While the words for plants and animals characteristic of mountainous regions are entirely original, the names for fish and for agricultural activities are often assumed to have been borrowed from other languages. However, considering the presence of some preserved old terms related to the sea fauna, some have proposed that this vocabulary might have been lost in the course of time after proto-Albanian tribes were pushed back into the inland during invasions. Wilkes holds that the Slavic loans in Albanian suggest that contacts between the two populations took place when Albanians dwelt in forests 600–900 metres above sea level. Rusakov notes that almost all lexemes related to seamanship in the Albanian language are loan-words, which may indicate that speakers of the proto-language did not live on the Adriatic coast or in close proximity to it.