routes: {
    'lions': 'animalsRoute',
    'animals': 'animalsRoute'
},
animalsRoute: function() {
    this.navigate("#/animals", { replace: true });
    // or using the global history object:
    // Backbone.history.navigate("#/animals", { replace: true });
}

routes: {
    'lions': 'lionsRoute',
    'tigers': 'tigersRoute'
},
showGenericRoute: function() {
    this.navigate("#/animals", { replace: true });
},
tigersRoute: function() {
    this.showGenericRoute();
    // handle the tigers route
},
lionsRoute: function() {
    this.showGenericRoute();
    // handle the lions route
}

routes: {
    'animals/:animal': 'animalsRoute',
},
animalsRoute: function(animal) {
    // removes the animal from the url.
    this.navigate("#/animals", { replace: true });

    // use the chosen animal
    var view = new AnimalView({ type: animal });
}

var Router = Backbone.Router.extend({
    routes: {
        'animals': 'animalsRoute'
    },
    animalsRoute: function() {
        // handle the generic behavior.
    }
});

var PussyRouter = Backbone.Router.extend({
    routes: {
        'lions': 'lionsRoute'
        // ...
    },
    lionsRoute: function() {
        // handle lions, then redirect
        this.navigate("#/animals", { trigger: true, replace: true });
    }
});