How to send an email to a node author after selecting a specific value in a selected list field?
You can use hook_node_presave() and in this function check if the select value is "new litter" and generate email.

function YOUR_MODULE_NAME_node_presave($node){
    if($node-&gt;type == 'YOUR_CONTENT_TYPE'){
        //get your field value
        $field = field_get_items('node', $node, 'YOUR_FIELD_MACHINE_NAME')[0]['value'];
        //check if value equals "new litter" key
        if($field == 'NEW_LITTER_KEY'){
            generate_email(); 
        }
    }
}


change uppercase strings with your module and machine names