How Vocalizations Evolve

Vocalizations, like any other phenotypic trait, can change over time.  These changes can have several causes which can be generally divided into drift (the random accumulation of mutations that are generally neutral in terms of fitness) and selection (changes that are under some directional pressure and tend to increase fitness).  The forms of drift and selection fall into five major categories.  In any given population, these categories can act independently of one another, they can act in concert, or they can oppose each other.  The five categories are Cultural Drift, Genetic Drift, Cultural Selection, Natural Selection, and Sexual Selection.

Cultural Drift is the process where changes in vocalizations occur by chance.  These changes can come from imitation errors as young individuals attempt to copy the sounds produced by adults.  They can also arise in the form of innovations where an adult incorporates a new sound element into its vocalization.  The accumulation of these changes can eventually lead to the formation of new vocal types.

Genetic Drift is the random accumulation of mutations at loci that regulate sound production.  Ass these mutations accumulate, the physiological and mechanical abilities of an organism to make sounds may be altered.  Due to this, genetic drift is likely to have a greater effect on the evolution of vocalizations when the mutations happen to effect the limits of performance for an animal.

Cultural Selection occurs when there is differential propagation of vocalizations across generations.  This can occur in the form of vertical transmission from parents to offspring, horizontal transmission between peer groups or siblings, or oblique transmission from adults to unrelated young.  Unlike the following two mechanisms, cultural selection is not directly driven be fitness.  Instead, variations in vocalizations can spread through a population for other reasons.  One example is because of a dominant individual using one particular variation and not others.  Another example is when a particular frequency transmits through a habitat better than others, such as how low frequency sounds travel through dense foliage farther tan high frequency sounds.  This would lead more young individuals to be exposed to low frequency sounds and so learn to imitate them.

Natural Selection can influence vocalizations directly, because of some fitness benefit that a particular vocalizations give the signaler, or indirectly, by altering some physical structure that is used is sound production (changing bill morphology adapting to different seed sizes, for example).  The most commonly discussed role of natural selection in vocal evolution is through the process known as reinforcement.  Reinforcement is where two populations have diverged to the point where hybrids between the populations are less fit than pure bred members of either population.  This might be because the two populations have split to use foods of two different sizes.  A hybrid might not be good and consuming either food size, and so be less fit.  If such hybrid disadvantage exists, natural selection is expected to favor individuals of each population that tend to avoid mating with individuals of the other population.  Vocalizations are frequently the first from of contact that two individuals have, and so they are in a unique position to moderate interactions and will tend to evolve towards greater species-level specificity.

Sexual Selection can take the form of intersexual selection or intrasexual selection.  Intersexual selection can drive the evolution of vocalizations by the preferences of one sex (usually the female) for particular vocalizations of the other sex (usually the male), by sensory bias where one sex (usually the male) uses a vocalization that the other sex (usually the female) is predisposed to respond to, when a display can only be produced by individuals of high fitness, when the production of a display carries some fitness cost such as increased risk of predation, or when a vocal display can inform the receiver as to their likely genetic compatibility with the sender.  Intrasexual selection on vocalizations come ins the form of members of same sex (usually males) using vocalizations to compete with one another.  Here, the evolution of vocalizations can occur when vocalizations contain information about the sender.  This information can be in the form of the senders size, strength, willingness to fit, social status, etc.  Facets of vocalizations that are often favored include increased vocal complexity, high amplitude, low frequency, and high calling rate.

These mechanism for the evolution of vocalizations are most thoroughly studied in bird songs.  However, bird calls may be susceptible to all of these types of evolution as well.  This would be particularly true of calls that are learned, as opposed to innate, for which more and more examples are being discovered.

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Ways to Maintain Honest Signals

Signals (auditory, visual, chemical, etc.) are the outcomes of co-evolutionary processes between signalers and receivers.  They involve the signaler attempting to influence the receiver in a particular way, and the receiver attempting to make the best decision possible based on the signal it receives.  The goals of the individuals involved in signaling are sometimes parallel, as is times of cooperation, but are quite often in some degree of conflict.  This conflict opens the possibility of cheating being advantageous to one or more of the individuals.  For example, a signal where the signaler and receiver are directly in conflict is between a predator and its prey.  Many prey species have specific signals that they give when they detect a predator.  This signal is given to warn the predator that it has been discovered, and that any chance of that predator ambushing the prey is now gone.  Predators receive these signals and go elsewhere in their search for food.  Given this situation, why doesn’t the prey give their predator-discovery signal all the time?  In other words, what stops the prey from cheating?  If they did, any predator who came close would receive the signal and then leave thinking it had been discovered.  How honesty is preserved has been studied using evolutionary game theory, and three types of signals have been discovered based on how cheating is prevented.

