Finding Love in the Animal World: Multiple Cues and Sexual Selection

Dating in the modern age can seem rather convoluted. Refer to any guide or book on the subject and you’re like to come across massive laundry lists detailing everything you must be mindful if you wish to snag the guy or girl of your dreams. Or at least, to make sure your outing isn’t a complete disaster. You must look good, smell nice, be confident, be a good listener, mind your posture, and as a beloved Disney villain once put it, never underestimate the importance of body language.

However, we are not alone. Other animals that seek to reproduce have interest in ensuring that they are as successful as possible. Choosing the best mate fundamentally requires some way of distinguishing good suitors from poor ones. Although the dynamics of mate choice in humans is considerably more complicated, across the animal kingdom it is typically the females that make these decisions. This is because reproduction is often more costly for females than it is for males in terms of producing gametes (eggs), birthing, and rearing young. Thus, females require as much information as possible about their potential mates to make good decisions. Males therefore communicate with females by producing a variety signals that serve as the basis for female mate choice decisions. This provides the foundation for preferential female selection for these male traits, which over time has resulted in a stunning array of adaptations and behaviors in males that in some cases serve no other purpose than to attract mates (Andersson, 1994).

But sometimes one signal isn’t enough. Different kinds of signals can communicate different kinds of information based on context. Carotenoids, a colorful pigment that is acquired in the diets of many birds, are a classic example of this principle in action. The length or complexity of a male bird’s song might indicate general healthiness or vigor to a potential mate. But bright, showy plumage may provide additional information on an individual’s ability to acquire nutritional resources, or the quality of territory he defends and forages on. Male house finches vary in coloration from yellow to red based on the amount of carotenoids in their diet. Although females do prefer that males have longer songs than shorter ones (Nolan & Hill, 2004), females will frequently choose to ultimately pair with the most colorful male regardless of other characteristics (Hill, 1990).

One question that this might raise is, if color seems the most important for females, why bother singing at all? The answer may be the medium your signal travels through. Auditory signals like song can be heard across long distances, while visual signals such as plumage and mating displays are relatively short-distance singals. A study on tree frogs suggested that this might be the case for some species. Tree frogs chorus at night to attract mates, and females show strong preferences for males that sing fast and at high frequency. However, when dummy frogs were presented to females along with recordings of a male chorus, females preferred males that had a prominent stripe on the side of their body (Taylor, et al., 2007). Thus, sexual selection in some species might be a two-step process. Traveling to meet a potential mate is costly, so females decide whether or not to make the trip depending on the quality of long-distance song. But anyone who has used a dating app would probably tell you that sometimes a once-promising suitor isn’t quite what you expected when you finally see them up close.

This issue of sending and receiving signals through the environment raises some potential concerns for conservation biologists. As urbanization, habitat fragmentation and human-induced climate change continue to alter and threaten habitats, so in turn does this fundamentally alter the physical mediums that signals are transmitted through. For example, excessive road noise may make it difficult for females to locate potential mates. Likewise, changes in the availability of nutrients like carotenoids as a result of urban development may also indirectly change male plumage coloration in some birds, thereby altering sexual selection dynamics. A great deal of research has been done on this kind of multi-modal communication in invertebrate models such as wolf spiders, jumping spiders and crickets. However, a great deal of resolution in how multiple cues interact in mate choice decisions, and how these sexual selection dynamics might potentially be affected by human disturbance, is still lacking in vertebrate systems.

 

Resources:

Andersson, M. B. (1994). Sexual selection. Princeton University Press.

Hill, G. E. (1990). Female house finches prefer colourful males: sexual selection for a condition-dependent trait. Animal Behaviour, 40(3), 563-572.

Nolan, P. M., & Hill, G. E. (2004). Female choice for song characteristics in the house finch. Animal Behaviour, 67(3), 403-410.

Taylor, R. C., Buchanan, B. W., & Doherty, J. L. (2007). Sexual selection in the squirrel treefrog Hyla squirella: the role of multimodal cue assessment in female choice. Animal Behaviour, 74(6), 1753-1763.

 

Images (In order of appearance):

Screen-Capture: Ursula, The Little Mermaid (1989), Disney.

Illustration: Male and Female House Finch, Diane Pierce, National Geographic.

Photo: Hardin Waddle, USGS.

Leave a Reply

Your email address will not be published.