TheRete tibiotarsaleand Arteriovenous Association in the Hind Limb of Birds: A Compartive Morphological Study on Counter-current Heat Exchange Systems

Arterio-venous association was investigated in the tibial region of 66 bird species representing 21 orders. An intermingled network of arteries and veins (rete tibiotarsale) was found in all stages of complication, ranging from the most simple with only 3 arteries and 5–7 veins (tinamou and owl) to the most elaborate types with more than 60 arteries and 40 veins (flamingo). As opposed to what has hitherto been believed, elaborate retia are present in many short-legged species, while most of the long-legged wading birds only have retia of a very simple type. Arctic forms of eider ducks were found to possess elaborate retia comparable to those in other ducks. In species lacking the rete tibiotarsale, the single artery and its counter-current veins are forming a venae comitantes system. The number of counter-current veins increases with foot size and in ravens and crows up to 8 veins were surrounding the artery, resembling the pattern described in the fins of whales. In species of gulls and guillemots, the contact surface between the veins and the artery appeared to be higher in arctic forms than in more southerly distributed birds. The findings are discussed in relation to heat economy, foot size, and ecology of the species. Further, theories for the evolution of the different types of heat exchange systems are presented. It is concluded that no single selective factor can explain the evolution and occurrence of the rete tibiotarsale.