Not sure why this hasn't saved

'use strict';

var mongoose = require('mongoose');

mongoose.connect('mongodb://localhost:27017/sandbox');

var db = mongoose.connection;

db.on('error', function(err) {
  console.error('connection error', err);
});

db.once('open', function() {
  console.log('db connection successful');
  //All database communication goes here.
  var Schema = mongoose.Schema;
  var AnimalSchema = new Schema({
    type: String,
    size: String,
    color: String,
    mass: Number,
    name: String
  });
  var Animal = mongoose.model('Animal', AnimalSchema);
  var elephant = new Animal({
    type: 'elephant',
    size: 'big',
    color: 'gray',
    mass: 6000,
    name: 'Lawrence'
  });

  elephant.save(function(err) {
    if (err) console.err('Save failed', err);
    else console.log('Saved');
    db.close(function() {
      console.log('db connection closed');
    });
  });
});