Animation issue caused by C# parameters passed by reference rather than value, but where?

Question

I'm having trouble with sprite animation in XNA that appears to be caused by a struct passed as a reference value. But I'm not using the ref keyword anywhere. I am, admittedly, a C# noob, so there may be some shallow bonehead error in here, but I can't see it.

I'm creating 10 ants or bees and animating them as they move across the screen. I have an array of animation structs, and each time I create an ant or bee, I send it the animation array value it requires (just [0] or [1] at this time). Deep inside the animation struct is a timer that is used to change frames. The ant/bee class stores the animation struct as a private variable.

What I'm seeing is that each ant or bee uses the same animation struct, the one I thought I was passing in and copying by value. So during Update(), when I advance the animation timer for each ant/bee, the next ant/bee has its animation timer advanced by that small amount. If there's 1 ant on screen, it animates properly. 2 ants, it runs twice as fast, and so on. Obviously, not what I want.

Here's an abridged version of the code. How is BerryPicking's ActorAnimationGroupData[] getting shared between the BerryCreatures?

class BerryPicking
{
    private ActorAnimationGroupData[] animations;
    private BerryCreature[] creatures;
    private Dictionary<string, Texture2D> creatureTextures;
    private const int maxCreatures = 5;

    public BerryPickingExample()
    {
        this.creatures = new BerryCreature[maxCreatures];
        this.creatureTextures = new Dictionary<string, Texture2D>();
    }

    public void LoadContent()
    {
        // Returns data from an XML file
        Reader reader = new Reader();
        animations = reader.LoadAnimations();

        CreateCreatures();
    }

    // This is called from another function I'm not including because it's not relevant to the problem.
    // In it, I remove any creature that passes outside the viewport by setting its creatures[] spot to null.
    // Hence the if(creatures[i] == null) test is used to recreate "dead" creatures. Inelegant, I know.
    private void CreateCreatures()
    {
        for (int i = 0; i < creatures.Length; i++)
        {
            if (creatures[i] == null)
            {
                // In reality, the name selection is randomized
                creatures[i] = new BerryCreature("ant");

                // Load content and texture (which I create elsewhere)
                creatures[i].LoadContent(
                    FindAnimation(creatures[i].Name),
                    creatureTextures[creatures[i].Name]);
            }
        }
    }

    private ActorAnimationGroupData FindAnimation(string animationName)
    {
        int yourAnimation = -1;

        for (int i = 0; i < animations.Length; i++)
        {
            if (animations[i].name == animationName)
            {
                yourAnimation = i;
                break;
            }
        }

        return animations[yourAnimation];
    }

    public void Update(GameTime gameTime)
    {
        for (int i = 0; i < creatures.Length; i++)
        {
            creatures[i].Update(gameTime);
        }
    }
}

class Reader
{
    public ActorAnimationGroupData[] LoadAnimations()
    {
        ActorAnimationGroupData[] animationGroup;
        XmlReader file = new XmlTextReader(filename);

        // Do loading...

        // Then later
        file.Close();
        return animationGroup;
    }
}

class BerryCreature
{
    private ActorAnimation animation;
    private string name;

    public BerryCreature(string name)
    {
        this.name = name;
    }

    public void LoadContent(ActorAnimationGroupData animationData, Texture2D sprite)
    {
        animation = new ActorAnimation(animationData);
        animation.LoadContent(sprite);
    }

    public void Update(GameTime gameTime)
    {
        animation.Update(gameTime);
    }
}

class ActorAnimation
{
    private ActorAnimationGroupData animation;

    public ActorAnimation(ActorAnimationGroupData animation)
    {
        this.animation = animation;
    }

    public void LoadContent(Texture2D sprite)
    {
        this.sprite = sprite;
    }

    public void Update(GameTime gameTime)
    {
        animation.Update(gameTime);
    }
}

struct ActorAnimationGroupData
{
    // There are lots of other members of this struct, but the timer is the only one I'm worried about.
    // TimerData is another struct
    private TimerData timer;

    public ActorAnimationGroupData()
    {
        timer = new TimerData(2);
    }

    public void Update(GameTime gameTime)
    {
        timer.Update(gameTime);
    }
}

struct TimerData
{
    public float currentTime;
    public float maxTime;

    public TimerData(float maxTime)
    {
        this.currentTime = 0;
        this.maxTime = maxTime;
    }

    public void Update(GameTime gameTime)
    {
        currentTime += (float)gameTime.ElapsedGameTime.TotalSeconds;
        if (currentTime >= maxTime)
        {
            currentTime = maxTime;
        }
    }
}