  Ontologies in the biomedical domain are numerous, highly specialized and very
expensive to develop. Thus, a crucial prerequisite for ontology adoption and
reuse is effective support for exploring and finding existing ontologies.
Towards that goal, the National Center for Biomedical Ontology (NCBO) has
developed BioPortal---an online repository designed to support users in
exploring and finding more than 500 existing biomedical ontologies. In 2016,
BioPortal represents one of the largest portals for exploration of semantic
biomedical vocabularies and terminologies, which is used by many researchers
and practitioners. While usage of this portal is high, we know very little
about how exactly users search and explore ontologies and what kind of usage
patterns or user groups exist in the first place. Deeper insights into user
behavior on such portals can provide valuable information to devise strategies
for a better support of users in exploring and finding existing ontologies, and
thereby enable better ontology reuse. To that end, we study and group users
according to their browsing behavior on BioPortal using data mining techniques.
Additionally, we use the obtained groups to characterize and compare
exploration strategies across ontologies. In particular, we were able to
identify seven distinct browsing-behavior types, which all make use of
different functionality provided by BioPortal. For example, Search Explorers
make extensive use of the search functionality while Ontology Tree Explorers
mainly rely on the class hierarchy to explore ontologies. Further, we show that
specific characteristics of ontologies influence the way users explore and
interact with the website. Our results may guide the development of more
user-oriented systems for ontology exploration on the Web.
