Heritable variation in thermal profiles is associated with reproductive success in the world’s largest bird

Organisms inhabiting extreme thermal environments, such as desert birds, have evolved spectacular adaptations to thermoregulate during hot and cold conditions. However, our knowledge of selection for thermoregulation and the potential for evolutionary responses is limited, particularly for large organisms experiencing extreme temperature fluctuations. Here we use thermal imaging to quantify selection and genetic variation in thermoregulation in ostriches (Struthio camelus), the world’s largest bird species that is experiencing increasingly volatile temperatures. We found that females that are better at regulating their head temperatures (‘thermoregulatory capacity’) had higher egg-laying rates under hotter conditions. Thermoregulatory capacity was both heritable and showed signatures of local adaptation: females originating from more unpredictable climates were better at regulating their head temperatures in response to temperature fluctuations. Together these results reveal that past and present evolutionary processes have shaped genetic variation in thermoregulatory capacity, which appears to protect critical organs, such as the brain, from extreme temperatures during reproduction.

Methods

General information and field setting

Data on head and neck surface temperatures of ostriches. For several years, we took repeated thermal images of individually marked ostriches in a pedigreed population at a research farm in the Western Cape Province in South Africa. We also recorded ambient temperatures at the time of each picture, allowing us to relate surface temperatures to ambient air temperatures.

Each year, from 2012 to 2017 and during October to December, we took 2744 pictures of 423 females between 5 am and 6 pm (average number of images per female = 6.5). This dataset was designed to maximize the number of individuals with repeated measures across years, in order to characterize individual thermal profiles across different thermal environments. Consequently, there was little repeated sampling within days (8 % of the pictures).