Convergent evolution in dippers (Aves, Cinclidae): The only wing‐propelled diving songbirds

Abstract

Of the more than 6,000 members of the most speciose avian clade, Passeriformes (perching birds), only the five species of dippers (Cinclidae, Cinclus) use their wings to swim underwater. Among nonpasserine wing‐propelled divers (alcids, diving petrels, penguins, and plotopterids), convergent evolution of morphological characteristics related to this highly derived method of locomotion have been well‐documented, suggesting that the demands of this behavior exert strong selective pressure. However, despite their unique anatomical attributes, dippers have been the focus of comparatively few studies and potential convergence between dippers and nonpasseriform wing‐propelled divers has not been previously examined. In this study, a suite of characteristics that are shared among many wing‐propelled diving birds were identified and the distribution of those characteristics across representatives of all clades of extant and extinct wing‐propelled divers were evaluated to assess convergence. Putatively convergent characteristics were drawn from a relatively wide range of sources including osteology, myology, endocranial anatomy, integument, and ethology. Comparisons reveal that whereas nonpasseriform wing‐propelled divers do in fact share some anatomical characteristics putatively associated with the biomechanics of underwater “flight”, dippers have evolved this highly derived method of locomotion without converging on the majority of concomitant changes observed in other taxa. Changes in the flight musculature and feathers, reduction of the keratin bounded external nares and an increase in subcutaneous fat are shared with other wing‐propelled diving birds, but endocranial anatomy shows no significant shifts and osteological modifications are limited. Muscular and integumentary novelties may precede skeletal and neuroendocranial morphology in the acquisition of this novel locomotory mode, with implications for understanding potential biases in the fossil record of other such transitions. Thus, dippers represent an example of a highly derived and complex behavioral convergence that is not fully associated with the anatomical changes observed in other wing‐propelled divers, perhaps owing to the relative recency of their divergence from nondiving passeriforms.

Modified skulls but conservative brains? The palaeoneurology and endocranial anatomy of baryonychine dinosaurs (Theropoda: Spinosauridae)

“…Others, as mentioned above, find no evidence for change in labyrinth shape being associated with such transitions and, furthermore, question previous form‐function inferences based on vestibular geometry (Bronzati et al, 2021; Evers et al, 2022). Furthermore, derived ecologies can arise in the absence of significant shifts in endocranial anatomy, as observed in the extant diving bird Cinclus (Passeriformes: Cinclidae), whose endocasts and inner ears are very similar to non‐diving passeriform relatives (Smith et al, 2022).…”

“…Furthermore, derived ecologies can arise in the absence of significant shifts in endocranial anatomy, as observed in the extant diving bird Cinclus (Passeriformes: Cinclidae), whose endocasts and inner ears are very similar to non-diving passeriform relatives (Smith et al, 2022).…”

Trends of avian locomotion in water – an overview of swimming styles

“…Among diving birds, a wide spectrum of uses of wings exists, and in contrast to footpropulsion, wing-propelled techniques are known only from the Cenozoic fossil record (Kristoffersen 2001). One of the most extreme morphological and structural responses among birds for locomotion-related mechanical loadings is present in the forelimbs of wingpropelled divers , Serrano et al 2020, thus this behavior (Figure 3) represents a strong selective pressure (Smith et al 2021).…”

“…While dippers do share some convergent features (e.g. related to musculature and the feathers) with other wing-propelled divers, the osteological modifications are subordinate (Smith et al 2021). Following Fish (2016) those taxa that perform underwater flight using wing-propulsion can be grouped regarding their aerial skills.…”

Diving into a dead-end: asymmetric evolution of diving drives diversity and disparity shifts in waterbirds

“…Diving as a foraging strategy in birds is relatively uncommon, and several studies have focused on the evolution of the strategy in specific taxonomic groups (e.g. penguins [ 28 ], Charadriiformes [ 29 ], loons and grebes [ 30 ], dippers [ 31 ], kingfishers [ 32 ] and Hesperornithiformes [ 33 ]). Many of these groups are found within Aequorlitornithes and several different diving techniques have evolved, including pursuit diving (wing and foot propelled) and plunge diving, with other taxa occupying various non-diving niches.…”