Movement ecology of mule deer in partially migratory herds

Abstract

The purpose of animal movement can be broadly partitioned into two categories of factors that either push or pull animals to move (Loe et al. 2009). Animals will move towards resources, such as food or mates, that are necessary to complete their life cycle. Likewise, they will move away from threats, such as predators or inclement weather, to survive. Movement therefore helps augment an animal's ability to access resources and avoid threats because these variables are typically also moving and changing across space and time. There is great diversity across taxa in how animals move. There is huge variation in the distances, regularity, and area over which animals move – certainly across species but often also within species. In all cases, we can expect that the strategy of movement that animals develop will have been shaped by selective processes where animals made choices about how to access resources and avoid threats that subsequently affected their fitness. In ungulates, a type of movement evolved independently across 17 different lineages (Abraham et al. 2022) that is thought to help them access high-quality forage throughout the annual cycle – i.e., migration. Ungulates that live in mountainous regions can typically maximize their nutrition by spending their winters at low elevation then following the green up as it progresses to higher elevations in the summertime. Selective pressures have shaped herbivore behavior so that they can learn to track food resources as they change in quantity and quality across a landscape. Migratory ungulates typically time their migrations to maximize access to high-quality food resources. Mule deer (Odocoileus hemionus) are unique among ungulates in that they tend to be extremely habitual in their seasonal movements. Like other cervids, they are plastic in terms of timing their movements but exhibit strong fidelity to their seasonal ranges and choice of movement strategy. Mule deer typically establish migratory or resident tactics as adults and stick to that behavior life-long. They also adhere closely to established migration routes and utilize the same extents of seasonal ranges year after year. Mule deer therefore can respond to a seasonally changing environment but are limited in doing so within a fixed spatial extent. In this work, I examined how migratory strategies of mule deer affect 1) resource selection of irrigated lands and 2) survival of adult females and fawns. The two chapters I produced were unified by an aim to understand the nuances of mule deer movement ecology in two partially migratory herds in south-central Colorado. In my first chapter, I examined how fluctuations in natural forage abundance influence the movements of mule deer in an agricultural system, where irrigation provides enhanced foraging opportunities for ungulates. Mule deer tended to increase their selection for alfalfa when their native forage was poor in quality. In my second chapter, I assessed how migratory strategies influenced survival of fawns and adult female mule deer. I detected apparent tradeoffs in survival of mule deer relative to their migratory strategy. Migrant fawns tended to have higher early survival while residents had higher overwinter survival. Additionally, there was interannual variation in the benefits of migration on survival of adult female deer. My research contributes to a broader understanding of how fluctuations in forage abundance drives mule deer movement and how the movement strategies that deer choose to obtain forage then influences their survival. Abraham, J. O., N. S. Upham, A. Damian-Serrano, and B. R. Jesmer. 2022. Evolutionary causes and consequences of ungulate migration. Nature Ecology & Evolution 6:998–1006. Loe, L. E., A. Mysterud, V. Veiberg, and R. Langvatn. 2009. Negative density-dependent emigration of males in an increasing red deer population.