Lecture Notes: Evolution and Traits of Tremarctine Bears

Introduction

• Presenter: Kallie
• Topic: Large bears of the Tremarctinae subfamily
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Spectacled Bear

• Habitat: Cloud forests of the Andes
• Characteristics:
  • Only native bear in South America
  • Distinctive markings and stout muzzle
  • Spends a lot of time in trees
  • Predominantly herbivorous

Tremarctinae Subfamily

• Spectacled bear is the last surviving member
• Includes extinct short-faced bears
• Misnomer about short muzzles
  • Distinctive quality: body size rather than snout length

Ancestors and Evolutionary History

Arctodus simus

• Lived during early Pleistocene Epoch in North America
• Height: Tall enough to look a full-grown person in the eye
• Widespread across North America (100+ sites found)
• Possible ancestor of South America's Arctotherium angustidens

Arctotherium angustidens

• Discovered in Buenos Aires; largest bear known
  • Upper arm bones twice the length of adult human's
  • Stood more than 3 meters tall on hind legs
• Lived in South America
• Connection to Great American Biotic Interchange (2.6 million years ago)
  • Influx of North American species to South America
• Initial thought to be related to Arctodus simus

Genetic Insights

• 2016 genetic study: Arctotherium was more closely related to the spectacled bear than Arctodus simus
• Suggests different evolutionary pressures for gigantism in North vs. South America
  • Independent acquisition of large size

Evolutionary Pressures and Adaptation

• Arctotherium angustidens: Response to South America's unique environment
  • Few large predators, large herbivores
  • Advantage of being a top predator or scavenger
  • Omnivorous diet suggested by skull morphology
    • Comparison with modern bears
• Arctodus simus: Varied hypotheses about lifestyle
  • Initially thought to be a hypercarnivore
  • Likely a kleptoparasite
  • Consumed a varied diet (meat and sugary plant foods)

Decline and Extinction

• Arctotherium angustidens: Died out around 800,000 years ago
  • Later species were smaller, more herbivorous
  • Possible factor: Increased competition from new predators (jaguars, cougars, dire wolves)
• Arctodus simus: Disappeared around 10,000 years ago
  • Unclear reasons; potential factors include loss of megafauna, competition from Ursus bears

Summary

• Tremarctine bears were highly adaptable in body size and diet
• Multiple theories exist regarding their extinction
• Spectacled bear remains as the last representative of this diverse and unique subfamily

Additional Information

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