Precipitous declines in moose populations in southern portions of its range have biologists and wildlife managers studying the animals to determine the cause of death. Moose numbers have declined so dramatically in Minnesota that the state cancelled the 2013 moose hunt.
Scientists involved in a study to solve the mystery of moose deaths link the high mortality rates in part to climate change. Evolved for surviving extreme cold, moose have little tolerance for temperatures even slightly above their comfort zone. In summer, 60-degree days can push moose to seek shade; warmer than that and moose may forego foraging, leading to malnutrition and to lack of stored fat on which to survive the winter.
Heat stress is complicated by pests and diseases that climate change makes more prevalent. Surviving milder winters, tens of thousands of ticks may infect a single moose. While infestations can result directly in anemia and death, ticks can weaken moose resiliency to temperature fluctuations and infections as moose may rub off nearly all of its fur in an attempt to rid themselves of the arachnids.
White-tailed deer are benefitting from warmer winters and their numbers are increasing in moose habitats. Pests and the diseases that deer carry pose an increasing threat to the large herbivores. While deer are largely unaffected by liver flukes and brainworms, the parasites they spread can be fatal to moose.
Moose may also be falling victim to two mosquito-borne diseases, West Nile virus and Eastern Equine Encephalitis. With fewer days of killing cold, mosquitoes increasingly over-winter and expand their numbers.
Other researchers believe that toxic levels of sulfates in the foods moose are eating are a factor in moose decline, but this too has a climate-change component. Toxic hydrogen sulfide is produced when ruminant animals consume excess sulfate; the gas is absorbed into the blood stream or is inhaled when the animal belches. In an effort to keep cool in warmer temperatures, moose may spend more time basking in mud high in anaerobic sulfates, and ingest more aquatic plants growing in that environment. Evaporation, intensified in droughts, concentrates sulfur in mineral licks and water sources.
A moose digestive system is capable of converting mercury into methylmercury, another neurotoxin. Increased uptake of sulfur and mercury by the plants that moose feed on supply the components of the gas. While there is as yet scant evidence that moose are dying directly from neruotoxic poisoning, scientists posit that they are adding to stresses that are reducing moose resiliency and making the huge animals of the north more susceptible to preditors and disease.