Moose Evolution and Adaptation

Characteristics

Fig. 1 Alaskan Bull Moose, (Alces alces gigas).

Moose (Alces alces) are large mammalian herbivores found in the northern regions of North America and Europe. They are the largest member of the deer family (Cervidae), and adult males can grow up to seven feet tall and weigh over half a ton. The name “moose” comes from the Native American tribe the Algonquin, and translates to “twig eater”. Moose prefer forested habitat near lakes, marshes, and wetlands, and are able to withstand cold winters due to their great size. This makes the countries of Russia, Canada, the United States, Sweden, Finland, and Norway most suitable. Despite their large range and forage diversity, moose populations have experienced increasing pressure in the contiguous United States due to climate change and disease, although populations remain stable in arctic and subarctic climates.

Moose have shaggy, dark brown coats, and are long-legged with broad shoulders. They have prominent muzzles with an overhanging upper lip, and a large flap of hair-covered skin that hangs beneath the throat called a "bell" or a dewlap. Adult males have flat broad antlers that shed and regrow every year to display dominance and protect their eyes when competing for a mate. The average adult male moose weighs 1,106 pounds, and an adult female weighs 836 pounds.

The moose diet consists largely of browse: the leaves and twigs of woody plants. Willow, aspen, birch, maple, pin cherry, and mountain ash are important, high quality browse utilized by moose throughout the year. Since leaves are absent from hardwoods in the winter, balsam fir provides additional nutritional value for moose. Sodium is important for moose, and a diet consisting of aquatic plants such as pondweed and water lilies provides the necessary amount. This is a problem in the winter, however, because moose will lick roads and cars to meet their necessary salt intake, which cannot be obtained from the seasonal aquatic plants.

Moose are seasonal breeders whose mating season begins in late September and lasts into early October. Female moose (cows) give birth the following May to one to two calves. Cows begin breeding at age two and can continue into their teens, whereas male moose (bulls) usually do not breed until they are older and can compete with larger bulls for a suitable mate. The average life expectancy of moose is eight years for a cow, and seven years for a bull. They can live into their teens, but rarely survive past twenty years old. Predation of adult moose is limited due to their large size, but calves and ageing moose are particularly susceptible to attacks from wolves, brown or grizzly bears, black bears, and sometimes cougars.

Evolution

Fig. 2 Map depicting the home ranges of 8 subspecies of moose (Alces alces).

Fig. 3 Graph depicting geographical origin of 21 moose (Alces alces) specimens and Bayesian phylogeny based on sequenced genomes.

Fig. 4 Graph depicting the differences among five complete moose (Alces alces) genomes generated using Principal Component Analysis (PCA).

Over thousands of years, moose have evolved and adapted to suit the northern environments they roam today. They are relatively young in the evolutionary scheme of large mammals; the genus Alces first appears in the fossil record 2 million years ago, and moose fossils were first recorded approximately 100,000 years ago. Originating in Europe, the genus Alces was never diverse; only one species was present in the fossil record at any particular time. Despite this limited species diversity, the precursor of the modern moose, the broad-fronted moose (Alces latifrons), was distributed across Eurasia and northwestern North America before going extinct at the end of the Pleistocene, which highlights the evolutionary success of the genus.

Modern moose populations are divided into a Eurasian, or North American clade. This divergence, based on short mtDNA data from 194 ancient radiocarbon dated samples, dates between 119 and 42 ka BP (thousand years before present). Within these clades are 8 numerous subspecies specific to certain regions. In Eurasia, moose populations comprise of the subspecies: A. a. alces, A. a. pfizenmayeri, A. a. cameloides, A. a. burturlini, and in North America: A. a. gigas, A. a. andersoni, A. a. shirasi, and A. a. americana. Most Eurasian moose have a karyotype of 2N = 68, whereas North American moose have 2N = 70. The genomic data shown in Fig. 3 suggests that all modern North American moose derive from a single migration event. However, we find some genetic differentiation among North American subspecies in Fig. 4, with Eastern moose (A. a. americana) and Alaskan moose (A. a. gigas) being distinct from the other North American samples which form a tight cluster. This differentiation could be a consequence of founder effects beset upon moose populations during the European colonisation of North America.

Glacial cycles greatly influenced the evolution of the modern moose. Large extinction events near the end of the Quaternary killed off many large mammals, save for a few ungulate species—including moose, who recolonized the Holarctic in subsequent years. Genetic analysis supports the existence of various moose glacial refugia during the last glacial maximum (LGM), primarily in the Alps, the Caucasus, Carpathians, Balkans, and northern Italy, as well as in western Siberia, the Ural Mountains, and Russian plains. More recent data suggests several more fragmented populations in southern Ukraine and the Urals, western Siberia, northern Mongolia, and eastern Siberia. This diversified the species into the various clades and subspecies viewable today, and most notably created striking differences between the Swedish moose (Alces alces) and the North American moose (Alces americanus).

Gut microbiome and Climate Change

Fig. 5 Graph depicting the bacterial families and organic compounds found in moose (Alces alces) rumen and colon microbiomes.

Like other ruminants, moose have a specialized digestive system with a four chambered stomach that allows a complex system of microorganisms to ferment plant matter that the animal cannot break down on its own, especially cellulose. During the fermentation process, hydrogen, ammonia, carbon dioxide, and methane gas are produced, as well as volatile fatty acids (VFAs) such as acetate, butyrate, and propionate. These VFAs are released into the rumen where they can be absorbed and used as a source of energy. Fig.5 shows the OTU microbial rumen and colon contents of moose analyzed through PhyloChip microbial screening. These species all play a role in the fermentation process which allows moose to sustain themselves as herbivores, feeding on forage.

Recently, due to global warming caused by human-environment interactions, moose gut microbial structure and function has been changing in response to climate-change related dispersal patterns. The data seen in Fig.6 suggests that there are significant differences between dispersing and isolated moose populations in the abundances of the bacteria: Roseburia (P = 0.039), Faecalibacterium (P = 0.039), rc4-4 (P = 0.011), Bulleidia (P = 0.030) and Escherichia (P = 0.035). This implies that moose have strong adaptive capability, but are undergoing artificial changes due to human-environment interactions, and in the future may not find sufficient levels of forage to sustain populations in certain regions.

Fig. 6 Graphs depicting the difference in gut bacterial diversity of dispersing and isolated moose (Alces alces) populations.

Conclusion

Overall, moose are hardy animals who have maintained an impressive existence despite their lack of species diversification. As the largest members of the deer family, moose are among few animals able to call arctic environments home—primarily due to their great size. This, combined with their herbivorous diet, make moose unique creatures, and model organisms for adaptability. As humans continue to impact the environment, moose, their microbiomes, and their subspecies will without a doubt continue to adapt to the changing world.

References

Edited by [Markus Roehrdanz], student of Joan Slonczewski for BIOL 116, 2024, Kenyon College.