Effects of allometric growth and toe pad morphology on adhesion in hylid tree frogs

Smith, Joanna McLellan (2003) Effects of allometric growth and toe pad morphology on adhesion in hylid tree frogs. PhD thesis, University of Glasgow.

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Abstract

Many species of arboreal frog, over a wide range of taxa, possess specialised expanded digital pads which are remarkable similar in structure. It has long been accepted that the function for which these toe pads have evolved is to facilitate the ability seen in 'tree frogs' to adhere to smooth, vertical surfaces. Recent consensus is that tree frogs adhere through wet adhesion, with the forces produced by this mechanism scaling directly with pad area. If frogs are geometrically similar and scale following isometric predictions, with weight increasing with the linear dimension cubed and toe pad area with the linear dimension squared, then the problem arises as to how an area dependent adhesive mechanism is able to cope with the pressures of increasing size? Adhesive forces recorded in adult frogs from twelve species of Trinidadian hylid support earlier studies, scaling directly with toe pad area. Weight increases at a lesser rate than that expected through isometry, but not sufficiently to be matched by toe pad area, which increases as the linear dimension squared. The ability to adhere is thus affected so that large species are not able as small species to maintain angles significantly beyond the vertical on smooth surfaces. Nevertheless, force per unit area increases with the linear dimension, suggesting that the pads of larger species are able to adhere more efficiently. Toe pad morphology and epidermal structure are roughly similar but there are small interspecial differences; particularly in the delineation of grooves separating the pad from the ventral surface of the toe and in cell size, which have significant effects on the adhesive efficiency of the pad. In large species there is an extension in the distribution of specialised pad cells to include subarticular tubercles, perhaps increasing the area available for wet adhesion. The increase in size in the period of growth from metamorphosis to adulthood is substantial for most species of frog. With similar metamorphic sizes in all species in this study, the degree to which growth will put pressure on the adhesive mechanism was dependent on the adult size and so species were considered separately according to their eventual size: In four small-sized species of hylid, there is a lowered rate of weight increase to that expected with isometric growth, though the degree to which this is the case is not sufficient to be matched by the equivalent increase in pad area, which increases with the linear dimension squared. There is, nevertheless, a greater increase in adhesive force with growth than can be explained by the equivalent increase in toe pad area and adhesive ability is not affected by growth, with no significant differences in detachment angles recorded in adults and juveniles. Furthermore, all four small species show an improvement in toe pad efficiency as adults. There are changes in pad morphology that might be expected to facilitate in this; particularly significant being the inverse correlation between the reduced pore density seen in larger frogs and increasing adhesive force.

Item Type: Thesis (PhD)
Qualification Level: Doctoral
Additional Information: Adviser: Jon Barnes
Keywords: Morphology
Date of Award: 2003
Depositing User: Enlighten Team
Unique ID: glathesis:2003-74064
Copyright: Copyright of this thesis is held by the author.
Date Deposited: 23 Sep 2019 15:33
Last Modified: 23 Sep 2019 15:33
URI: http://theses.gla.ac.uk/id/eprint/74064

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