Charles Darwin’s fame is often attributed to his description of natural selection, but his work on sexual selection is often overlooked. Experimental evidence to support Darwin’s ideas was only provided one hundred years later, using the simplest of methods.
In a letter to a colleague, Darwin wrote “the sight of a feather in a peacock’s tail, whenever I gaze at it, makes me sick!”. The elaborate tail of the peacock just should be a hindrance to its “struggle for existence” – as explained through his theory of natural selection.
In 1871, Charles Darwin wrote the book he considered to be his biggest work: The Descent of Man, and Selection in Relation to Sex. He developed the theory of sexual selection to explain why traits that seem to reduce survival chances, like the peacock’s tail, could persist. Sexual selection differs from natural selection – explained as the struggle for reproduction, not the struggle for existence. If a trait ensures you have children, it doesn’t matter that you die quicker. This can occur via male-male competition, or by matching female preference. The peacock’s tail and other secondary sexual ornaments, traits that are either detrimental or neutral to survival, will evolve through female preferences.
Alfred Russel Wallace, the co-founder of natural selection, strongly disagreed with Darwin’s views on sexual selection. With Darwin’s health deteriorating, the debate over sexual selection became rather one-sided. Wallace believed that sexual selection was less important, with the evolution of all traits best explained using natural selection. He thought that female mate choice could not develop in animals, as they weren’t capable of aesthetic feeling.
Later work by Fischer hypothesised runaway selection to explain sexual ornaments. A female preference for a trait arises, with female preference being beyond the optimal male survival benefit. This results in males developing a trait that leads to increased reproductive success but has a detrimental effect on survival chances.
The theory of sexual selection is now widely supported by the scientific community. In the 1980s, over 110 years since Darwin’s book, research in Kenya finally provided experimental field evidence to support Darwin’s ideas on sexual selection. Dr Malte Andersson used simple equipment, scissors and glue, to alter the tail length of widowbird males. It is important to note that only feathers were trimmed and that the birds were not harmed during the experiment.
Widowbirds were chosen as a likely case of sexual selection, with clear differences between male and female tails.
- Average female tail length = 7cm
- Average male tail length = 50cm
- There seemed to be no survival benefit linked to having a longer tail, with longer tails making flying harder
Andersson created four experimental groups of males:
- Males with artificially shortened tails – using scissors
- He then glued the trimmings from that group to another group of males. These males had elongated tails.
- He then created two control groups:
- A group with unaltered tails
- A final group had their tails cut and glued back on. This meant that the tails appeared the same length, allowing Andersson to determine if the scissors and glue caused any reproductive differences
Andersson found that males with elongated tails had significantly more nests than other males. He concluded that this elevated success was due to female preference rather than changes to male behaviour, as males with longer tails had shorter display flights – which would have a detrimental effect on mating success. Therefore, the experiment also showed that female preferred males with tails longer than normal – fitting with Fischer’s runaway selection hypothesis.
But why do females choose?
- Tail length might be involved in male-male competition, with males with the longest tails ranking higher in the hierarchy. Though this is unlikely due to the decreased display times, and no evidence of correlation between tail and territory size.
- It is more likely that longer tails are a sexual ornament that displays the quality of the male’s genes. If true, long tails must be linked to a desirable trait.
Andersson’s investigation provided evidence to settle a one-hundred-year-old debate, using just scissors and glue.
The paper: Female choice selects for extreme tail length in a widowbird, was published in Nature volume 229, although unfortunately access is locked behind a paywall.