Relationships between territory size, food availability and body size in the Torrent Duck (Merganetta armata)

Torrent Duck (Merganetta armata) is a fast flowing river specialist which inhabits exclusively the Andes Mountain Range, from southern Argentina and Chile to Venezuela. The species is highly territorial and uses a river section of one to two km long which they defend year-round if water levels remain stable. We studied territory size (n=25), food availability, and body size of dominant individuals in a latitudinal gradient of 25° between Jujuy and Santa Cruz provinces. We found that food availability decreases southwards while territories become bigger, as is to be expected regarding optimal foraging models. Within each river studied the largest male and female ducks occupied territories with greater food availability. This relates to male and female territorial behaviour and the territory obtaining process, which takes place outside the breeding season and without the partner’s help. Finally, we did not find differences in body size (culmen length, body mass) in relationship with the latitudinal gradient. This turns out to be contrary to what was expected according to Bergman´s and Allen’s rules that predict an increase in body size and decrease in length of limbs respectively, as temperature drops. Probably, the difference in altitude between the studied sites could explain these differences.