The dryness of the Dahomey Gap is unusual, given that it lies surrounded by a very wet monsoon belt on all sides, without mountains to block moisture.
Yet, Accra, which is in the heart of the Gap, receives only 720 mm (28 in) of rainfall per year — less than half the amount needed to sustain tropical rainforest (which would be expected at a latitude of 6° N).
Murphy and Collier, in their analysis of two aplocheiloid fish genera, show a split in the African species which they attribute to the presence of an epicontinental sea between the late Cenomanian and early Eocene.
[2] This discontinuity had earlier been noted in plant species by White[3] and is supported by an analysis of the Coffea clade by Maurin et al.[4] The Dahomey Gap has existed in its present form for only about four thousand years.
[5] For most of the Quaternary, dry conditions due to a much colder Atlantic Ocean (aided by extensive cold currents from ice sheets in Europe and North America) have meant that the present-day forest zone has supported very little or no rainforest.