Female finches can afford to be picky when selecting a mate

In many animal species, females apparently choose male mates based on the presence of adornments, such as large antlers or bright colors, or the use of ritualized displays that in some way indicate a male’s fitness. This is known as sexual selection and is thought to be responsible for the presence of extravagant and unusual male characteristics. 

But when the pool of possible mates is limited, females who are too choosy run the risk of ending up with no partner and no possibility of reproducing. The risks associated with female choosiness were the subject of a recent study by Wolfgang Forstmeier of the Max Planck Institute for Ornithology in Germany, and colleagues, who used populations of captive zebra finches (Taeniopygia guttata) to test their hypotheses.

Zebra finches are socially monogamous songbirds that have regionally distinct songs. Females prefer to select mates that have the same song dialect. 

The researchers capitalized on this situation and set up 10 aviaries, each containing 12 male and 12 female finches from the two different dialect lineages. In each captive breeding colony, one-third of the females (4 birds) could choose from two-thirds (8 birds) of the males that shared the same dialect. This was considered to be a situation of relaxed competition as there was an excess of males from which the 4 females could select a suitable mate. 

However, in each aviary the remaining two-thirds of females (8 birds) had only one-third of the males (4 birds) that shared their song dialect. This was thus a situation of high competition as these females had a shortage of preferred males from which to select a mate. They could either choose a male with a different song dialect or choose not to breed at all.

The researchers hypothesized that females that experienced high competition for mates would have less reproductive success (measured in terms of offspring that reached independence). They also theorized that these females would lay fewer eggs and start egg-laying later – because they would be busy competing for mates.

The results showed that 31 percent of females in the high-competition treatment chose to pair with a male of a different song dialect, while 26 percent refused to settle for this and remained unpaired throughout the experiment. The researchers termed these unpaired females “wallflowers.”

The surprise came, however, when it turned out that the “wallflowers” produced, on average, the same number of eggs and successful fledglings as the breeding pairs. It seems that there was no shortage of mating opportunities between unpaired females and paired males. 

The unpaired females then chose either to build a nest and rear the young on their own, or they sneaked their fertilized eggs into the nests of pair-bonded individuals and left the chick rearing to them, like a cuckoo does.

Some previous studies have suggested that there will be a cost to a female that is too fussy when selecting a male, and that this cost (not reproducing) would act against the evolution of sexual selection as a process. However, the current study found that females who refused to pair with a male of the wrong song dialect would suffer no cost at all in terms of offspring produced. 

In addition, unpaired females chose to mate with, and to deposit their eggs in the nests of males that had the same song dialect. With this reproductive strategy, these unpaired females were achieving the desired result even though they were not able to form pair-bonds with the preferred males.

The study is the first to quantify the fitness costs to females of being too picky. And indeed, the results indicate that, at least in zebra finches, these costs are non-existent, as long as the females are able to employ alternative reproductive strategies. This behavioral flexibility can facilitate the evolution of female choice and male sexual selection in monogamous species, the authors say.

“Our study asks how females cope with the situation that their mate preferences are difficult to satisfy,” said Forstmeier. “The answer is: more successfully than we had expected.”

The study is published today in the journal PLoS Biology.

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By Alison Bosman, Earth.com Staff Writer