You can use "looking time," then, as a rough but reliable proxy for what captures babies' attention: what babies are surprised by or what babies like.
The more one has of the trait to be touted (fitness, say), the less costly the signal (feathers), making the display of the signal a reliable proxy for the trait.
It can inform modern human medicine, because studying the fragmentary, squashed and hard-to-identify fossilised remains of distant relatives that lived millions of years ago in a different climate are definitely the most reliable proxy for understanding human health.
In fact, for all its cliched image, California is an increasingly reliable proxy for the nation as a whole, with high-tech industry and giant farming operations, international trade and small start-up businesses, a shrinking white majority and an ideological spectrum that runs from gun-owning ranchers to yuppie entrepreneurs and aging liberals who came for the 60's revolution and never went home.
Furthermore, "advanced age is not a reliable proxy for poor health," Zulman and Humphreys argue.
For simplicity, initial testing was conducted with 100%% CO2, which represents a reliable proxy for the Martian atmosphere.
Electric power generation figures are viewed by many analysts as a more reliable proxy for China's economic activity than official growth numbers.
A tree-ring δ18O chronology for the period 1828 2000 was developed from four Pinus merkusii trees, to find a reliable proxy for reconstructing precipitation in northwest Thailand.
In particular, nighttime luminosity data has been shown convincingly to be a useful and reliable proxy for socioeconomic statistics [15 20].
The δ13C signature of tree material is widely regarded as a reliable proxy for water-use efficiency (Kruse et al. 2012).
