Sexual selection for extreme physical performance in a polygynous bird is associated with exceptional sex differences in oxygen carrying capacity

In many animal species, males compete for access to fertile females. The resulting sexual selection leads to sex differences in morphology and behaviour, but may also have consequences for physiology. Pectoral sandpipers are an arctic breeding polygynous shorebird in which males perform elaborate displays around the clock and move over long distances to sample potential breeding sites. We examined the oxygen carrying capacity of pectoral sandpipers, measured as the volume percentage of red blood cells in blood (haematocrit, Hct). We found a remarkable sex difference in Hct levels, with males having much higher values (58.9 ± 3.8 SD) than females (49.8 ± 5.3 SD). While Hct values of male pectoral sandpipers are notable for being among the highest recorded in birds, the sex difference we report is unprecedented and more than double that of any previously described. We also show that Hct values declined after arrival to the breeding grounds in females, but not in males, suggesting that mal..., Study site and general procedures
We studied pectoral sandpipers at a ~2 km2 site at the northern tip of the Alaskan Arctic Coastal Plain near Utqiagvik, Alaska (71°18′ N, 156°44′ W) in the years 2004-2009, 2012, 2014, and 2018. We caught pectoral sandpipers during the breeding season using hand-held mist nets (males and females) or nest traps (females only). We assigned each bird a metal leg band and a unique combination of colour leg bands, weighed them (to the nearest 0.1 g), measured their tarsus (to the nearest 0.1 mm), and sampled 200–300 μl of blood using brachial venepuncture. We collected blood in 70 μl heparinized microhematocrit capillary tubes and centrifuged the samples at 5,000 rpm for 10 min. on the day of collection, thus separating plasma from cellular blood. Haematocrit levels were measured for each full capillary as the percentage of packed red blood cells over the total blood sample. For statistical analyses, we used the mean value from all capillary tubes obtained f..., , ### Description of data files

data\_PESA.csv (contains hematocrit values of individual pectoral sandpipers)

ID - unique identity for each individual
date\_ - date of sample collection (NA=no data)
year\_ - year of sample collection (NA=no data)
dayofyear\_ - day of year of sample collection (1 = January 1st, NA=no data)
hema - hematocrit value (%)
tarsus - tarsus length (in mm, NA=no data)
weight - body mass (in g, NA=no data)
cond - body condition (residuals of weight over tarsus, NA=no data)
sex - sex(1=male, 2=female)
lattitude - lattitude (degrees°, NA=no data)
longitude - longitude (degrees°, NA=no data)
breed - factor indicating whether an individual bred at the study site (0=no, 1=yes)
date\_cent - mean-centered date of sample collection (NA=no data)
tarsus\_cent - mean-centered tarsus length (in mm, NA=no data)
weight\_cent - mean-centered body mass (in g, NA=no data)
cond\_cent - mean-centered body condition (residuals of weight over tarsus, NA=no data)

data.comp (contains a...