Dominance-subordinate relationship in Zulu sheep rams*.
Social ranking is usually caused by limited access to resources such as feed, water as well as mating partners. In rams, social dominance is mostly related to physical traits such as body weight, horn size, body length and scrotal circumference. The objective of the study was to determine the relationship between physical traits of Zulu sheep rams and the establishment of social rankings. The social dominance rank was determined by a feed competition test using rams of the same age. Physical traits such as body weight, chest girth, horn length, scrotal circumference and withers height were measured for each ram. Sheep A was ranked first with a 100% number of wins (P < 0.01) followed by sheep E with an 86% number of wins (P < 0.05). A positive linear relationship between time spent on the feeder against the number of wins was not significant (P > 0.05). There was a significant positive correlation between the proportions of wins against horn length (P < 0.05) and chest girth length (P < 0.05). Time spent at the feeder was positively correlated with body weight and withers height (P < 0.05). Social dominance in Zulu sheep can be determined by particular physical traits such as horn length and chest girth.
- body weight and chest girth
The importance of reproductive success in mammals, specifically in males, has caused the notion of social dominance to be investigated . Social rankings are associated with inadequate access to various resources such as water, food, territory and shade . Ungerfeld and González-Pensado  reported that mating performance is affected by hierarchical relationships in rams. Lower-ranking rams have restricted access to on-heat ewes . Social rank is achieved by provoking other members in dominance fights .
Social rank is determined by physical traits such as body size, horn size and body condition score . However, the authors further pointed out that rams with larger testicular circumference have a higher mating rate. Kabiraj et al.  reported that larger rams have larger testicular sizes. Larger and dominant rams have a tendency to suppress submissive rams to mate ewes .
Under an extensive farming system, where animals are reared in one herd, dominance ranking is likely to occur . Research to understand the relationship between social behaviour and body developments in male ungulates is limited. In general, very little is understood about determinants of the individual rank of male animals of any ungulates . However, some studies were conducted on Merino and Border Leicester sheep , Bighorn sheep  and with three breed crosses, Wurttemberg, Ile De France, and Pirot Pramenka . Such study has not been documented in indigenous Zulu sheep. Therefore, the objective of the study was to investigate the relationship between the physical traits of Zulu sheep rams and the establishment of the dominance hierarchy.
2. Materials and method
The experiment was conducted at the University of Zululand farm (South Africa), 28.8500° S, 31.8333° E in the small ruminant section. Eight rams of the same age (3 years) were used. To determine the social rank, the feed competition method by Maksimović et al.  was used. To initiate aggressive behaviour rams were subjected to fasting for 12 hours before data collection session. However, water was provided
3. Statistical analysis
Data were analysed using SPSS. The number of contests and wins was recorded for each sheep. The number of wins was converted to a proportion and the binomial test was used to compare against the expected number of wins. A Pearson correlation analysis was used to test the relationship between body measurements, proportions of wins and minutes spent at the feeder.
Tables 1 and 2 show the dominance-subordinate relationship in Zulu sheep rams. Ram A was ranked first with a 100% number of wins (
|Subject||Opponent sheep||Number of contests||Number of wins||Proportion of wins||Rank on proportion of wins||p-value|
|Time spent (%)|
|Sheep Tag||At the feeder||Displaced from the feeder||Attempting to re-enter feeder||Time spent on the edge of the feeder|
As shown in Figure 1, a relationship between time spent on the feeder against the number of wins was not significant (
|Body weight (kg)||Withers height (cm)||Scrotal circumference (cm)||Horn length (cm)||Chest girth (cm)||Time spent at the feeder|
|Proportion of wins||0.165ns||0.332ns||0.147ns||0.634**||0.461**||0.434ns|
|Body weight (Kg)||0.572**||0.513**||0.160ns||0.639**||0.552**|
|Withers height (cm)||0.115ns||0.109ns||0.361ns||0.474**|
|Scrotal circumference (cm)||0.696**||0.099ns||0.188ns|
|Horn length (cm)||−0.247ns||0.220ns|
|Heart girth (cm)||0.060ns|
According to Keeling , in the absence of ewes, subordinate rams tend to initiate some of the agonistic interactions challenging high-ranking rams. After losing an encounter, the subordinate ram may display submissive behaviour to the winner. In the present study, agonistic interactions involved pushing, horn threat, head butt, chasing and low stretch. However, these behaviours were not quantified. These results were in accordance with the behaviours observed by Keeling  where low stretch and horn threat were observed as an agonistic behaviour in rams. The author described the low stretch behaviour as a threat display in which a ram lengthens its neck forward and horizontal to the ground. Pelletier and Festa-Bianchet  observed similar agonistic interactions during contests in rams. The observed behaviours included front kick, frontal clash, rubbing, butt, non-contact displacement, and horn threat. Similar to the present study, Roberts et al.  observed head butt behaviour as common in rams. This is when rams are slamming their heads together until one ram withdraws from the encounter. Squeezing was another behaviour, which was commonly observed in the present study where a ram would squeeze itself between closely aligned rams. This was similar to the observation by Erhard et al.  where a feeding ram would stand almost parallel to the wall holding the feed hopper and eventually other rams squeezed in between the feeding rams.
Roberts et al.  described ‘win’ as a situation where a sheep wins an encounter, either by initiating and displacing another sheep or fending off another ram trying to displace it. The ‘loss’ is a situation where one sheep loses an encounter, either by starting an encounter and failing to displace the other sheep or by being displaced by another individual initiating an encounter. The insignificant linear relationship between the proportion of wins and time spent at the feeder suggests that rams with a higher number of wins did not necessarily spend more time at the feeder. Some rams with a low proportion of wins were able to spend more time at the feeder compared to rams with higher proportions of wins. Low-ranking rams might have gained more access to the feeder by shifting laterally and squeezing themselves between aligned rams, or they would wait for the dominant ram to turn its head down and they would quickly rush into the available space. Squires and Daws  reported results similar observations. Dwyer  suggested that when the feeding space is limited there is an increase in displacement at the feeder, and some of the sheep will stop feeding and become non-feeders. Thus, a decrease in time spent at the feeder was due to forceful displacement and disturbances at the feeder of low-ranking rams by high-ranking rams. In dairy cattle, the reduction of feeding space per cow in dairy cattle increases agonistic encounters even if the feed is provided
A significant correlation between the proportion of wins against horn length and chest girth suggests that rams might have used their larger horns and wider girth to fight and gain feeding space in the feeder, thereby increasing the proportions of wins. Body mass and horn length mainly affect social rank . Bergeron et al.  stated that the heaviest males with long horns are generally at the top of the hierarchy. In this study, there was strong positive correlation existed between the proportion of wins and chest girth. Body weight was positively correlated with withers height, scrotal circumference, horn length, and time spent feeding but strongly and positively correlated with chest girth. In accordance with findings in this study, Maksimović et al.  also obtained a significant relationship between body weight and chest girth.
The social rank status of the ram can be determined by the proportion of wins. High proportions of wins seem to be related to high social rank status. Body weight was associated with chest girth whereas horn length was associated with scrotal circumference. The social rank of Zulu sheep is not affected by body size.
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