Short communicationSynchronization and leadership in switches between resting and activity in a beef cattle herd—A case study
Introduction
In group living ungulates, synchronization of activity is essential for maintaining herd coherence, which in turn provides advantages such as improved protection from predators (Krause and Ruxton, 2002) and reduced insect loads (Hart, 1992). However, the specific time when it would be optimal to switch from activity to resting and back would often differ between individuals. For instance, Ruckstuhl (1998) suggested that different demands for grazing time between females and males can lead to a split in behavioural synchronization resulting in spatial sexual segregation. Conversely, animals with similar metabolic needs (in terms of body size, condition, reproductive status, etc.) should have similar demands for grazing and resting and therefore should be more synchronized with each other.
On the proximate level, an important question is who will decide about when activity switches take place. One possibility is a democratic decision-making process in which all animals contribute to a communal decision (Conradt and Roper, 2003). Alternatively, specific individuals in the group can determine when a change is made (Sato, 1982). Within this “leadership” paradigm, it is usually assumed that leaders are older, more experienced animals or animals with higher dominance status.
In this study, we examined how the strength of synchronization was influenced by between-individual similarity in body weight and lactation status. Specifically, we tested the following hypotheses: H1 Synchronization will be stronger between cows of similar body weights. H2 Synchronization will be stronger between cows with the same reproductive status (lactating/non-lactating).
The second goal of our study was to check the time course of activity switches for evidence of leadership by specific individuals. The following hypotheses were tested: H3 Individual animals differ in their capability to incite activity switches in others. Therefore, inter-individual variability will explain a substantial proportion of variability in intervals between activity switches of subsequent cows. H4 More dominant animals more readily induce activity switches in others. Therefore, dominance index will correlate with the mean latency between the animal's activity switch and the activity switch by the next animal.
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Location, animals and observation
The study was carried out on grass pastures (ranging from 1.9 to 6.8 ha in size) on the outskirts of Prague in August 2004 and June 2005. The rainfall was 56.6 mm in August 2004 and 65.4 mm in June 2005, and the average daily temperature was 19.8 °C in August 2004 and 17.8 °C in June 2005. We observed a herd of 15 Gasconne cows, of which 14 were identical in both years. Ten of the 15 cows had a calf in 2004, and 12 calved in 2005. The cows were between 2 and 9 years old and weighed 540–894 kg at the
Results
The average observed synchronization over all pairs over the 2 years was 88.7%, whereas the randomly expected synchrony amounted to only 56.7%. Each of the 91 pairs was synchronized much stronger than expected by chance in both years. Pairs of cows with smaller differences in body weight were more synchronized than pairs with larger differences in body weight (Fig. 1, GLMM, F1,65 = 6.79; p < 0.05). Conversely, identical or different lactation status in the pair did not have a significant effect on
Discussion
Synchronization of activity is only observable at the group level. Data in this study come from a single herd, and therefore the paper should be understood as a case study documenting whether the tested hypotheses were upheld in this specific herd.
Activity in our herd was highly synchronized, much above the randomly expected level. However, not all animals were equally synchronized with each other. In agreement with our first hypothesis, pairs of cows with small differences in body weights were
Acknowledgements
This study was supported by grants GA AVČR IAA 6307402 and GAČR 523/03/H076. We thank Robert Kněz, Martina Vondráková, Tereza Vojáčková, Barbora Božděchová, Kristýna Bělská, Lýdie Máchová, Renata Kolovratová, Helena Chaloupková and Jitka Šilerová for their help with data collection and Pavel Firla for his cooperation. We also thank Cassandra Tucker and Becky Hothersall for helping with the English and two anonymous reviewers for their very valuable comments.
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