Competing Natural and Sexual Selection: Variation in the Visual Signals of Wall Lizards

Date published: July 28, 2014

Source: Behavioral Ecology

Summary: A study suggests that coloration and conspicuousness of wall lizards evolved under conflicting demands of natural and sexual selection.

            A study suggests that coloration and conspicuousness of wall lizards evolved under conflicting demands of natural and sexual selection, according to a study published in 2014 by Kate Marshall and Martin Stevens from the University of Cambridge in the journal Behavioral Ecology.

            Three populations of Aegean wall lizards were studied from the islands of Skopelos, Syros, and Foelgandros. The authors investigated four main points: whether the lizards were more conspicuous to conspecifics (members of the same species) than to predators, if males were more conspicuous than females as a result of sexual competition, if there was variation in visual signals between different parts of the body, and whether the conspicuousness of the lizards differed among the three populations. Lizards must be noticeable enough to attract potential mates but they must also remain somewhat camouflaged to avoid detection by predators, resulting in conflicting natural and sexual selection.

            The results showed that lizards were more noticeable to conspecifics than to avian predators across all body regions in all three populations. Avian predators can only perceive relatively high wavelengths of UV light while lizards can perceive a much higher range. The increased conspicuousness to conspecifics was caused in part by the use of short UV wavelengths to remain noticeable to potential mates yet avoid detection by predators.

            In the Skopelos population, males exhibited increased conspicuousness over females, but in the Syros population, both males and females displayed higher conspicuousness of certain body regions. The Syros population has a higher population density, which can lead to increased sexual competition, even for females, resulting in the need for both males and females to attract mates. This is indicative of evolution because the conspicuousness varied among populations to best suit the needs of the lizards.

            It was also shown that there was variation in conspicuousness between body parts of the lizards. In the Skopelos and Syros populations, the flanks of the lizards were more conspicuous than the backs. Avian predators view the lizards from above, so their backs were more camouflaged, and because potential mates view the lizards from the side on the ground, the sides of the lizards were more noticeable.

             The Foelgandros population of lizards did not exhibit the significant differences in conspicuousness between body parts that was found in the other two populations. This is likely because the Foelgandros island lacks a number of the avian predators that are on the other islands, so the lizards have a lower risk of detection and therefore less of a need for camouflaged backs.

            The findings of the study indicate that the coloration of the lizards has evolved to make the lizards better suited to their environment. The competing demands of natural selection, which would make the lizards less noticeable to their predators, and sexual selection, which would result in lizards being more conspicuous to potential mates, led to different conspicuousness between certain body regions and differences between males and females. These differences are indicative of evolution.

Journal Reference:

Marshall, K.L.A, and Stevens, M. 2014. Wall lizards display conspicuous signals to conspecifics and reduce detection by avian predators. Behavioral Ecology 25: 1325-1337.

Evolution of Echolocation in Bats: An Antagonistic Interplay between Natural and Sexual Selection

Date:  July 30, 2014

Source:  PLoS ONE

Institutions Involved:  Max Planck Institute for Ornithology, University College Dublin, Tabachka Bat Research Station, MTA-ELTE-MTM Ecology Research Group, Naturalia environnement, Tunis Superior Institute for Biological Applied Sciences, and Max Planck Institute for Evolutionary Biology

Summary:  Research suggests that sexual selection plays a role in the evolution of high frequency echolocation in bats, possibly against natural selection, according to a study published on July 30, 2014 in the open-access journal PLoS ONE by Sebastien J. Puechmaille and his colleagues. 

Animals generally use echolocation, or acoustic signals, as a means of communication.   Unlike bird songs or frog calls, bats are very interesting when studying the evolution of signaling systems because echolocation is used for both foraging (food detection) and communication.  While it was common belief that echolocation is influenced by ecology through natural selection alone, Puechmaille et al. investigated the role of echolocation calls with regards to sexual selection in bats. 

This study was conducted using 90 Rhinolophus mehelyi or Mehely’s horseshoe bat (75 females and 15 males) collected at two caves in North-Eastern Bulgaria.  This bat species was an ideal choice because unlike other species of the Rhinolophidae family that produce lower frequencies as they increase in body size, R. mehelyi differ greatly by producing frequencies higher than expected given their size.  Attributing this piece of information to sexual selection, researchers hypothesized that an individual’s echolocation peak frequency should be related to its body size and condition, and therefore indicate mate quality.  They predicted that females should prefer males with high frequency calls during the mating season.  As a result, males with high peak frequency should have a higher reproductive success, and therefore have higher relatedness with other members of the colony (more relatives). 

The experiments were performed in a one cubic meter box, lined with sound-absorbing foam.  The bat was placed on a listening perch (wooden basket) at an equidistant position from the two compartments, with each containing a speaker.  The set-up was also equipped with infrared light, and an infrared-sensitive camera to monitor the activity of the bat.  The assignment of low or high frequency playback to the left or right speaker was randomized before testing each individual. 

The data demonstrated that echolocation peak frequency is representative of body size and body condition, with the larger and heavier individuals have higher frequencies.  It was found that the female bats are more attracted to males with higher peak frequency during the mating season.  This result is in accordance with the theory of sexual selection that females prefer to mate with good quality males to produce fitter offspring.  Along with female mate choice, the researchers had observed a significant positive correlation between relatedness and peak frequency in male bats. 

