With usual teat
The baby opens slightly his mouth to receive the teat areola, as its diameter is reduced and its shape conical, what is limiting the lower jaw movements. His lips all around the teat close it tightly, to avoid leaking out.
The chewing muscle effort is 2 to 3 times less important than during the breast feeding and, consequently, we don’t see any “move” of the ears. Then, the tongue point makes a piston movement, which, repeated several million times, is pushing up the palate to the detriment of the nasal pits.
At the beginning of the feeding, the milk is pouring freely under the gravity effect with a poor chewing muscle solicitation, not allowing a complete posterior closing of the oral cavity, what is often generating breathing holds. To breathe, the baby is placing his tongue against the extremity of the teat in order to stop the milk flow. He is breathing by mouth, where the aerial flow is more important that the one of the nasal pits, as he has a very short breathing time to inspire a sufficient volume of air.
This leaking defect in the oral cavity also entails the absorption of an air-milk mixture generating many ear infections and diarrheas. The important volume of the nipple teat drives the tongue to a too low and rear position, limiting the right closing of the rear of the baby’s mouth. By the end of the feeding, the chicks’ muscles are working hard to create an important intra- mouth depression allowing the milk getting out the feeding bottle, having as effect to limit the transversal growing of the dental arcades.
Important points :
-- Limited opening of the oral cavity:
      => Milk is pouring freely under gravity effect = poor solicitation of the chewing muscles, absence of growing stimuli and non-complete closing of the upper oral cavity  (risks of wrong route and ear infection)

- Maximum effort from chicks’ muscles
- Piston move from the tongue generating palate distortion
      =>teeth position modification.
- The baby is subject to breath holding and breathes by the mouth.
- The baby’s mouth is forced to adapt its shape to the teat shape.