Sex allocation and sex-specific offspring performance under variable food conditions in the color polymorphic tawny owl (Strix aluco)

Abstract

Previous research into variation in sex allocation strategies in birds has provided evidence for sex ratio adjustments by parents to maximize fitness, based on differences in the costs of producing males and females. Additionally, studies of melanin-based color polymorphism indicate that life history strategies, physiology, and behavior may vary among color variants of the same species. This study investigates the effect of parental color morph and timing of breeding on offspring sex ratios and sex-specific offspring condition in a Finnish population of the color polymorphic tawny owl (Strix aluco). Reproductive and physiological parameters were collected annually in Southern Finland during 2009-2019, and offspring sex was molecularly determined using blood samples. Data was fit to generalized and/or mixed-effects linear models to statistically test for effects of parental characteristics (brown or gray morph, condition) and timing of breeding (i.e., laying date or food availability) on offspring sex ratio and condition. I found that laying date negatively affected brood sex ratio, and this effect was dependent on parental color morph: late broods of brown mothers were more strongly male-biased than those of gray mothers. Regarding offspring condition, offspring of early broods and of brown mothers were heavier than those of late broods or gray mothers, respectively; daughters of brown mothers were the heaviest offspring overall. Mass at fledging correlates positively with probability of recruitment, and costly daughters appear to benefit most from being born to brown mothers under good food conditions (early broods). Under poor food conditions (late broods), brown mothers may maximize their reproductive success by producing more cheap sons, while gray mother face fewer challenges and favor daughters. These findings provide evidence of morph-specific sex ratio adjustment in tawny owls, and suggest that variation in reproductive strategies between morphs may be adaptive.

Popular Abstract

Sex Allocation in Strix aluco: Variation across Color Morphs

The tawny owl (Strix aluco) is a medium-sized woodland owl species in which individuals of either sex display two different plumage color types (morphs), brown and gray, in varying frequencies across their Eurasian geographic range. In tawny owls and numerous other animal species that display color morphs, these are determined by heritable genetic variation in the expression of a reddish-brown pigment called pheomelanin, which has also been associated with variation in other aspects of physiology and behavior, including immune response, aggression, and conversion of food to body mass.

Previous studies into this type of color variation, called melanin-based color polymorphism, in the tawny owl and other Strix species also suggest differences in life-history strategies between color morphs, including parental investment in offspring based on food availability. In this study, I aim to understand how color morph and food availability may affect both parental adjustment of offspring sex ratios in broods prior to egg-laying, and sex-specific condition of hatched offspring.

Data on tawny owl physiology and reproductive behaviour, including individual body condition, timing of breeding, and parent and offspring color morphs, were collected on an annual basis during 2009-2019 in southern Finland, along with seasonal estimates of food availability. Blood samples were also collected from all identified offspring each year, and offspring sex was determined using molecular analysis of DNA. Statistical analysis was used to test the effect of these variables on brood sex ratio and offspring condition.

Evidence for adaptive sex ratio adjustment
I found that, while there was no significant annual deviation from equal sex ratios (male : female) at the population level, brood sex ratios varied significantly with timing of breeding. Late breeding is associated with poor food conditions, which suggests that tawny owl parents adjust their brood sex ratios based on food availability. Additionally, this effect differed between parent color morphs, particularly females: late broods of brown mothers were more male-biased, while the late broods of gray mothers were more female-biased. In tawny owls, females are the larger sex, which means they require more resources and are costlier for parents to produce. Brown offspring have been shown to be larger than gray offspring, and I found that the difference in size between sexes is largest in brown offspring. Early-laid offspring were also heavier than late-laid offspring, which suggests that brown female offspring may benefit most from being laid early.

Overall, I show that sex ratio adjustment occurs in the tawny owl (Strix aluco) and is dependent on female color morph and food availability. There are inherent differences between the morphs, including their responses to food conditions, which may lead to variation in their reproductive strategies. I suggest that morph-specific sex ratio adjustment may be adaptive, because it allows individuals of each morph to allocate reproductive resources in a way that optimizes the condition and survival of their offspring, which ultimately increases individual reproductive success.

Master’s Degree Project in Biology 45 credits 2020
Department of Biology, Lund University

Advisors: Patrik Karell, Chiara Morosinotto
Evolutionary Ecology

• author: Tooth, Amandine
• supervisor: Patrik Karell; Chiara Morosinotto
• organization: Degree Projects in Biology
• course: BION02 20191
• year: 2020
• type: H2 - Master's Degree (Two Years)
• subject: Biology and Life Sciences
• language: English
• id: 9006994
• date added to LUP: 2020-03-23 11:46:48
• date last changed: 2020-03-23 11:46:48