Smart is the new sexy

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by Nicole Dib (CLAS ’20, Biology), May 2019

During the Spring 2019 semester, I lead a lab meeting discussion regarding a recent paper published on the relationship between chickadees spatial cognition, mating, and reproductive investment: Branch et al. (2019) found that for food-caching Mountain Chickadees, females mated to males with better spatial cognition had greater reproductive success. The study provides important information regarding possible causation of mating habits of chickadees, which can be beneficial to our Black-capped Chickadee and Carolina Chickadee cognition and reproduction studies here in the Curry Lab. The study specifically gives insight regarding how the spatial cognitive abilities of chickadees impact their reproductive success. This information will supplement the research being conducted in the Curry Lab on other factors such as social behavior, spatial memory, and plumage brightness and their relation to reproductive success. Together the research of Branch et al. and ongoing work in the Curry Lab can lead to greater conclusions being made regarding factors influencing chickadee mating choice and reproductive success.

Chickadees are known for their exceptional spatial cognitive abilities, which are instrumental in their food caching throughout the winter. Those with better spatial cognition are more likely to survive the winter, and as a result more likely to reproduce in the breeding season (Sonnenberg et al. 2019). Female chickadees, the ‘choosier’ sex, may choose to mate with higher quality males—in this case those with better spatial cognition—and as a result invest more in reproduction. Branch et al. hypothesized that food-caching females increase their reproductive investment when mated to males with better spatial cognitive abilities. Branch et al. conducted the study on wild food-caching Mountain Chickadees, Poecile gambeli, over 2 years in the Saeghan Experimental Forest in California. The first of the years had favorable winter conditions with average precipitation and snow levels and the highest recorded overall reproductive output in chickadees at the site, while the second year had incredibly harsh winter conditions with high snow levels and low overall reproductive output in chickadees. Only results from the first year were statistically significant.

The new study consisted of two areas of measurement—spatial cognition and breeding—that Branch et al. recorded for chickadees in low and high elevation sites. Branch and her colleagues conducted spatial cognition tests using radio frequency identification (RFID)-enabled feeders that respond to the birds’ passive integrated transponder (PIT) tags. (We have used the same RFID approach in the Curry Lab for work on chickadee social networks over the last several years.) Branch et al. tested the birds on their spatial learning and memory with a system that assigned each bird to a specific feeder, which with the use of the RFID would only open its door and supply food to the bird with the specific programmed PIT tag. After investigators assigned each bird to a specific feeder, the RFID systems counted the number of visits to a non-rewarding feeder prior to visiting the assigned, rewarding feeder during a trial, with a trial beginning once the bird visited any feeder and ending once visiting the assigned feeder. The researchers estimated the mean number of location errors per trial during the first 20 trials of the spatial task, which represents how accurately the birds learned and remembered the task. Additionally, Branch et al. monitored the breeding of the birds at 350 nest boxes across both elevations, and recorded with four reproduction parameters: first egg date, clutch size, brood size at nesting age 16 days, and mean nestling mass at age 16 days.

The relationship between breeding variables and spatial cognitive performance was assessed using general linear models (GLMs) on both years of data including testing and breeding at both elevations. In each GLM, one of the four breeding parameters was the dependent variable while spatial learning and memory performance and elevation were two independent variables. The results between the two years varied greatly, as a result of large differences in reproductive output due to the large discrepancy in winter conditions. The results show in the 2016 breeding season, better spatial learning in males was associated with larger clutch size and significantly larger brood size, which stayed significant when accounting for total number of trials completed during the spatial task and age (juvenile vs adult). There were no significant associations between male spatial learning and other reproductive parameters. Additionally, in 2016, better spatial learning in females was associated with a higher mean nestling mass, however there were no associations with any of the other reproductive parameters. However, the higher mean mass results became statistically insignificant when accounting for total number of trials completed during the spatial task and age (juvenile vs adult). There are no significant results for associations between spatial learning of males nor females and reproduction parameters in 2017.

The significant findings of the study were that females mated to males with better spatial learning laid larger clutches and fledge larger broods and females with better spatial learning fledged young at a higher body mass. These results were only found statistically significant in one of the two years of the study, which renders them slightly inconclusive. Another interesting finding of the study is that there was no correlation between spatial learning and start date of breeding. There is a usual correlation between an earlier laying date and a bigger clutch size; however, there is no data in this study to support that claim. Overall, the results provided support for the original hypothesis that females may increase their reproductive investment when mated to males with better spatial cognitive abilities.

Despite these results, it is still unclear if females are truly able to determine the spatial cognition abilities of males and discriminate against them based on this characteristic. It is difficult to prove that females knowingly mate with males which have better spatial cognitive abilities, or if this is simply a side effect of females choosing mates based on other traits.

Overall, the study is incredibly enlightening and very relevant to our own work here in the Curry Lab. Within our own lab, we are studying other behavioral, physical, and genetic components of specifically Black Capped Chickadees, Carolina Chickadees, and their hybrids. Specific components that our lab members are studying include boldness and exploratory behavior by Taylor Heuermann (CLAS ’19, MS ’20); social organization and spatial memory by Michael Rowley (MS ‘20); problem-solving by Breanna Bennett (MS ‘20); and plumage brightness by Caraline Gammons (CLAS ‘20). We’re studying these components in relation to mating habits and reproductive success of the chickadees. Additionally, basic research on social and reproductive ecology icontinues, including study of social network relationships, mate choice, parental behavior, and song. Branch et al. (2019) surpassed any previous knowledge of the correlation between spatial cognitive abilities and reproductive investment in chickadees, and it is very beneficial to know if there is a direct correlation between the “smartness” of both males and females and the later reproductive investment and success of the female. It is difficult to prove that the females are truly basing mate choice on the cognitive abilities of the male; however, the research done and conclusions of Branch et al. are certainly useful additions to the research we are doing regarding other aspects of chickadee mating habits.

Literature Cited
Branch, C. L., A. M. Pitera, D. Y. Kozlovsky, E. S. Bridge, and V. V. Pravosudov. 2019.
Smart is the new sexy: female Mountain Chickadees increase reproductive investment when mated to males with better spatial cognition. Ecology Letters 22:897–903.
Sonnenberg, B. R., C. L. Branch, A. M. Pitera, E. Bridge, and V. V. Pravosudov. 2019. Natural
selection and spatial cognition in wild food-caching mountain chickadees. Current Biology 29: 1–7.