Illegitimate offspring are a better investment for female house wrens

Birds have a reputation for monogamy, but it is often only an appearance. In fact, males and females alike engage in extra-pair copulations in many cases. A recent study set out to find out what’s in it for unfaithful female house wrens.

The benefit of cheating is easy to see for males: their illegitimate offspring gets reared at no expense by another pair in another nest. The rewards aren’t so obvious for the other sex. Females always provide care, and a cuckolded father might even invest less energy in the chicks; not to mention the risk of contracting STDs that comes with mating. Despite this inequality, female birds still cheat on their social mates.

House wrens are small, short-lived, passerine birds that are common throughout North America. Each year, pairs form for the breeding season, the female picking the males she desires. Males and females look alike, so she cannot assess a potential mate’s quality through showy displays or colorful attributes.

The oldest males get cuckolded the most, found the researchers. This is surprising: in the absence of other information, choosy females should consider older males better quality. This is because, well, they’re survived so far. Consistent with this, younger males also get cheated on more than intermediate aged ones.

Older birds produce lesser quality sperm. They make less of it, and it can be less motile and more mutation ridden. The clutches they produce contain fewer chicks with weaker immune systems. So why pair up with old males at all?

It turns out that female house wrens may not be as picky as we think they are. House wrens nest in cavities, and a good territory might be the most important quality for a male (the ones that have one rarely get cheated on regardless of age). However, because she won’t encounter very many mates before the breeding season, a female might just settle for the first one with a decent nesting location and start making chicks. She would then mate with the other males she encounters.

This settling and cheating strategy is a great one. The researchers found that chicks born of female extra pair mating were better quality! They were the most likely to return breeding, making more chicks themselves. Mating is a complex game, but apparently cheating pays off for female birds too.


Source: Bowers et al. 2015. Increased extra-pair paternity in broods of aging males and enhanced recruitment of extra-pair young in a migratory bird. Evolution.

Photo: Wikipedia commons


What is Natural Selection?

To start at the beginning and lay the foundation for future posts, I am discussing a term familiar to many people, but important to grasp in all of its nuance (I am assuming a very basic understanding of evolution here). So then, what is Natural Selection?

Natural selection is a way in which evolution can happen, with evolution being the change in life forms (or organisms) over time. Darwin wasn’t the first person to suspect that the life that surrounds us (and ourselves) had changed in history, but is contribution was to suggest a mechanism through which it happens: Natural Selection.

What enabled Darwin to figure out how evolution can work was a change in perspective. Instead of thinking about individual organisms, like his predecessors, he envisioned change at the level of groups of organisms. To clarify, as opposed to looking at change during the life time of one organism, he looked at the change in the composition of groups of such organisms (populations) over successive generations. This “simple” shift in thinking led him to one of the greatest and simplest ideas in biology.

Resources are finite on our planet. There is only so much sunlight for plants to share, so much grass to eat, so many prey to consume… Darwin realised this meant that whichever organisms were better at obtaining said resources would survive longer and reproduce more. Those individuals would then pass on the traits that enabled them to do so to their offspring, and over time the composition of the population would shift towards organisms possessing those traits.

Note that this excludes changes that occur during the organism’s lifetime. Evolution by Natural Selection is therefore simply a shift in trait frequency in a population over time. It’s important to note that Natural Selection does not really exist in a sense, it is not a force that acts on species to shape them. To me, this is at once confusing and wonderful: rather, Natural Selection is a property that emerges from each individual’s “struggle for existence”, each individual’s fight to survive and reproduce.

Over time, this “culling” of bad traits and “selection” of beneficial traits will lead to groups of organisms being more and more in tune with their environments, better at exploiting it. This is adaptation by Natural Selection.

Natural Selection implies three properties about the population we are looking at:

That there is variation in the population. Without variation, there can be no change in the frequency of traits.

That this variation affects survival and/ or reproduction.

That this variation is heritable. If the character is acquired throughout an organism’s lifetime but not passed on to future generations, then if there is change in the trait’s frequency it will have nothing to do with Natural Selection.

The simplicity yet complexity of the result of this process is amazing. However, we must delve in much deeper to understand the complexity of life that surrounds us. This process underpins many things that happen in evolution, but things get tricky. I’ll get to some of that in future discussions!

Bye for now,