The combination of behavioral, ecological, and genetic studies provided researchers with the first documented evidence that female bats indeed choose their mate based on echolocation frequency.  The ability of bats to precisely locate the source of acoustic signals even in complete darkness further establishes the idea that echolocation is essential for mating.  

Citation:

Puechmaille, S.J., I.M. Borissov, S. Zsebok, B. Allegrini, M. Hizem, S. Kuenzel, M. Schuchmann, E.C. Teeling, and B.M. Siemers. 2014.  Female Mate Choice Can Drive the Evolution of High Frequency Echolocation in Bats: A Case Study with Rhinolophus mehelyi. PLoS ONE 9(7): e103452. 

Cost for a Sexually Selected Trait in Male Guppies

Summary: Jean-Guy J. Godin and Heather E. McDonough of Mount Allison University conducted an experiment to gather empirical evidence on whether predators prefer guppies of a brighter coloration. Results support the idea that the guppies’ sexually selected trait of bright coloration turns out to be a disadvantage in terms of survival. 

In many species males use obvious sexual signals, like bright coloration or large plumage, to attract females. It is well known that such traits are an advantage in sexual selection because they are female preferred but do not translate to an advantage when it comes to natural selection since such traits also attract the attention of predators. However, the direct fitness cost of a sexually selected trait has not been thoroughly proven via experimentation.

Godin and McDonough decided to examine the effects of a sexually selected trait on the rate of mortality in a predator-prey system. They chose to test male guppies and their natural predators, the blue acara cichlid fish. Male guppies are genetically polymorphic. They express patches of different color, hue, number, size, and reflectivity. It is known that the guppies with brighter coloration tend to be more active and bold than their dimmer counterparts. Although it seems that bright males have a mating advantage, evidence exists that fish predators, like the blue acara cichlid, also select prey with conspicuous color patterns so it may be that these bright guppies face higher predation intensity. It was found in one study that populations of male guppies in the waters of Trinidad were less colorful since there was an increased population of their natural diurnal predators.

In the first experiment, two male guppies of the same length but difference in brightness were kept restrained in separate Plexiglas tubes. The tubes were equidistance from the cichlid, which was acclimated to the environment for 24 hours before being exposed to the prey. The second experiment was set up in much the same way. However, a larger aquarium was used and the cichlid was left to assimilate to the environment for 36 hours. The guppies in this case were free-swimming which more accurately replicates the natural interactions between the predator and prey.

The end results of the experiment indicated that the brightness of the male guppies affects the frequency in which they get approached and attacked by predators. For the first experiment the bright guppies were attacked significantly more than the drabber guppies. In the second experiment, bright guppies were once again attacked significantly more than the drab guppies irrespective of distance. The results further support the theory that sexually selected traits have a cost in terms of survival.

The study pertains to what we have been learning in class about the apparent paradoxical relationship between sexual selection and natural selection. It is interesting that nature requires organisms to balance mate preference and predator evasion in order to survive. 

Citation: 

Godin,J.J.,McDonough, H.E. 2003. Predator preference for brightly colored males in the guppy: a viability cost for a sexually selected trait. Behavioral Ecology 14(2): 194-200

Energy Cost of Heavy Peacock Plumages: Is it worth it?

Summary: Peacocks were tested to see if their feathers present a disadvantage in respect to their ability to fly even though it is an obvious sexual advantage in attracting peafowls. Tests were conducted on this question and results show that in fact, their heavy plumages do not present much of a disadvantage to peacock flight.

Peacocks are one of the most famous examples used to describe the definition of sexual selection. Their grand and colorful tail feathers attract peafowls who use these tails as an indicator for a mate with good health. However, these colorful feathers that drag behind the peacock are heavy and for a long time, researchers have hypothesized that these birds have a physical disadvantage when flying as their plumages take up the energy that could potentially be used to fly longer or escape from predators faster.

However, recent research on this topic by Graham N. Askew with assistance from professors at Loughborough University, tested to see if this hypothesis was valid. Two groups of peacocks from the UK population of captive bred-birds were observed using high speed video cameras. One group was in its natural state with their full plumages while the other group was made of peacocks whose plumages were removed. Data on the energy expenditure and aerodynamics of these two groups revealed that there wasn’t much of a difference in their ability to fly. The total energy that is required for take-off was slightly higher in those with the plumage but not significant enough to support the old hypothesis. The power required to generate the same velocity was not significantly different between the two groups either.

To ensure that flight muscles weren’t exerting more energy (in the group with the plumage) to allow the bird to fly just as fast as it would without the train of feathers, researchers also checked the mechanical power output of the flight muscles but there was no evidence for this hypothesis. Based on the data and the observations of how these two groups take-off for flight, it is understood that the drag force on the plumage is dependent on the flight velocity and the angle at which the plumage is when the bird takes off. The drag force (the force of the plumage feathers weighing down the bird) was calculated to be only 0.12-0.15 Newtons. This force increases the parasite power (the power required to overcome the force of drag) which is only 0.1% of the total power required for flight. This shows how insignificant the slight increase in the power needed to take off, is, in birds with the plumage. 

Despite these studies, it cannot be confirmed that the sexual advantage that peacocks have with their plumages is not a disadvantage. In respect to flight and escaping predators, there is now evidence to support the claim that heavy plumages are not a drag, however, in other respects these ornaments may be a disadvantage to these birds.

Citation:

Askew, Graham N. 2014. The elaborate plumage in peacocks is not such a drag. The Journal of Experimental Biology 217: 3237-3241