Performance Signals are signals that are inherent in the signaler. They can only be displayed by individuals that possess a specific capability or knowledge.  Body size limits how low a frequency a frog can call at, with larger frogs able to make lower frequency calls.  Therefore, the minimum frequency of a frog call is constrained by the size of the frog, and in this way a receiver can know something about the signaler.  These types of signals are impossible to fake, and so is no way for the signaler to cheat.

Handicap Signals are costly to make or use.  Performing a display that is highly energetic can be costly to a signaler because the energy they use to display is energy they could be using for feeding or other activities that would benefit their survival.  An example of this kind of display could be the flight displays of male Red-winged Blackbirds.  Possessing a display that attracts the attention of predators can carry a high predation cost.  An example could be the calls of Tungara Frogs that are at the same frequency as that which predatory bats use in echolocation.  Forming a display that requires a specific nutrient that is difficult to acquire can carry a high cost because of the time and energy used in location the nutrient cannot be used for other activities.  All these signals can only be used if the signaler can tolerate the handicap of the increased cost.  Signalers that are unable to tolerate the cost are unlikely to survive if they use the signal and so cheating is prevented.

Conventional Signals are signals that have no specific cost, but that have been chosen arbitrarily to convey a given meaning.  An example of this type of signal is the head posture of many sparrow species.  By thrusting the head forward at an opponent, a sparrow indicates that it is willing to fight for a resource.  If that individual’s opponent is also willing to fight both sparrows could be injured.  Cheating is prevented because an individual does not know the condition of their opponent.  If the signaler indicates that it is will to fight, but is cheating and not actually willing or able to fight, they will trigger an unknown response from their opponent.  If their opponent is willing to fight the cheater will incur the costs of the fight with no chance of getting the benefits of the resource.  Cheating with conventional signals is also prevented when individual interact repeatedly.  In this way, a cheater will be discovered over time, and their signals will come to be ignored.

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Raptorthon!

It is time for Raptorthon!  This is an event, like a bird-a-thon, where hawkwatchers go out into the field to count as many species of birds of prey (and other bird species) that they can in a 24 hour period.  Raptorthon is an event organized by HMANA, the Hawk Migration Association of North America, and also like a bird-a-thon this event is designed to gather support for the continuing monitoring and preservation of the raptors of North America.  This year there is a featured hawkwatcher.  Laurie Goodrich, Ph.D., is the Senior Monitoring Biologist for Hawk Mountain Sanctuary and co-chair of HMANA.  The funds that she gathers from sponsors like you will be split between Hawk Mountain and HMANA.  These funds will allow both organizations to continue monitoring the population levels of all of North America’s birds of prey.  HMANA is the largest network of raptor migration monitoring site on the continent and Hawk Mountain was the first site to be set aside as a sanctuary for raptors.  So, Raptorthon is great way to make a wonderful donation to the raptors of North America and to support two fantastic organizations at once!  Donations can be made in the form of a flat sum, on a per raptor species basis, or on a per bird species basis.  Please consider how much enjoyment you get from the raptors you see be that seeing Swainson’s Hawks fill the central valley of California each summer or watching Golden Eagles stream through Pennsylvania each spring or standing beneath a sky filled with Brad-winged Hawks in Veracruz or seeing your neighborhood Cooper’s Hawk hunting starlings, and think of how much poorer the world would be without them and make a donation to the cause by following the link below.

http://www.hmana.org/raptorthon/

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Mechanisms of Mate Choice Evolution

Mate choice, or intersexual competition, is the non-random selection of sexual partners of one sex by the other sex.  In most vertebrates, this is expressed as females choosing which males to mate with.  Many mechanisms have been proposed for how and why mate choice might evolve.  These mechanisms can generally divided into five categories, which are direct phenotypic effects, sensory bias, Fisherian, indicator mechanisms, and genetic compatibility mechanisms.  Some of these mechanisms give direct fitness benefits to the chooser (direct phenotypic effects and sensory bias), others give indirect benefits to the chooser by benefiting their young (Fisherian and indicator mechanisms), and others may give other genetic benefits (genetic compatibility mechanisms).  These mechanisms are not mutually exclusive.  In fact, it is likely that several operate simultaneously in any give system.  Each mechanism is described in more detail below.

Direct Phenotypic Effects are when females display a preference for a male ornament that gives the female some benefit.  This benefit can be in the form of the male holding a superior territory, providing more or better food, increased protection from predators, or higher contribution to parental care of offspring.  All these benefits are good for the female herself, directly.  As such, signals that males can display that indicate that the male will provide these benefits can be preferred and selected for.

