I have seen a lot of people talking about Alastor emoting with his ears in ways that make it apparent they assume deer emote like cats and dogs. They do not. Deer are prey animals and they do not emote like predators. So let's clear up some things!
To begin, because this is the reason I'm making this post: yes! Deer do, in fact, pin their ears back when they feel threatened, or as an aggressive display. Here is an example of what that looks like for Alastor.
Note how angular the shape language is, with a sharp bend at the top of the ear. This creates a sense of tension, especially when paired with the hunched posture. This is not a happy deer.
With that established! Let's proceed.
The ears of a happy deer will be relaxed. They may droop, lie flat, or move independently of each other. This deer is paying attention to his left side, but he is calm and comfortable.
Let me repeat that. The ears of a happy deer may lie flat.
The ears here are laid back, but they are gently tapered from base to tip. They are not pressed to the skull, and the overall body language is relaxed. This deer. Is fine. He is not upset. He is not bothered by what's going on, or what's being said. He is not annoyed by Vox imitating him. A brief touch to the shoulder draws his eyes down, but there is no other reaction. If he dislikes it, it's a momentary irritant that doesn't ruin his mood.
Want to know something else laid-back ears can indicate, specifically in a social context? Search images of "deer showing affection". You will get so many laid-back ears. At worst, Alastor is comfortable with Vox, in spite of the touching, and it would not be a stretch to suggest that he enjoys his company.
Ears upright but still relaxed. This deer appears mildly interested, but not bothered. He offers no reaction at all to being touched this time, maintaining the above posture and ear angle both before and after.
Ears upright but no longer relaxed, accompanied by a slight growl. A line has been crossed. This deer is no longer comfortable, and may take action to escape the unpleasant situation if it persists.
From the moment Vox proposes a partnership, ears upright becomes Alastor's default for the rest of the scene. Which might not seem noteworthy because that's their default position anyway, but it hasn't been their default for this scene. The ears of a happy deer are relaxed, and they move freely; rigidly upright indicates alertness and alarm. There is a non-zero chance that this deer is having a crisis over a label being put on his relationship.
Socially speaking, ears up is also a display of dominance. Which is doubtless the interpretation Alastor would prefer, and a valid one considering he spends the rest of the scene verbally tearing Vox apart.
I choose to interpret it as a bit of both. Maybe one more so than the other.
