No rumpy pumpkin as calls fall on deaf ears

Natural selection is untidy, it often leaves ‘loose-ends’ behind it. Whale front legs evolved into fins, writes Richard Collins

The hind limbs disappeared altogether, but the pelvic bones of the whales’ hoofed ancestors remain; modern right whales have vestigial legs inside their bodies. A dog whirls about on himself before lying down, as his wolf antecedents did to create beds in long grass or snow. Your pet no longed hunts, yet he rolls in filth to mask his scent from potential prey. Guillemots have two brood patches, although they lay only one egg. In the past, they produced two. Shedding the feathers of the breast, the better to warm the egg, has heat-loss implications but evolution hasn’t caught up with the change.

Our bodies bear what Elaine Morgan called “the scars of evolution”, remains of
redundant adaptations. The coccyx, at the base of the spine, is a remnant of our tree-dwelling ancestors’ tails. Men still have nipples. To free their arms for tool use, four million years ago, our ancestors stood and walked upright. The back-pains slipped disks and falls which plague us today are legacies of that botched adaptation. Humans can do without the little pocket between the small and large intestines known as the appendix. Rabbits can’t.

Sometimes, however, discarded adaptations can be put to other uses. Dr Sandra Goutte and colleagues have been studying pumpkin toads in Brazil’s Atlantic forest. Less than 15mm long, the orange-coloured male frogs produce high-pitched sounds when courting. Goutte played recordings of these calls to females of two toad species and observed their behaviour. To her surprise, they didn’t respond at all to the arias. This led the researchers to examine the frogs’ ears. Tests showed that their “hearing sensitivity range and their inner ears are partly undeveloped, which accounts for their lack of high-frequency sensitivity”. Serenading, it would seem, is a complete waste of time for the aspiring Casanovas. They are, literally, broadcasting to the birds; the would-be mistresses can’t hear the music.

Frogs worldwide rely on vocal communication and their hearing is well developed. The pumpkin toads’ female ancestors, presumably, once responded to the passionate vocalisations of males. That their descendants can no longer do so raises intriguing questions. Calling would seem to be a waste of valuable energy, placing vocal males at a disadvantage compared to silent ones. Being noisy is risky. It draws attention to the caller, putting him at risk from predators and parasites. The ruthless forces of natural selection, therefore, should have eliminated such behaviour in pumpkin toads once the females had become deaf. Why hasn’t it?

Irish wasps may offer a clue to the answer. Rather than relying on camouflage to evade their enemies, the social wasps opt for deterrence; ‘don’t mess with me’ their conspicuous yellow markings warn. The threat of a nasty sting ensures that everyone gives wasps a wide berth. As Kim Jong-un knows, nuclear weapons are an effective deterrence, but you must leave you enemies in no doubt that you have them.

Like our wasps, pumpkin toads are active by day. Their bright yellow-orange colouring renders them conspicuous. The frogs’ don’t sting, but toxic substances are embedded in their flesh, which would-be predators can’t risk swallowing. Calling, the researchers suggest, acts as a further deterrent, warning off potential enemies. Meanwhile, courtship is conducted visually. “We suggest that protection against predators, conferred by their high toxicity, might help to explain why calling has not yet disappeared, and that visual communication may have replaced auditory in these colourful diurnal frogs,” Goutte and her colleagues conclude.

  • Sandra Goutte et al. Evidence of auditory
    insensitivity to vocalization frequencies in two frogs. Scientific Reports 7, September 2017.


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