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

                radius:      @radius
                bodyColor:   @bodyColor
                inheritMark: @inheritMark
                belly:       if @inheritMark then @belly else null
                generation:  @generation + 1
              }

  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
