by Andrew Anderson (@AndersonEvolve)
Imagine you’re on a dating site looking for a potential partner. You browse through a couple of individuals and find a few that merit a further look. This particular site allows feedback from individuals who have contacted or dated a person to be viewed on that person’s page. Would you read the comments? Would you weigh the comments in your decision to engage in further conversations/dates with the person? If so, you have employed a mate choice strategy called mate copying.
Clunky hypotheticals aside (AOL Instant Messenger was the social media of choice when I last dated), mate copying has been observed in mammals (yes, possibly humans), birds, fishes, and even insects. Most often, these are confirmed by testing if a female’s interest in a male is altered if she observes him with another female. Personally, I would love to see if this occurs in role-reversed systems, but the research that have studied this pattern generally find females to be the choosier sex. So why would a female rely on another female’s choice? There are some hypotheses that have been proposed, such as: searching for a mate is costly (i.e. lose energy/time or become a target to predators inspecting each potential mate), a female may not have enough experience to determine male quality, or distinguishing between quality males is challenging. Females do not have to directly observe males being successful with other females; they can use other, more subtle signals to indicate the desirableness of a male. In rats, there is some evidence that the smell of a male who has recently copulated is a potential driver of female choice.
Mate copying is something that occurs across taxa; but, in my opinion, fishes have the most interesting behaviors associated with it. As a reminder, the dads are more likely to take care of the young in fishes that engage in brood care. Even though dads care for the young,males are still more likely to engage in competition for mates rather than have females compete for them (although I study a few awesome exceptions). One possible reason for this is that some species of males can tend nests larger than one female can fill with eggs. Some males will have eggs from many females and others have no eggs to care for (unequal mating success is an impetus for sexual selection). As you might be piecing together, females can use the amount of eggs already present in a male’s nest as an indicator of how “sexy” other females have found him. In fact, in some species of fishes females prefer males who have eggs already in the nest. This has been tested in several species by adding or removing eggs from males’ nests and observing the resulting female choice.
Now the evolutionary mayhem begins.
In three-spined sticklebacks, the males engage in a hurly-burly of activity centered around mating. Before deciding to mate with a male the female will inspect the male’s nest, his bright colors, his swimming behaviors, and if his nest has eggs in it. If she is satisfied he has met her criteria, she will lay eggs in his nest. While she’s doing so, other males will try to “sneak” a mating in by releasing sperm next to her. Such sneak behavior is fairly common among fishes, but some males will also steal eggs from the nest and bring them back to their own. These eggs are not their own and therefore that male has no paternity, but he will care for them and raise them as if he did. Since females use the presence of eggs in a nest to judge a potential mate’s quality, such behavior may end up actually increasing the total number of offspring they father.
Other species,such as the river bullhead, don’t even bother stealing eggs. Males nest in close proximity to each other, and females choose which nest to lay eggs in –again with consideration for the presence/absence of eggs. Instead of stealing a few eggs, males who haven’t mated will attempt to evict egged males from their nest and take over the entire clutch. The expectation, again, is that males who engage in that behavior might do better in overall reproduction than those that don’t, even though some of the eggs they invest in aren’t their own.
There is another example that is rather bizarre. Darters are small fishes found in creeks that sometimes engage in egg-raiding. One species, the striped darter, has evolved a unique coloration on its fins. This coloration creates a design that could be considered a facsimile of eggs. During courtship, the male will display these markings, and there is a correlation between mating success and number of egg-spots on their fins. The hypothesis is that these “egg spots” stimulate the female the same way that seeing eggs might, making her more likely to mate.
As you can see, fish have a wide diversity of adaptations to one stimulus: a preference for eggs in nest. It’s worth pointing out that the explanations of what’s been observed have varying degrees of confirmation through experimentation. Here I have presented three species as examples; indeed, there are more species that have these behaviors and traits which lends credence to the explanations given here. That is what’s so awesome about evolutionary biology: when something exists in nature that grabs your attention, you get to work to try to piece together what might have led to those traits.