     @radius = options.radius || 5
     @tick   = @universe.tick
     @babies = 0
+    @mark ||= null
     @belly ||= 'white'
+    @topicColor = null
   describe: ->
     description = super()
     description.health = @health()

  drawHunger: -> # show emptiness of belly
    h = @health()
    r = ( @radius - 1 ) * ( h - @hp ) / h
    @drawCircle @x, @y, r, @belly
    if @sickness?
      @drawCircle @x, @y, 3 * r / 4, @sickness.disease.color
  # maximum health points an organism can retain

  setTopic: ( x, y ) ->
    candidates = ( t for t in @thingsAt( x, y ) when t instanceof Animal )
    if candidates.length
      @topic.topicColor  = null if @topic
      @topic             = candidates[0]
      @topic.topicColor  = @outline
      @topic.inheritMark = false
      @draw()
  # highlight an organism by coloring its belly
  highlight: ( x, y, color, inherit ) ->
    for t in @thingsAt( x, y ) when t instanceof Organism
      t.mark        = color
      t.inheritMark = inherit
      t.draw()
  # remove all highlights
