For a topic like "maternal instinct in alouatta palliata in Costa Rica", start your search with "maternal alouatta" or "mother howler"
Try different combinations of search terms
Search for articles by the same author you've already found useful
Be flexible with search terms
Try common names (mantled howler) and scientific names (Alouatta palliata)
Use synonyms/related terms (mother, maternal, infant, juvenile, etc.)
Look at terms used by researchers- try those too!
"Zoom out" for bigger picture
The perfect article may not exist
Consider looking for articles that are more broad (i.e. "howler" instead of "mantled howler", "primate maternal instinct" instead of "alouatta palliata maternal instinct")
Sometimes an article with a similar methodology to your approach can be useful to include in a literature review
Use your sources' sources!
Look at your current research sources for more sources!
Read literature reviews or skim reference lists
Also consider looking at "Cited by" lists (some databases, like Natural Science Collection and Google Scholar, have this option to follow the research forward in time)
