Social Dominance in South African Indigenous Zulu Rams

Abstract

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.

1. Introduction

The importance of reproductive success in mammals, specifically in males, has caused the notion of social dominance to be investigated [1]. Social rankings are associated with inadequate access to various resources such as water, food, territory and shade [2]. Ungerfeld and González-Pensado [3] reported that mating performance is affected by hierarchical relationships in rams. Lower-ranking rams have restricted access to on-heat ewes [4]. Social rank is achieved by provoking other members in dominance fights [5].

Social rank is determined by physical traits such as body size, horn size and body condition score [3]. However, the authors further pointed out that rams with larger testicular circumference have a higher mating rate. Kabiraj et al. [6] reported that larger rams have larger testicular sizes. Larger and dominant rams have a tendency to suppress submissive rams to mate ewes [7].

Under an extensive farming system, where animals are reared in one herd, dominance ranking is likely to occur [2]. 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 [2]. However, some studies were conducted on Merino and Border Leicester sheep [8], Bighorn sheep [9] and with three breed crosses, Wurttemberg, Ile De France, and Pirot Pramenka [2]. 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. [2] was used. To initiate aggressive behaviour rams were subjected to fasting for 12 hours before data collection session. However, water was provided ad libitum. The feed was put in an immovable concrete container. Before the experiment commenced, body weight, pelvis length, horn size and chest girth were measured in all rams. To simplify observations and analysis, the animals were recorded by a CCTV camera giving a clear view as the animals entered the feeder site and during feeding [8]. After the experiment, the video was watched and analysed for evidence of dominant interactions. The position and time spent at the feeder by animals were also analysed. Furthermore, the activities of each ram were described using the method of Squires and Daws [8]. Each ram’s activities were categorised as follows: (a) retained a fixed position at the feeder trough, (b) evacuated from the feed trough, (c) attempted re-entry on the feed trough and (d) on the edge. As ram entered the feeder, their identification number was noted and time spent in all four categories was recorded. Time spent by rams was recorded as they forced their way to the feeding trough, or attempted to enter the feeder, between adjacent aligned rams at the feeder. The ram trying to search for a space at the feeder was also noted. The number of contests between two rams was recorded, with a dominant and subordinate ram identified.

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.

4. Results

Tables 1 and 2 show the dominance-subordinate relationship in Zulu sheep rams. Ram A was ranked first with a 100% number of wins (P < 0.01) followed by Ram E with an 86% number of wins (P < 0.05). Ram D was ranked number 6 with only one win out of seven (P < 0.01). From the behavioural observations, rams divided themselves into two groups when they were introduced to the feed. The first group rapidly entered the feeder whereas the second group remained on the edge of the feeding trough or tried to find a space in the feed trough. Rams at the edge of the feeding trough struggled to push their heads in it by fighting rams, which were already feeding, but while trying to get to the feeding spot other rams pushed them away. Throughout the feeding, there were contests and pushing between individuals. Due to the overcrowding at the feed trough, very few rams were able to occupy one spot without being pushed away. However, some rams were able to maintain their position at the feeder by positioning themselves laterally. In some instances, due to the need to feed and the pressure at the feeder, some rams (second group) gained access to the feeding trough by forcefully pushing their heads between two closely aligned individuals. Rams, which struggled to enter the feeding trough, were hindered by adjacent individuals, which were closely aligned to another ram, thus subsequently blocking entry to the feed trough. The behavioural sequences were complex at the feed trough, when one ram forced his way to the feeder; one or more rams lost their spot at the feeder. A dominant-ranking ram (i.e., Ram A) caused low-ranking rams to be submissive and flee. Submissive or subordinate rams were reluctant to try to find a feeding spot near the dominant ram due to the threatening behaviour displayed by the high- ranking ram. However, the submissive ram sometimes would wait for the dominant ram to stop its threatening behaviour and turn its head to feeding, and then a submissive ram would quickly move into the available feeding spot by doing so it would be allowed to stay and feed.

