Having lots of children isn’t all it’s cracked up to be
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Southern pied babbler (Turdoides bicolor) at Marakele National Park, South Africa. (Credit: Derek … [+]
An international team of scientists made a strange discovery recently. They found that as female southern pied babblers age, they produce more offspring — but they become stupider in the process.
Southern pied babblers, Turdoides bicolor, are medium-sized black-and-white songbirds that reside in arid and semi-arid regions of Africa’s savannah and Kalahari desert (more here). They live in groups and are cooperative breeders, each group consisting of a dominant reproductive pair and variable numbers of subordinate helpers. The dominant pair produces approximately 95% of the offspring in the group and, on average, only 4% of subordinate helpers originate from non-natal groups each year.
The study population in the Kalahari desert has been intensively monitored since 2003 and is habituated to the presence of people, so researchers can test the wild birds with a variety of cognitive tasks. In this study, the scientists captured, tested and released multiple generations of pied babblers to assess their cognitive abilities as the birds mature. During the study (2018–2021), the population comprised 14 groups of birds with a group size of 2–7 adults. Over time, the scientists tested different individuals: 13 individuals from six groups in 2018, 18 from 10 groups in 2019 and seven from four groups in 2021.
They compared the birds’ test scores to the number of chicks produced by each of the females who participated in the study.
Cognition is the process of acquiring knowledge and understanding through thought, experience, and the senses. The cognitive tests used in this study consisted of three tasks designed to quantify associative learning, reversal learning, and inhibitory control (Figure 1). These cognitive tasks were designed to take advantage of the natural terrestrial foraging behaviors of babblers, requiring each test bird to peck downwards at a lid or move around a barrier on the ground to access a food reward (a mealworm).
F I G U R E 1 : (a) Wild pied babbler interacting with the cognitive task used to quantify … [+]
Perhaps not surprisingly, the researchers found that individuals that did well on one cognitive task generally performed well on all of them. This general cognitive performance varied considerably across all birds of both sexes and peaked early, but declined in females as they matured. The researchers also found that as the birds’ cognitive abilities declined, they became more fertile.
“Analyzing over 10 years of breeding data, we found that individuals with lower general cognitive performance produced more fledglings per year”, lead author, evolutionary biologist Camilla Soravia, a PhD student at the University of Western Australia, writes in the newly published paper (ref). Ms Soravia’s doctoral research investigates how rising temperatures due to climate change impact animals’ ability to learn from their environment and adjust to new conditions.
“Collectively, our findings support the existence of a trade-off between cognitive performance and reproductive success in a wild bird”, Ms Soravia and her colleagues surmise in their study (ref).
Cognitive abilities are energetically demanding. To conserve energy, the birds are apparently reducing the energetic investment into the brain, thereby increasing the amount of energy available for producing eggs and raising chicks. But females also invest energy into competing with each other for breeding rights — they fight each other for dominance, and clutches of eggs can get destroyed in the process and must be replaced. All of this also consumes a lot of energy too.
It’s interesting to note that the females’ fertility increases as they age, so they end up with yet more chicks to raise in each clutch. Further, as females grow older, they may redirect more of their energy into reproducing rather than into maintaining their nervous system and brain — as previously reported from butterflies (ref).
Male babblers have no such energetic demands, so they do not suffer cognitive declines. Curiously, living in larger groups (with more helpers) does not alleviate the females’ cognitive decline, as one might suppose. But cognitive declines were linked with reproduction: females with less cognitive decline also tended to produce fewer chicks.
But this raises the counterintuitive question: are females with lower cognitive scores simply outliving smarter females? The study was unable to answer this question — which of course is a scenario worth investigating further.
“In order to understand how selection acts on cognition we need to consider not only its benefits but also its costs”, Ms Soravia and colleagues point out.
They already are making plans for the next series of experiments to better understand the details of the energetic compromises that pied babblers are making.
“Future studies should consider expanding the test battery by including, for example, tasks assessing social cognition, the ability to make inferences and reaction time.”
Source:
Camilla Soravia, Benjamin J. Ashton, Alex Thornton and Amanda R. Ridley (2022). General cognitive performance declines with female age and is negatively related to fledging success in a wild bird, Proceedings of the Royal Society B: Biological Sciences | doi:10.1098/rspb.2022.1748
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