Stay home order: why do some ungulates return to the same places?

This blog post is provided by Thomas Morrison from the Institute of Biodiversity, Animal Health and Comparative Medicine, University of Glasgow and tells the #StoryBehindThePaper for the article “Drivers of site fidelity in ungulates“, which was recently published in the Journal of Animal Ecology.

In a classic study, inauspiciously titled “A lesson in the limitations of field experiments: shrubsteppe birds and habitat alteration”, John Wiens and co-authors set out to assess how three bird species would respond to experimentally imposed habitat loss. With brushcutters in hand, the team spent roughly 340 person-hours clearing sagebrush from a large study area in Central Oregon. They expected birds to strongly avoid these manipulated areas, whilst changing their territory location, size and configuration. To the authors’ surprise, over the following 7 years many birds patently ignored the destruction and continued returning to the same territories with a high degree of affinity. They conclude that their study was complicated by “…site tenacity by breeding adults, leading them to return to previous breeding locations… despite the habitat changes.” Limitations of ecology field experiments notwithstanding, the broader lesson, perhaps, is that scientific progress can emerge from the discarded brushpiles of unanticipated results. 

The study exposes a commonly observed, though poorly understood, feature of animal behaviour: the tendency for individuals to return to previously visited sites, a.k.a. ‘site fidelity’. This behaviour has been reported in a wide variety of taxa, from salmon to starfish to ungulates, though is perhaps most associated with migratory passerine birds, and is thought to benefit animals, evolutionarily, by making them familiar with the locations of food, predators and shelter. Importantly, the sagebrush experiment shows that the animals can be attracted to the site itself, regardless of site quality. In contrast, site fidelity can also emerge when animals repeatedly choose high quality sites, so long as site quality remains predictable over time.

Research shows that wildebeest faithfully returned to the same wet season ranges each year, despite increasing levels of cultivation. Photo by Tom Morrison.

This distinction between attraction for sites versus attraction for quality has subtle, yet profound conservation implications. If animals continually return to sites even if the sites become degraded in quality, the animals will presumably incur survival and reproductive costs. In one example, Wyckoff et al. (2018) showed that mule deer in western Wyoming, USA continued using the same migratory routes despite increasing construction of oil and gas infrastructure within those routes over the last several decades. Unsurprising, this population has undergone a long-term population decline. 

Recognizing the importance of site fidelity, our group came together to better understand the phenomenon in ungulates (i.e. mammals with hooves). Many ungulates, such as caribou, wildebeest and mule deer, famously move long distances in search of food, though only some exhibit strong site fidelity. We were interested in why. We took an observational approach to this problem and compiled as many datasets of GPS-collared populations and species as we could manage, borrowing heavily from the Wyoming Migration Initiative database and other collaborators. In total, nearly 700 GPS collared animals from eight species with data collected over the past two decades.

Example trajectories of the eight study species showing varying strengths of site fidelity across years. Year 1 is in black and Year 2 is in colour. Credit: Tom Morrison, with data from Morrison et al. 2021, Drivers of site fidelity in ungulates, Journal of Animal Ecology.

Not surprisingly, ungulates seem to vary in their strength of site fidelity. Species like mule deer and moose faithfully follow the same routes each year with great spatial fidelity. We estimated that for an average mule deer, any given GPS location is within <60 meters of a location that they visited 12 months previously, and often much closer (see Figure above). This extreme site fidelity is really quite astonishing given that some of these mule deer migrate hundreds of kilometres each year. On the other end of the spectrum, species like barren-ground caribou in Alaska’s Porcupine herd and wildebeest in the Serengeti Ecosystem in East Africa deviate tens or hundreds of kilometres on average each year from the year before. We think this lack of site fidelity may be a consequence of a couple of factors. First, the food resources in these areas can be very unpredictable from year to year because of chance snowfall, fire and rainfall events, which affect the spatial availability of food. Second, these animals move in large herds, and are perhaps under social control, the strength of which possibly trumps any affinity for particular sites or routes.

We wanted to dig deeper into why some individuals and species have strong site fidelity while others do not. Therefore, we measured resource predictability within each individual’s home range using satellite imagery of vegetation quality (called the normalized difference vegetation index – NDVI for short) and determined whether it explained site fidelity. Indeed, in landscapes where vegetation becomes green at the same time and in the same place in a predictable way, ungulates have stronger site fidelity. The fact that ungulates respond to resources in this way suggests that site fidelity is at least somewhat shaped by how animals respond to site quality, suggesting some degree of flexibility in their response to habitat change. 

Measuring innate attraction for sites is more difficult. As you might imagine, using brushcutters to experimentally manipulate large portions of an ungulate’s range in the name of science is logistically unfeasible. Instead, we compared patterns of site fidelity across populations within species, reasoning that if species have a fixed strength of attraction to specific geographic locations, this attraction should remain consistent across different populations living in different environments. We found only weak evidence that this was the case.

Our working hypothesis is that in species with strong site fidelity like mule deer and moose, animals learn the locations of food early in life, possible the first year as they follow their mothers, and then retain those locations in memory. However, if environmental conditions deteriorate at those locations – for example because of human activities – these animals can adaptively move to new sites or ranges. While this is generally good news for conservation, we do not know the threshold at which animals decide to move to new areas. In many landscapes, there is only a handful of routes still available, so moving to new areas is increasingly not an option.

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