Slim Framework and doctrine, problem to generate json

I'm building a API where I register animals and colors and get animals in return, I'm using Doctrine to treat entities and I've created a many-to-many relationship between animals and colors.

/**
 * @Entity @Table(name="Animais")
 **/
class animais {

    /**
     * @var int
     * @Id @Column(type="integer") @GeneratedValue
     */
    public $id;

    /**
     * @var char
     * @Column(type="string") 
     */
    public $sexo;

    /**
     * @var char
     * @Column(type="string") 
     */
    public $tamanho;

    /**
     * @var char
     * @Column(type="string")
     */
    public $tipo_animal;

    /**
     * @var string
     * @Column(type="string") @GeneratedValue
     */
    public $coordenada;

    /**
     * @var string
     * @Column(type="string") 
     * @GeneratedValue
     */
    public $obs;

    /**
    *  @var temascores
    * @ManyToMany(targetEntity="Cores", inversedBy="animais")
    */
    public $cores;

    public function __construct()
    {
        $this->cores = new ArrayCollection();
    }

Colors class

/**
 * @Entity @Table(name="Cores")
 **/
class cores {

    /**
    * @var int
    * @id @Column(type="integer") @GeneratedValue
    */
    public $id;

    /**
    * @var string
    * @Column(type="string")
    */
    public $cor;

    /**
    * @var string
    * @ManyToMany(targetEntity="Animais", mappedBy="cores")
    */
    public $animais;

    public function __construct() {
        $this->animais = new \Doctrine\Common\Collections\ArrayCollection();
    }

The real problem of the application is when I'm going to make a get of animals, it returns a json perfect except colors

{
    "id": 2,
    "sexo": "macho",
    "tamanho": "grande",
    "tipo_animal": "gato",
    "coordenada": "12340-12303",
    "obs": "achado ao lado",
    "cores": {}
}

I would like the colors to be contained in json since I already confirmed that they exist in the class when I search for them by EntityManager method code of API

$app->get('/animal', function (Request $request, Response $response) use ($app) {

    $entityManager = $this->get('em');

    $animaisRepository = $entityManager->getRepository('App\Models\Entity\Animais');

    $animais = $animaisRepository->findOneById(2);
    $return = $response->withJson($animais, 200)
        ->withHeader('Content-type', 'application/json');
    return json_encode($animais);
});