Positive allometry is likely with mate choice, competitive display and other functions

Abstract

Petrie (1988) argued that structures important in displaying competitive ability between individuals should be developed in a positively allometric manner in species where large animals usually win fights, with larger animals developing relatively larger structures, as with the frontal shield of the moorhen, Gallinula chloropus. Petrie's reasoning is that animals of low competitive ability have little to gain from paying the costs of developing a large structure because any structure advertising low ability will do little to reduce the number and/ or intensity of contests. In contrast, characters important in mate choice are considered unlikely to be positively allometric. Referring to moorhen shields, Petrie said 'If shields were important in mate choice it is hard to see why they should be positively allometric, since it would pay all individuals to invest maximally in structures that advertise their quality as mates, and this would result in shields being isometric.' In my view, positive allometry will often occur in characters used in mate choice or with various other functions, and is therefore not evidence that a character is used to display competitive ability. As animals grow, they will partition investment into both bodies and characters so that bigger animals have bigger characters. How much bigger they are will depend on both the costs and benefits of a larger body size, and the costs and benefits of a larger character. The key question is, as an animal acquires more resources to invest in growth, how much should it invest in the character and how much in body size? Positive allometry will result if the net benefit of larger characters increases faster than the net benefit of larger bodies. Negative allometry will result if the net benefit of larger characters increases more slowly. In other words, if the same resource investment into a character or body size brings a greater net benefit from the former, that is where investment will be concentrated, resulting in positive allometry. If, however, a greater net benefit comes from the latter, negative allometry will result. This applies to characters with mate choice, competitive display or any other functions. A character developed for mate attraction may be positively allometric for at least two reasons. First, smaller animals may be less able to obtain mates and so have little to gain from paying the costs of developing a large character that does little to increase the number of matings. This logic is similar to that used by Petrie (1988) for characters displaying competitive ability, and will apply whether body size influences mating success through intersexual competition or mate choice. There may be cases where competition between males prevents small males from gaining access to females, while females choose males with larger characters (e.g. in the pinyon jay, Gymnorhinus