Mongoid subclass indicies not created in Atlas

Animal.rb

class Animal 

  field :color, type: String
  field :mammal, type: Boolean
  ...
  
  index({ mammal: 1, color: 1 }, background: true)
end

Bear.rb

class Bear < Animal
  include Mongoid::Document
  
  field :hibernates, type: Boolean
  field :favorite_honey_type, type: String
  index({ mammal: 1, favorite_honey_type: 1 }, background: true) ### this index is not showing up in Atlas

  ### is adding index line in file the "correct" way to add a new index? or via Atlas? or both?
end