinant of the male quality in mate selection process.
Conclusion
The reviewed articles clearly depict a multifactorial complexity nature of the mate selection process among Red Deer. Reviewed article focussed on different perspective with the same goal. It can be seen clearly from the articles that both male and females compete for the process of mate selection. Population density seems to serve as the main key factor in determination of the sex that would be competing for mate. This is because, in a female sex biased situations which is seasonal dependent, population of the females will be more and there will be a subsequent fight for male red deer. In situation where the population density is normal, males tend to fight for control hence leading to emigration of some males. Other factors that are being considered can be regarded as supporting factors in mate selection.
Work cited
Charlton, B., David, R. & McComb, K. Female red deer prefer the roars of larger males. Biol. lett. (2007) 3, 382-385.
Clutton, T.H., Rose, K.E., & Guinness, F.E. Density-related changes in sexual selection in red deer. Ecology of sexual selectioin. Proc. R.Soc. Lond. (1997) 264, 1509-1516
Cristina et al. Reproductive behavior in female iberian red deer: effects of aggregation and dispersion of food. Journal of mammalogy, 85(4): 761-767, 2004.
Malo et al. Antlers honestly advertise sperm production and quality. Proceeding of the royal proceedings. Proc.R.Soc.B (2005) 272, 149-157.
Nicole Bebie & McElligott, A.G. Female aggression in red deer: Does it indicate competition for mates?. Original investigation. Mamm. biol. 71 (2006) 6. 347-355.
