Poisonous colours increase chances of sexual success
Warning colours ensure that predators remember that certain prey are poisonous. But now it appears that the colours of poisonous Panamanian frogs are also influenced by fastidious females: partner choice can also lead to colour change in these frogs, as Leiden researcher Martine Maan has reported in PNAS.
It is surprising that sexual selection leads to brighter colours in male frogs because animals with warning colours generally look very similar. Poisonous animals often use the same warning colours, such as red, yellow and black. This makes it easy for predators to remember which creatures are poisonous and which are not. But the Panamania strawberry poison frog is an exception to the rule: this species demonstrates enormous colour variation: green, blue, red, with or without spots, etc.
Maan is studying sexual selection by fastidious females as a possible explanation for the variation. A series of behavioural experiments has provided experimental evidence. By manipulating light conditions, she was able to show that female frogs prefer to associate with brightly coloured males. This preference then leads to sexual dimorphism, where males are more brightly coloured than females. The colours of poisonous frogs play a role in communication with both enemies and members of the same species.
Colour differences between males and females occur in many species of animals, but such differences have never previously been observed in poisonous creatures bearing warning colours. This is probably because predators select strongly on uniformity. Prey that differ from the average are immediately eaten. But this ideal also means that researchers look no further. Together with co-author Molly Cummings, Martine Maan made precise colour measurements and calculated how the frogs observe members of their own species. According to their findings, there is a difference between males and females, and this should be visible to the frogs.
How predators react to these differences is as yet uncertain. But there are indications that they are more flexible than was originally thought: once they have tried a highly toxic frog, they subsequently avoid everything that resembles them. The venom of one strawberry poison frog is powerful enough to kill a mammal 300 times larger than the frog itself, whichimay be the reason why these frogs can allow themselves so much colour variation. Some colours are brighter than others, and the brightness or conspicuousness of a colour is also partly determined by the environment. This leads the researchers to suspect that sexual selection for striking males can lead to colour variations among populations.
Martine Maan and Molly Cummings' article entitled Sexual dimorphism and directional sexual selection on aposematic signals in a poison frog was published on 26 October in the Proceedings of the National Academy of Sciences of the USA.
Martine Maan is currently working at the University of Texas in Austin, on a research grant provided by the NWO.
Please also see the article A well-developed eye for males (9 May 2006) about the PhD reserach carried out by Martine Maan.