Undocumented migrant deaths disproportionately clustered within regions of greatest physiological stress

Deaths of undocumented migrants crossing the deserts that span the southern United States border between Mexico and Arizona are disproportionately clustered within regions of greatest physiological stress, including those where dehydration is likely, reports a new study.

Based in part on models previously used to study physiological stress in nonhuman animals, the analysis presents a new application of biophysical modeling to human systems and could help inform the impacts of past, present, and future climate on human physiology, stress, and evolution, say the authors. Their modeling suggests journeys by undocumented migrants across the southern United States will become much more dangerous in the coming decades, as the climate changes.

For those seeking to cross the southwestern U.S. border, avoiding apprehension often requires perilous treks across a rugged, hot desert landscape with very little water availability. Personal accounts of individual journeys across this landscape suggest that in many instances, migrants do not have the knowledge to identify a least-cost path of travel. Although the true cost of human life on such journeys is currently unknowable, roughly 350 undocumented migrant deaths in this region are reported annually. Firsthand accounts of surviving migrants depict experiences of extreme heat stress and dehydration, but empirical evidence of these hazards, and how they relate to migrant mortality, is lacking.

Using an approach more commonly used to understand the dynamics of environmentally driven physiological stresses in nonhuman animals, Shane Campbell-Staton and colleagues modeled physiological stresses associated with migration across a commonly traversed section of the Sonoran Desert. They found that documented locations of migrant deaths are often clustered within areas of greatest predicted physiological stress, particularly evaporative water loss. According to Campbell-Staton et al., minimum values of estimated water loss in these areas were enough to cause severe dehydration and death in humans, indicating a root cause for observed patterns of undocumented migrant mortality.)