I was lucky enough to be invited as a co-author on a recent study by the excellent Dr. Saeideh Esmaeili at Colorado State University. We contributed tracking data from our collared Przewalski’s horses from Mongolia (mentioned here) and I contributed some thoughts to the manuscript but this paper and the analysis were led by Saeideh. I will summarize the main points here but if you are interested you can click here for more info.
Foraging animals need to balance the expenditure of energy needed to find their food with the energy they get from it and this balance can change depending on body size and digestive system. Smaller bodied ungulates (mostly ruminants like gazelles) have higher metabolic rates and should prioritise the intake of highly digestible foods with high energy and nutrient concentrations whereas large bodied ungulates (larger ruminants and hind-gut fermenters like horses) need to take in large amounts of food and should prioritise areas with lots of biomass, where they can eat all they need. The forage maturation hypothesis (FMH) states that energy intake for ungulates is maximised when forage biomass is at intermediate levels.
This figure shows Illustrated predictions for resource selection by equids and different sizes of ruminants during a hypothetical growing season and in a hypothetical landscape. Source: Esmaeili et al 2021, Ecology Letters
This paper used GPS relocations from studies on a 30 populations of hindgut fermenters (equids) and ruminants across the world in different habitats. This movement data was then
matched with satellite data on forage characteristics and surface water. While the smaller ungulates maximise energy intake regardless of their digestive system, for horses, the need to stay close to water overrides the desire to maximise foraging efficiency. This is something that is definitely reflected in our data on the movements of P-horses as they rarely stray far from water and stallions will vigorously defend their access to key watering holes. The paper suggests that the FMH should be updated to include these kind of subtleties and specifically the importance of water intake balanced with foraging. I am very proud to have played a small part in this excellent paper and encourage you to read the full paper if you can access it.