no req.params sent in the requests

In a practice example, i'm trying to create a restfull API, very simple. The plain GET and POST methods works well, but the GET, PUT and DELETE method pointing to /api/bears/:bear_id just stay there, waiting...

// CONFIGURACION INICIAL //
// ===================== //
var express = require('express');
var app = express();
var bodyParser = require('body-parser');
var mongoose = require('mongoose');
var db = mongoose.connection;

// CONFIGURANDO APP //
// ================ //
app.use(bodyParser.urlencoded({ extended: true }))
app.use(bodyParser.json())

mongoose.connect('mongodb://localhost:27017/bears');
db.on('error', console.error.bind(console, 'connection error:'));
db.once('open', function cb () {
  console.log('conexion establecida');
})

var Bear = require('./models/bear_model.js');

var port = process.env.PORT || 8080;      // seteo del puerto

var router = express.Router();          // instancia del ruteador

Above, the simple config, below, the snippet that is causing me problems:

router.use(function (req, res, next) { // simple logger
  if (req.method === 'GET')
    console.log('executing query on id %s', JSON.stringify(req.params));
  else if (req.method === 'PUT')
    console.log('executing query on id %s', JSON.stringify(req.params));
  else
    console.log('executing query on id %s', JSON.stringify(req.params));
});

router.route('/bears/:bear_id')

  .get(function (req, res) {

    Bear.findById(req.params.bear_id, function (err, bear) {
      if (err)
        res.send(err);
      res.json(bear);
    });
  }) // end GET /bears/:bear_id

  .put(function (req, res) {

    Bear.findById(req.params.bear_id, function (err, bear) {
      if (err)
        res.send(err)

      bear.name = req.body.name; // Update bear_id of Bear

      bear.save(function (err) {
        if (err)
          res.send(err);

        res.json({msg: 'Bear actualizado!'});
      });
    });
  }) // end PUT /bears/:bear_id

  .delete(function (req, res) {

    Bear.remove({
      _id: req.params.bear_id
    }, function (err, bear) {
      if (err)
        res.send(err);
      res.json({ msg: 'Bear eliminado' });
    });
  }); // end DELETE /bears/:id && router /bears/:id

app.use('/api', router);               // la api usará como base el prefijo /api

Executing one route with a param log me: executing query on {}, so, the req.params.bear_id simply is not captured, and if i change req.params by req.params.bears_id, obviously i get an undefined log, so i read de docs and think i'm doing generally well the process but don't catch the issue.