Red-capped plovers (Frankzed via Flickr)
The title of a research article says it all: The bright incubate at night.
Only male red-capped plovers have a red cap on their heads; female red-capped plovers aren’t quite as showy, having just a light brown cap.
You might think the best explanation for why females don’t have bright colors on their heads is that they don’t need them. After all, when it comes to mating, the males attract females with their bright colors, and all females have to do, really, is be there.
But evolution and natural selection don’t often take a passive role in the survival of a species. The simple “lack of need” for color on the part of females is not a very satisfying explanation in evolutionary terms.
Therefore, working in Australia, Kasun B Ekanayake and fellow researchers hypothesized that the female plovers had dull caps because dull colors gave plovers some sort of survival advantage as a species. If they could prove that hypothesis was correct, it would be much more satisfying in terms of evolution.
Their research findings were published on April 8 for the May 2015 edition of Proceedings B from the Royal Society of London.
Experimental approach
To test their hypothesis, researchers set up nests filled with pheasant eggs, which look the same as plover eggs, and in front of the nests, they stationed a model of either a male or female plover.
Then, they set up video cameras and recorded ravens swooping down to eat the eggs. If using the male plover models caused more eggs to be eaten by the ravens, then the hypothesis would be supported—having a red cap is a mixed blessing in that it attracts not only mates but predators as well. If, on the other hand, more eggs were eaten when the female plover model was guarding the nest—of if there was no significant difference between male and female models—no evidence would have been found to either support or refute the hypothesis.
Corroborating evidence
Scientists knew that male plovers watch over the nest and the eggs therein at night, while females guard the nest during the daytime.
As a bit of background research for their hypothesis, it made sense that females, who need to leave the nest in order to gather food at least occasionally, would leave when it didn’t affect the species’ survival if they left the males guarding the eggs.
Sure enough, 80 percent of the eggs were eaten with males guarding the nest, and only 20 percent of the eggs were eaten with females guarding the nest. This difference was considered significant, and scientists then concluded that the dull color on the females wasn’t just unnecessary but that it actually gave the species an increased survival likelihood:
The male can guard the nest at night, when male and female plovers look the same to ravens or any potential predator. Females are better off guarding the nest during the day, since their dull caps don’t attract predators as much as the bright red caps do on the males during daylight.
Article reference
Kasun B. Ekanayake , Michael A. Weston , Dale G. Nimmo , Grainne S. Maguire , John A. Endler , Clemens Küpper. “The bright incubate at night: sexual dichromatism and adaptive incubation division in an open-nesting shorebird.” Proc. R. Soc. B (April 8, 2015) 282:20143026; DOI: 10.1098/rspb.2014.3026.
Evaluate their experimental design and suggest other approaches these scientists might have taken to address the same scientific question. See Next Generation Science Standard HS-LS4-4 for more information. “Emphasis is on using data to provide evidence for how specific biotic and abiotic differences in ecosystems (such as ranges of seasonal temperature, long-term climate change, acidity, light, geographic barriers, or evolution of other organisms) contribute to a change in gene frequency over time, leading to adaptation of populations.”
