Horses are herd animals, but as individuals they experience different costs and benefits associated with group living.
Horse owners get to see this daily, with the most dominant horses often opting for the choicest pickings in newly available grass, or shooing away their companions from hay.
Does this mean that the less dominant horses get to eat less? Or do they simply spend more time foraging to make up for any shortfalls in eating time?
University of Bristol researcher Sarah Giles and her colleagues, in a fresh study published in the open-access journal PeerJ, set out to investigate associations in horses between social dominance, body condition and interruptions to foraging behaviour.
Their work involved the monitoring of a total of 116 domestic horses, across 20 herds, all of which were assessed on the Henneke nine-point body condition scale. The animals scored from 4 to 8.5.
The study was conducted over winter when pasture availability was limited and some competition for supplementary forage was evident.
Social dominance was measured for each individual alongside observations of foraging behaviour.
During foraging sessions, the duration, frequency and number of interruptions (such as movements, social displacement or vigilance behaviour) were recorded, with the total interruption time taken as a proxy measure of foraging efficiency.
The study team noted that while the foraging success of individual animals in social groups may be partly influenced by their social status, the relationship between herd behaviours, dominance and body condition is not yet fully understood.
“In a socially foraging herbivore, the benefits of group living outweigh the costs. Individual animals living within groups follow behavioural rules which allow them to function as a social unit.
“These rules are likely to depend upon both aspects of their own body condition and also the actions of other individuals within the group.
“Rules governing social interaction (e.g. dominance) may be important for a well-functioning group in terms of minimising costly conflict over resources.”
The authors noted that previous work involving horses had shown that higher-ranking individuals spent more time eating hay and had a higher body condition during the winter, but the mechanisms behind this association had not been researched.
The researchers, after analysing behaviours during 120 hours of monitoring, found that total foraging time was not influenced by body condition or social dominance.
Horses ranked higher in terms of social dominance tended to have higher body condition scores, but the most dominant factor for those with higher scores was foraging efficiency.
The horses that showed signs of greater vigilance — that is, those who more frequently raised their head and pricked their ears toward an area of interest — tended to have lower body condition scores.
The authors noted that, across the 120 hours, about 92 of them were devoted to foraging.
They found no effects for age, sex or height in the study.
Discussing their findings, Giles and her fellow researchers said that whilst social dominance explains some variation in body condition, the results highlight the potential role of other factors that could influence foraging efficiency.
“Factors such as a tendency to show vigilance behaviour have been little explored to date but have the potential to greatly influence the ratio of energy gained vs energy expended during bouts of foraging,” they said.
Vigilance, they said, was not associated with dominance status.
“These results suggest that certain individuals may be more likely to conduct vigilance, perhaps on behalf of the group, regardless of their social status.”
The results seem to support the suggestion that vigilance is an inherently costly activity, given the link to lower body scores.
“However, lower body condition individuals may also be more stressed or nervous individuals, which would also explain the association with increased vigilance.”
There was no evidence that subordinate or low body condition individuals compensated for less efficient foraging by increasing total foraging time.
“Another recent study found that horses with low body condition tend to adopt more passive behaviour. Potentially, such results may be due to a strong motivation to feed as a group in this species and thus synchronise feeding and resting behaviour.”
Subordinate or lower body score individuals were unlikely to remain foraging when their companions were not, supporting suggestions that social factors may result in stable differences in body condition within group-living animals.
Indeed, the tendency to synchronous feeding and resting, as reported in sheep, may be hard-wired as adaptive behaviour, they said.
“The lack of a compensatory change in total foraging time means that any variation observed in foraging efficiency could plausibly have an effect on body condition.”
The study team described their results as novel and exciting, in that they present the first behavioural evidence confirming a broad body of influential theoretical work linking condition and behaviour in a group-living species.
The results suggest that differences in energy reserves (body condition) can emerge simply through a reduction in energy intake by subordinates when dominant animals are present.
“This hypothesis could be further tested in a future prospective study.”
Information on individual horse dominance status could be included as a relevant factor when addressing health problems associated with equine obesity, they said.
The study team comprised Giles and Sean Rands, with the University of Bristol; Pat Harris, with the Equine Studies Group, which is part of the Waltham Petcare Science Institute; and Christine Nicol, with the Royal Veterinary College.
Giles SL, Harris P, Rands SA, Nicol CJ. 2020. Foraging efficiency, social status and body condition in group-living horses and ponies. PeerJ 8:e10305 https://doi.org/10.7717/peerj.10305