Introduction
Charles Darwin once said, “The sight of a peacock makes me sick.” Why did male peacocks’ extravagant feathers provoke Darwin? He could not understand why male peacocks had developed such a disadvantageous trait, for their tail feathers consume significant energy to grow and carry during flight. Also, the bright colors make peacocks vulnerable to predators. Natural selection states that advantageous adaptations permeate species’ gene pools to increase their survival and reproductive abilities, so peacocks seemingly contradict Darwin’s revolutionary theory. However, the scientist did not yet understand that sexual selection is another mechanism of evolution and thus works alongside natural selection.
What is Sexual Selection?
Sexual selection is the process by which a female picks high-quality mates with whom to reproduce. Thus, the traits she finds desirable become more widespread within the gene pool. Male peacocks impress females by displaying their tails and thus increase the number of offspring they produce, or their fitness. Since organisms’ reproductive success determines their evolutionary success, male peacocks’ tails are, in fact, advantageous. As you can see, sexual selection favors traits that natural selection may or may not. When I looked at evolution through the lens of reproductive success, I realized the importance of sexual selection.
Frigate Birds
As I explored the Galápagos, I observed more examples of sexual selection in action that deepened my understanding of which traits it favors. For instance, male frigate birds take about 15 minutes to inflate their gular pouches, red sacs located near their throats, in order to attract mates. Magnificent frigate birds display these pouches year round while other species have orange spots when they are not in breeding season. Males tap their beaks against their gular pouches to produce drumming sounds, which urge females to pick them as their mates. Are gular pouches simply a result of sexual selection as peacock tails are, or does natural selection also favor them? I learned that the latter is true, for they help frigate birds cool off.
Sexual Dimorphism
Since female frigate birds pick mates rather than attracting them, they have not developed gular pouches. This difference between the male and female versions of a species is called sexual dimorphism. Frigate birds reproduce every two years and split up after their baby matures. Organisms that pair for life exhibit lower sexual dimorphism or are more similar. Since the males of monogamous species seek only one mate during their lifetimes, they do not have to invest as much energy in being more attractive by developing extravagant traits.
In contrast, males of polygamous species engage in a type of intrasexual selection called precopulatory competition, the process by which a species’ males fight each other so that the victor can mate with the female. To be a victor, males must develop impressive characteristics, and if the competition is aggressive and physical, they must exhibit traits such as larger canines or body size. Thus, it makes sense that frigate birds, as polygamous organisms, display high sexual dimorphism.
Boobies
Blue-Footed Boobies
In contrast, blue-footed boobies usually pair for life and are thus less sexually dimorphic; both sexes have blue feet, but only males use them and mating dances to attract females. As we have discussed, since female frigate birds prefer males with impressive gular sacs, their young inherit this feature along with the advantages of collecting fish and cooling off. Likewise, female boobies not only favor bright blue feet because they are visually attractive but also because they signal good health. For many animals in the Galápagos, such as flamingos, brighter colors indicate nutrition in addition to reproductive readiness.
Interestingly, blue-footed boobies’ feet can be different shades of blue. In Mexico, scientists painted males’ feet different colors to see which attracted the most females and discovered that turquoise was most attractive. They observed that when the bird is feeding babies, its feet are always dark blue. This color indicates that they are not as healthy as when they are single, with more energy and thus turquoise feet.
Red-Footed Boobies
Another species of boobies is red-footed boobies, which have blue beaks because they lack blue feet. Blue-footed boobies, which live on the ground, attract mates by dancing with their feet. Red-footed boobies live in bushes and trees instead to ensure their niches do not overlap, a process called resource partitioning. Thus, they attract mates with their beaks. An even less common booby species is the Nazca booby. Male Nazca boobies capture females’ attention through a gesture called sky-pointing. They pull their wings back and whistle while looking upward. In general, male boobies’ mating calls are whistles while females make honking sounds as ducks do. Nazca boobies are together for only a year because they split up and look for new mates once their babies can fly. As a result, they exhibit more sexual dimorphism.
Galápagos Lava Lizards
One of the most interesting facts I learned is that Galápagos lava lizards demonstrate reverse sexual dimorphism, for females are more brightly colored although males usually are in order to attract mates. In the case of this unique species, the purpose of females’ bright red is not to attract mates but rather to indicate inavailabilty. The female we saw on Floreana Island signaled that she was already fertilized and did not want any suitors bothering her. As my father said, her bright coloring was like a wedding ring.
Humans
Sexual selection is also visible in humans all over the world. Human males are usually attracted to young women they consider beautiful while females usually select mates who exhibit power, status, and resources. Since women give birth to and nurse babies, they should be young and healthy to have reproductive and thus evolutionary success. Men assess women’s health through their physical appearance while women assess men’s ability to provide for their dependent young through their power and status.
We often view other animals and plants as wildly different from ourselves. However, the goal of this post and others about the Galápagos is to highlight the similarities between humans and other organisms.
Sources
- Thank you to our Abercrombie & Kent guide in the Galápagos for sharing the information upon which this post is based.
- https://www.nps.gov/cabr/blogs/excuse-me-what-do-you-keep-in-your-gular-pouch.htm
- https://carta.anthropogeny.org/moca/topics/sexual-body-size-dimorphism#:~:text=Sexual%20body%20size%20dimorphism%20is%20correlated%20with%20intermale%20competition%20and,mating%20systems%2C%20exhibit%20greater%20dimorphism
- https://www.britannica.com/animal/booby#ref19949
- The Great Courses – Biological Anthropology: An Evolutionary Perspective