Carleton PhD candidate Sarah Harrison has published new research that suggests male members of various species are not entirely honest when it comes to sexual signals about their quality as a mate. The paper was published in PLOS ONE, an online peer-review publication.
Males of various species produce signals to attract females to mate. These sexual signals come in many forms, from the songbird’s call to the antlers of a deer. These signals are thought to help females choose between different males by providing signs of potential mating benefits.
“If a male bird has bright and colourful plumage, for example, it might signal that a male is healthy and good at finding high quality food,” said Harrison. “Mating with males that have colourful plumage might benefit females directly if males share their food, or indirectly if they pass on good genes for health and foraging ability to their offspring.”
Sexual signals like these have long been thought to honestly indicate a male’s mating benefits since poor quality males should be incapable of cheating and producing sexual signals that rival those of high quality males. But are all sexual signals honest, or do males sometimes cheat when trying to attract a mate?
Harrison set out to answer this question by looking at fall field crickets (Gryllus pennsylvanicus), a species that is common in much of the U.S. and southern Canada. Males of this species produce mating calls by rubbing their hind wings together. The calls are made up of a series of sound pulses called chirps. She examined the relationship between the energy of these chirps and both body size and plumpness, two indicators of a male’s condition.
“Higher condition males should have more energy to devote to signalling than low condition males,” said Harrison.
In examining the crickets, Harrison found evidence that males dishonestly signal their condition. Because it costs more energy to call at faster pulse and chirp rates, these faster rates should be harder for low condition males. But the study found that small males called with faster chirp rates than large males, while lean males produced calls with pulse and chirp rates similar to plump males. Males of intermediate plumpness had the lowest pulse and chirp rates.
“We found that males might be more likely to produce dishonest signals, or cheat, if they have a bleak reproductive future,” said Harrison. “These small and lean males might be starving, diseased, old or dying, and so may be investing all of their remaining energy into attracting females at the risk of dying sooner.”
"Harrison's work tests a key evolutionary theory - that sexual signals provide honest information to females about mating benefits,” said Sue Bertram, an associate professor in the Department of Biology and Harrison’s co-supervisor. “We are excited about her findings because they suggest poor condition (lean or small) males signal more attractively than they should be capable of - essentially they cheat. If males cheat then sexual signals are not always honest and a key evolutionary theory is not supported.”
In the end, however, these males would not be able to provide females with the mating benefits (e.g. good genes for offspring body size and foraging ability) that they were dishonestly advertising.
Though Harrison’s research has been published, her work is not yet complete.
“Because our study was conducted on field-captured crickets, we assume lean and small males are bad at acquiring food,” said Bertram. “This assumption must be tested. Currently, Sarah is manipulating condition by feeding crickets diets varying in carbohydrates and proteins to definitively test whether poor condition male signals rival the signals of high condition males, but at the risk of an early death."
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