Subject	Opponent sheep								Number of contests	Number of wins	Proportion of wins	Rank on proportion of wins	p-value
Sheep	A	B	C	D	E	F	G	H
A		+	+	+	+	+	+	+	7	7	1.00	1	***0.0041
E	−	+	+	+		+	+	+	7	6	0.86	2	**0.0284
B	−		+	+	−	−	+	+	7	4	0.57	3	^ns 0.3555
F	−	+	+	+	−		+	+	7	5	0.71	3	^ns 0.1332
H	−	−	−	+	−	−	+		6	2	0.33	4	^ns 0.2025
C	−	−		−	−	−	+	+	7	2	0.29	5	^ns 0.1332
D	−	−	+		−	−		−	6	1	0.17	6	^ns0.0630
G	−	−	−		−	−		−	6	0	0.00	7	***0.0072
Losses	0	3	5	5	1	2	6	5

Table 1.

Dominance-subordinate relationship in Zulu sheep rams*.

*

These results emanate from our own experiment.

**

P<0.05.

***

P<0.01.

Reading across: + = dominant and − = subordinate. Reading down: + = subordinate and − = dominant. Blank spaces indicate that no contest was observed. NS = not significant.

	Time spent (%)
Sheep Tag	At the feeder	Displaced from the feeder	Attempting to re-enter feeder	Time spent on the edge of the feeder
Ram A	98.7	0.7	0.6	—
Ram F	90.2	6.0	3.8	—
Ram E	85.9	10.2	3.9	—
Ram B	73.6	15.7	10.7	—
Ram H	60.0	13.3	26.7	—
Ram G	59.6	9.8	17.9	12.8
Ram C	50.3	15.0	34.7	—
Ram D	33.6	19.3	35.3	11.8

Table 2.

Percentage of time spent at the feeder and in seeking re-entry*.

*

These results emanate from our own experiment.

As shown in Figure 1, a relationship between time spent on the feeder against the number of wins was not significant (P > 0.05). Table 3 exhibits a correlation between physical traits of Zulu sheep rams with proportions of wins and minutes at the feeder. There was a significant positive correlation between the proportions of wins against horn length (P < 0.05) and chest girth length (P < 0.05). Body weight was positively correlated with wither height (P < 0.05), Scrotal circumference (P < 0.05), chest girth (P < 0.05) and time spent at the feeder (P < 0.05). Withers height was positively correlated with time spent at the feeder (P < 0.05). There was a positive correlation between scrotal circumference and horn length.

Figure 1.

Association between the proportion of wins and time at the feeder for each ram*. *These results emanate from our own experiments.

	Body weight (kg)	Withers height (cm)	Scrotal circumference (cm)	Horn length (cm)	Chest girth (cm)	Time spent at the feeder
Proportion of wins	0.165^ns	0.332^ns	0.147^ns	0.634**	0.461**	0.434^ns
Body weight (Kg)		0.572**	0.513**	0.160^ns	0.639**	0.552**
Withers height (cm)			0.115^ns	0.109^ns	0.361^ns	0.474**
Scrotal circumference (cm)				0.696**	0.099^ns	0.188^ns
Horn length (cm)					−0.247^ns	0.220^ns
Heart girth (cm)						0.060^ns

Table 3.

Correlation between physical traits of Zulu sheep rams with proportions of wins and minutes at the feeder*.

*

These results emanate from our own experiments.

**

P < 0.05.

***

P < 0.01.

NS = not significant.

5. Discussion

According to Keeling [10], 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 [10] 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 [9] 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. [11] 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. [12] 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. [11] 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 [8] reported results similar observations. Dwyer [13] 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 ad libitum [14]. The same was concluded in dairy goats by Jørgensen et al. [15], suggesting that there is a decrease in time spent at the feeder if there is limited feeding space.

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 [5]. Bergeron et al. [16] 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. [2] also obtained a significant relationship between body weight and chest girth.

6. Conclusion

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.