What would be a secular interpretation of animal rebirth For people who don't believe in literal rebirth as animals,(secular or non)  how would they describe the mental state associated  with rebirth in the animal realm?  E.g, if stress is associated with hell rebirth, what mental state is associated with animal rebirth .
'Animal birth' is described in many places in the suttas. It refers to ignorance (stupidity), lacking reflective reasoning (ayoniso manasikara), immorality &amp; the merciless devouring of other beings, including mutual devouring. 

Animals merely blindly follow &amp; act upon instinctual drives &amp; desires without having the capacity to reflect upon the consequences. This is the meaning of 'animal birth'. 

Below are some quotes. 


  Bhikkhus, a god, a human or any other good state would not be evident from actions born of greed, hate and delusion. Yet, bhikkhus, from
  actions born of greed, hate and delusion a hellish being, an animal
  birth a ghostly birth or some other bad state would be evident. AN
  6.39
  
  
  
  Sooner, I say, would that blind turtle, coming to the surface once every hundred years, insert its neck into that yoke with a single hole than the fool who has gone once to the nether world would regain the human state. For what reason? Because here, bhikkhus, there is no conduct guided by the Dhamma, no righteous conduct, no wholesome activity, no meritorious activity. Here there prevails mutual devouring, the devouring of the weak. For what reason? Because, bhikkhus, they have not seen the Four Noble Truths. What four? The noble truth of suffering … the noble truth of the way leading to the cessation of suffering. SN 56.47
  
  
  
  Now on that occasion the wanderers of other persuasions had come together in a gathering and were sitting, discussing many kinds of
  bestial (animal) topics, making a great noise and racket. AN 10.93
  
  
  
  Bhikkhus, these two bright principles protect the world. What are the two? Shame and fear of wrongdoing. If, bhikkhus, these two bright
  principles did not protect the world, there would not be discerned
  respect for mother or maternal aunt or maternal uncle’s wife or a
  teacher’s wife or the wives of other honored persons, and the world
  would have fallen into promiscuity, as with goats, sheep, chickens,
  pigs, dogs and jackals. But as these two bright principles protect
  the world, there is discerned respect for mother… and the wives of
  other honored persons. AN 2.9
  
  


The follow quote is important because it shows 'animals' can realise the Four Noble Truths, which also show this quote cannot refer to real animals but to people:


  Beings (sattā) are few who, when they pass away from the animal realm (tiracchānayoniyā) are 'reborn' (paccājāyanti) among humans
  (manussesu)… those beings are more numerous are reborn in hell…. For
  what reason? They have not realised the Four Noble Truths. SN 56.102
  to 131
  
  


This video might help understand, which shows Angulimala, the mass-murderer, as a 'animal', being reformed or changed into a 'human' by the Buddha:

https://www.youtube.com/watch?v=uNzrKAzxipY


  THE FIRST OF the Four Woeful States is hell. Hell is anxiety (in Thai, literally “a hot heart”). Whenever one experiences anxiety,
  burning, and scorching, one is simultaneously reborn as a creature of
  hell. It is a spontaneous rebirth, a mental rebirth. Although the body
  physically inhabits the human realm, as soon as anxiety arises the
  mind falls into hell. Anxiety about possible loss of prestige and
  fame, anxiety of any sort — that is hell.
  
  Now rebirth in the realm of beasts is stupidity. Whenever one is inexcusably stupid about something: stupid in not knowing that Dhamma
  and nibbāna are desirable, stupid in not daring to come into contact
  with or get close to Buddhism, stupid in believing that if one became
  interested in Dhamma or Buddhism it would make one old-fashioned and
  odd. That is how children see it, and their parents too. They try to
  pull back and move far away from Dhamma and religion. This is
  stupidity. Regardless of what sort of stupidity it is, it amounts to
  rebirth as an animal. As soon as stupidity arises and overwhelms one,
  one becomes an animal. One is a beast by spontaneous rebirth, by
  mental rebirth. This is the second Woeful State.
  
  The third Woeful State is the condition of a peta...
  
  Buddha-Dhamma For Students
  
  


Please note: this is not a 'secular interpretation' but, instead, the true meaning of the suttas.