Sensory Bias is when mate choices are made on the basis of a preference that evolved for some other reason.  For example, many animals have been shown to prefer specific colors.  These colors are often associated with common or preferred food types.  If a male develops the same color on his skin, fur, feathers, etc. females may tend to prefer him over other males that have not developed that color display.  Biases could exist for other reasons (associated with habitat selection, predator avoidance, etc.), and also using other senses (scent, sound, etc.).  The point is that the females sensory system already existed and was set up with a bias for certain displays that the males happened on and it then proved successful.

Fisherian (or Fisharian Runaway or Sexy Sons) is a process by which there is a genetic link between the display of a male and the preference for that display in the female.  Once this link occurs (and it could occur through selection or genetic drift), females will be more likely to choose males that have that trait.  They will then have sons that posses that trait and daughters that prefer it.  In this way, the mate choice process becomes self reinforcing.  This positive feedback between preferences and traits can drive the male trait to become quite extreme.  The tail of a male Peacock is thought to be an example of Fisherian selection.  This explanation of mate choice evolution was first developed by R. A. Fisher, a very famous evolutionary biologist, hence the name.

Indicator Mechanisms (or Good Genes or the Handicap Principle) is when a display trait in one sex has a selective disadvantage to the individual who has that trait.  In this way, having that trait is a cost, and males who possesses such a trait are indicating that they are able to survive despite the added cost of the display trait.  In other words, that they can succeed even with a handicap.  These traits are also condition-dependent, meaning that the condition of the display trait is dependent on the condition of male himself.  Females use these traits to determine which males have broadly high genetic quality.  An important aspect of this mechanism of  mate choice evolution is that it can favor males with a variety of genes because different males will be able to succeed in different way (some will be fast, some will strong, etc.).  This idea of maintaining genetic variation is sometimes referred to as the genetic capture hypothesis.

Genetic Compatibility Mechanisms are ways in which one individual may choose a mate based on how the genes of that mate will interact with their own.  For example, an individuals’ immune systems has been shown to mount a better defense against infections when the individual has a large amount of genetic diversity.  This diversity generally comes in the form of being heterozygotic (having two different versions, or alleles, of a gene on each chromosome) at many loci.  Females have been shown to prefer to mate with males who have fewer similar immune systems genes.  Since the female and male immune system genes are so dissimilar they will tend to compliment each other, and this will increase the heterozygosity of their offspring.

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ABirdingNaturalist Turns 1

One year ago, today, I published my first entry on this blog on the Swainson’s Hawks that had recently returned from their wintering grounds in South America.  Well the Swainson’s Hawks are again filling the central valley as another annual cycle is completed, and this time around the sun I have been doing my bit to chronicle it.  Over the course of the last year, I have published 70 posts and this blog has received 1886 views from people in 36 countries around the world.  For me, it has been an interesting time, and I have had a lot of fun thinking and writing for this blog.   I think that writing for this blog has helped me to clarify ideas and make them more clear for myself, which was one of the reasons I started it.  So for me, this blogging experience has definitely been a success, and I hope that others have gained something by it as well.

So thanks to all the people who have read a post or decided to follow my blog.  It will exciting to see what the next year brings!

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Tinbergen’s Four Questions

Tinbergen’s four questions are categories that when taken together are used as a scheme for thinking about animal behavior.  They were developed by Nikolaas Tinbergen in the early 1960s, who adapted them from the work of Konrad Lorenz.  They represented areas of research that should be conducted on any aspect of behavior, and have become a fundamental part of the science of animal behavior.  The four questions are function, mechanism, ontogeny, and phylogeny.  These can be divided into two broad categories.  Ultimate Questions (function and phylogeny) are the larger picture, evolutionary time scale, ‘why’ questions.  Proximate Questions (ontogeny and mechanism) are the more immediate, individual, ‘how’ questions.

Function, or Adaptation, Questions are those that ask why a species has particular structures.  These questions set out to explain a current form (like why does a bird have a particular song)

Phylogeny Questions are those that ask why a structure evolved.  These questions set out to explain why a form has occurred as set in its historical context (a particular bird songs is especially efficient at being heard in a particular habitat).

Ontogeny Questions are those that ask how a structure develops.  These questions set out to explain how a structure forms as an individual grows (how does a birds’ song change throughout its life).

Mechanism, or Causation, Questions are those that ask how a structure is actually made and used.  These questions set out to explain the fundamental processes that create a structure (what neurons and hormones are used in a birds’ brain to actually trigger a song to be produced).

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