The handicap principle of Amotz Zahavi, Russell Lande and W. D. Hamilton, holds that the fact that the male is able to survive until and through the age of reproduction with such a seemingly maladaptive trait is taken by the female to be a testament to his overall fitness. Such handicaps might prove he is either free of or resistant to disease, or that he possesses more speed or a greater physical strength that is used to combat the troubles brought on by the exaggerated trait. Zahavi's work spurred a re-examination of the field and several new theories. In 1984, Hamilton and Marlene Zuk introduced the "Bright Male" hypothesis, suggesting that male elaborations might serve as a marker of health, by exaggerating the effects of disease and deficiency. In 1990, Michael Ryan and A.S. Rand, working with the Túngara frog, proposed the hypothesis of "Sensory Exploitation", where exaggerated male traits may provide a sensory stimulation that females find hard to resist. Subsequently, the theories of the "Gravity Hypothesis" by Jordi Moya-Larano et al. (2002), invoking a simple biomechanical model to account for the adaptive value for smaller male spiders of speed in climbing vertical surfaces, and "Chase Away" by Brett Holland and William R. Rice have been added. In the late 1970s, Janzen and Mary Willson, noting that male flowers are often larger than female flowers, expanded the field of sexual selection into plants.