Shelter use of horses during Swedish summer in relation to weather conditions and insect abundance

Outdoor housing of horses’ best fulfils the horses’ need for physical activity and it is an alternative to the more cost and energy demanding indoor housing in stables. Furthermore, if outdoor housed horses have access to shelter they can generally cope well with adverse weather conditions such as high or low ambient temperature, heavy rain or strong winds. In this study, the daytime shelter-seeking behaviour of three groups of horses housed outdoors was studied during the summer. The aim was to evaluate whether shelter use is related to weather variables (e.g., ambient temperature and wind speed) and insect harassment. The shelter-seeking behaviour was studied for three different groups of horses: Group 1) eight individually housed horses in paddocks that had access to three different shelter types (C: closed on three sides with roof, R: open on three sides with roof, W: closed on three sides without roof), Group 2) 25 group housed mares with foals on pasture with access to shelters C, and Group 3) ten mares without foals on pasture without access to shelter. Each group was studied for eight days. Behaviour, including shelter use and insect defensive behaviour (e.g., tail swish, shake head) were recorded every five minutes from 09:00-16:00 (Group 1) or once every hour from 10:00 to 15:00 (Group 2 and 3). Respiration rate and rectal- and skin temperature were measured three times per day in group 1 and two video cameras were installed to monitor shelter use at night (Group 1). Climatic conditions were measured at regular intervals of ten minutes and insect activity was monitored by catching flying insects on sticky paper traps. Seven of the eight horses in Group 1 used at least one shelter once; one horse never used any shelter neither during day nor night. The longest duration one horse was observed inside shelters was 546 minutes in total during two days at daytime; the lowest 15 minutes. The longest use of shelter during night was 405 minutes, whereas the lowest was 1 minute. Most of the horses favoured shelter R over shelter C, which in turn was favoured over shelter W. It was significantly warmer in shelters C and R than outside during daytime but not during night. There were no significant differences in the number of insects caught inside the shelters and outside, neither during day nor night. The most frequently performed insect defensive behaviour (IDB) was tail swishing, accounting for 63% of the observed IDB. The IDB performed outside the shelters was significantly higher than inside the shelters with roof, and there were also significant more IDB recorded in shelter C than in shelter R. The rectal and skin temperature and respiration rate did not differ between horses that had uses shelters before measurement and those that did not. In Group 2, the shelters were used during four of the eight days. The highest usage recorded was 40.0% (ten mares) and as its lowest 10.7% (less than three mares). The remaining horses that could not fit inside the shelters stood gathered nearby during sunny, warm days with high shelter usage. During sunny days, tail swishing averaged 59.3 /min for Group 2 and 50.0/min for Group 3, whereas during cloudy days, it was 44.7/min, and 40.9/min, respectively. Generally, the more tail swishing was performed in groups 2 and 3, the less time the horses spent on grazing and vice versa. In conclusion, horses may benefit from having access to shelter during warm and sunny days as they seem to be less harassed by insects, reflected in lower IDB when using shelters. We can also conclude that the horses preferred a shelter that had a roof, and thus provides shade. There is still a need for further research on the subject of shelter seeking behaviour during warm weather conditions to better understand how shelter affects both the behavioural and physiological responses.