During the 1960's and 70's, scientists were concerned and puzzled by a large gap in the human fossil record. The "aquatic ape theory" gained prominence as an explanation for this gap. This theory posited that primitive humans were forced toward a littoral lifestyle by competition for arboreal resources. Analogies were made to seal populations, who sleep on land at night but spend most of their days in coastal waters. Proponents pointed to various physiological human attributes, such as bipedalism and the webbing between human toes, as extant adaptations. However, the aspect of the theory that captured the public imagination and undoubtedly boosted its standing was the point that this hypothesis explained human hairlessness; as with dolphins, this streamlining would facilitate swimming and diving. Proponents noted that the remaining body hair would match the flow of water, and extreme advocates explained the gender difference in hair by suggesting that females much more rarely ventured out of the shallows and into the putatively more dangerous forests and savannahs. Nonetheless, despite the popular stature of the theory, the scientific community almost unanimously rejects it as mere conjecture not only because of the lack of supporting evidence but also because its claims do not withstand scrutiny. While bipedalism does facilitate swimming, it is even more of an advantage in terrestrial pursuits. Further, biomechanical analysis indicates that humans remain such inadequate swimmers that they could not so succeed. As for hairlessness, critics point out that other semi-aquatic mammals actually have dense fur and/or barrel shaped torsos for heat retention. Today, the theory, while still championed by a prominent writer but non-scientist, has no serious support among mainstream-trained paleoanthropologists.