Zebra Finches More Prone to Divorce When Living in Fluid Social Environments
Animals…divorce? While at first thought it might seem like a facet of only human relationships (or at least how they end), divorce is not unheard of in other animals. Take Zebra finches, Taeniopygia castanotis, for example. Like some 80% of birds, they form pair bonds, or sustained cooperative relationships between two individuals engaged in mutual care of dependent offspring. Understandably, when a pair is well matched, it brings about fitness benefits for both partners. In practical terms this means each partner can take turns caring for the offspring and thus be able to devote less time and personal effort than they would as sole parents. All the while raising a genetically fit individual to further their own bloodline. Poorly made matches, on the other extreme, bring costs. Divorce, or more accurately, partner switching, can act in an adaptive way to improve fitness.
Previous scientific work had established the fact that divorcing does indeed occur in captive Zebra finches, and that sex composition of the breeding group (more females or more males) played some role in determining it prevalence[1]. However, whether or not it occurred in wild Zebra finch populations (and what other factors might play a role) remained relatively unknown. Enter Kerianne Wilson and her fellow researchers. They recently published a study in Animal Behavior [2] offering some insight into this knowledge gap. In it, they predict that flocks with highly fluid social scenes (as are commonly found in wild Zebra Finch populations) would have higher divorce rates. And that makes sense, because, given a change in the pool of partner choices, it is totally conceivable that individuals might find a more preferable partner, drop the old one and mate with the new one: effectively getting divorced.
In the nature of all good science, Wilson needed to test the hypothesis. To do that, her team set up an experiment where they introduced 10 males and 10 females in a large aviary (12 groups of 10 females introduced to 10 males). The aviary presented conditions similar to those found in the Zebra Finch’s native Australian bush habitat. Each of the 12 replicates would act as a different flock: six replicate-flocks with highly fluid social environments, and six with low social fluidity. They gave all the individuals, across all flocks, a conservative 6 days to pair. 80% of individuals paired. The four individuals per flock that didn’t were removed. This left the total population per flock at four pairs each (Fig 1).
The researchers next had to create their social environments. For the high fluidity environment this meant that the team randomly shuffled half of the birds in each flock to another flock. Meanwhile they handled the remaining ½ and replaced them. Following this they introduced 2 handled unmated males and 2 handled unmated females: wild flocks often contain such individuals.
To create a less socially fluid environment, Wilson merely removed and handled all eight pairs per flock and then returned them to the same flock. While the researchers did add 2 unmated males and 2 unmated females as they did in the creation of the high social fluidity environment, overall, the less socially fluid environment remained just that.
Once all the pairs and lone individuals were in their new places the team watched what happened. They paid particular attention to pairing-indicative behaviors including clumping (resting in physical contact away from the nest), allopreening (one bird preening another) along with joint nest site searching and nest building. If these behaviors were observed among a pair consisting of one previously paired finch, a divorce had likely taken place.
As expected, Wilson found that pairs in highly fluid social environments were more prone to divorce. At least one divorce occurred in each highly fluid social environment. Interestingly though, physical features like beak color or cheek size (commonly used by females to choose mates) were found to not greatly influence divorce, i.e. the females were not just abandoning their former partner because they found another finch with a larger cheek.
Rather, Wilson speculates that while first pairings are usually determined by physical traits, potential second partners are evaluated on the basis of behavioral traits different from those the present mate has (and the other individual is struggling to cope with).
Applied practically, something as basic, but nonetheless vital, as choosing a new nesting site in the complex highly fluid social environment may have been the final straw for some finch’s already strained first pairings. Add in the sudden presence of bachelors, divorce might seem fashionable indeed. One way the finches may determine just who is available to become partner number two is by watching the signs of a failing couple (excess nest site searching etc).
Despite Wilson’s interesting findings, the extent to which divorce improves fitness is still not completely understood. Divorced pairs produced as many eggs as did never divorced pairings. However, merely finding a better suited partner to live with and help in raising offspring may be where the heart of the benefit lies; even if doing so incurs a short term cost while looking for them.
References:
- Hormones and Behavior 87, 155-163 (2017) doi:https://doi.org/10.1016/j.yhbeh.2016.11.004
- Animal Behavior 183, 51-60 (2021) doi:https://doi.org/10.1016/j.anbehav.2021.10.018
