The better, the choosier: A meta-analysis on interindividual variation of male mate choice (2022) Pollo et al. , Ecology Letters, https://doi.org/10.1111/ele.13981

Image credit: barloventomagico, CC BY-NC-ND 2.0

The Crux

Choosing who to reproduce with (mate choice – see Did You Know?) is a major player when it comes to the evolution of a species, yet it can be tough to know when individuals (and which individuals) should be choosy in their partners. A general trend is that when there are a plethora of potential mates available, too many for a given animal to mate with, they must make decisions on who to mate with. For many species, females tend to be the choosy sex, given the limited number of reproductive resources that are available to them (i.e., eggs) and how many males are usually available to mate.

Despite this commonality of female mate choice, male mate choice is also widespread in the animal kingdom. It is therefore important to know how different factors affect how a male chooses his mates. One factor that may play a key role is male quality, or the ability of a male to acquire mates. It could be that males that vary in their quality also vary in how picky they are. Today’s authors used a meta-analysis, or a “study of studies”, to understand how males make their decisions.

Did You Know: Mate Choice

Mate choice is when an animal is choosy about who they decide to mate with. The majority of systems tend to involve choosy females, but there are a number of systems in which the male is the choosy sex. Who a mate chooses can revolve around a number of different factors, but they all tend to relate to the “quality” of the potential mate. The featured image for today’s paper, the wattled jacana (Jacana jacana) is actually a species where females compete to mate with a male! Males build the nests, and then females fight one another for access to the males.

What They Did

The authors searched multiple online databases for studies investigating the influence of at least one male trait and one female trait on the mating decision of the male. The mating decisions included actions taken before mating (i.e., courtship intensity), during mating (i.e., duration of mating), and after mating (i.e., length of time guarding the mate to ensure no other suitors arrive). A key point is that the authors only used studies that conducted controlled lab or field experiments, and discarded any study utilizing field observations. They did this because such an uncontrolled setting may actually relate to other factors beyond male mate choice, like predator presence or food availability.

Then, the authors calculated effect sizes. For this study, they used the standardized mean difference (SMD), which in this case allowed them to compare male mate choice across studies to one another. Positive SMD values represent greater investment in high-quality females, while negative values represent greater investment in low-quality females. In addition, the authors related these effect sizes to male quality, to then test for a relationship between male quality and male mate choice.

What They Found

On average, males invest more in high-quality females than low-quality females, and this effect was relatively large. In addition, males that were of high and medium quality were choosier in who they picked than the low-quality males. An additional analyses conducted by the authors compared the choosiness of larger males in greater body condition to smaller males with lesser body condition, and they found that large, high-condition males were choosier than their smaller, lower-condition counterparts.

Problems?

Two points are worth addressing here. First, meta-analyses tend to check for publication bias, or the tendency for a certain “type” of result to get published. In this case, the authors checked if studies tended to be published were the males WERE choosier, as opposed to a more random set of results. While they didn’t find evidence for a publication bias, they did find that older studies tended to publish more “positive” results where males preferred high-quality females.

The second caveat is that the models the authors used didn’t explain much of the variance in the data. What this means is that while they did show that high-quality males tend to have a stronger preference for high-quality females than low-quality males, they didn’t have a good grasp of why this occurred.

So What?

I love a meta-analysis, and it is even more interesting to me when such a study analyzes a question that hasn’t been analyzed yet. Today’s paper showed that while high-quality males are choosier than low-quality males, such low-quality males are still exercising a bit of choice when it comes to who they mate with. Further, the high amount of variance in the data showed that while male-quality is an important predictor of male mate choice, this behavior is a complex and nuanced process that requires further study to better understand it.

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Dr. Adam Hasik is an evolutionary ecologist interested in the ecological and evolutionary dynamics of host-parasite interactions who tends to be choosy when it comes to coffee and pizza. You can read more about his research and his work for Ecology for the Masses here, see his personal website here, or follow him on Twitter here.