A primate with a Panda's thumb: The anatomy of the pseudothumb of Daubentonia madagascariensis

Abstract

Objectives: Accessory digits have evolved independently within several mammalian lineages. Most notable among these is the pseudothumb of the giant panda, which has long been considered one of the most extraordinary examples of contingent evolution. To date, no primate has been documented to possess such an adaptation. Here, we investigate the presence of this structure within the aye-aye (Daubentonia madagascariensis), a species renowned for several other specialized morphological adaptations in the hand, including a morphologically unique third digit. Materials and Methods: We combine physical dissection techniques with digital imaging processes across a sample of five individuals (four adults and one immature individual) to describe and visualize the anatomy of the wrist and hand within the aye-aye. Results: A distinct pseudothumb, which consists of both a bony component (an expanded radial sesamoid) and a dense cartilaginous extension (the "prepollex") was observed in all specimens. We demonstrate that this pseudodigit receives muscular attachments from three muscles which collectively have the potential to enable abduction, adduction and opposition. Finally, we demonstrate that the pseudothumb possesses its own distinct pad within the palm, complete with independent dermatoglyphs. Discussion: Pseudothumbs have been suggested to improve palmar dexterity in taxa with overlygeneralized first digits (e.g., pandas) and to widen the hand for digging (e.g., some fossorial moles), but This article is protected by copyright. All rights reserved. This is the author manuscript accepted for publication and has undergone full peer review but has not been through the copyediting, typesetting, pagination and proofreading process, which may lead to differences between this version and the Version of Record. Please cite this article as

Manual dexterity of mice during food-handling involves the thumb and a set of fast basic movements

“…Among rodents, pincer-type grasping may be common [12,19] but bimanual thumb-holding was previously reported only for squirrels [12]. Interestingly, bimanual thumb-holding has also been described for aye-aye lemurs [26], which furthermore possess a 'pseudo-thumb' (an extension of a radial sesamoid bone replete with its own pad) thought to be involved in gripping [27]-an independent evolutionary adaptation that, we speculate, could represent shared functionality with the thenar pads of mice and squirrels. Thumb-holding was preferentially used to manipulate smaller food items, and for the tiniest morsels, mice used "pure" bilateral thumb-holds; i.e., the only digits holding the food were the thumbs (and/or thumb-clefts).…”

To tell a different story: Unexpected diversity in local attitudes towards Endangered Aye‐ayes Daubentonia madagascariensis offers new opportunities for conservation

“…The Aye-aye is the only living member from the family of Daubentoniidae (Herrera & Dávalos, 2016;Perry et al, 2012) and is with an average weight of about 2.5 kg and a length of 74-90 cm the biggest nocturnal primate on earth (Feistner & Sterling, 1995;Glander, 1994;Oxnard, 1981). It has a suite of unusual, derived traits including an elongated, thin, highly flexible middle finger, a pseudo thumb, continuously growing incisors, large and orange eyes, long and dark fur and tail, the largest relative brain size of any lemur and a relatively slow life history including late weaning and a protracted learning period (Figure 1; Barrickman & Lin, 2010;Hartstone-Rose et al 2019;Jouffroy, 1975;Owen, 1863;Simons, 1995).…”

“…It has a suite of unusual, derived traits including an elongated, thin, highly flexible middle finger, a pseudo thumb, continuously growing incisors, large and orange eyes, long and dark fur and tail, the largest relative brain size of any lemur and a relatively slow life history including late weaning and a protracted learning period (Figure 1; Barrickman & Lin, 2010; Hartstone‐Rose et al. 2019; Jouffroy, 1975; Owen, 1863; Simons, 1995).…”

Locomotion of an adult female and juvenile male aye‐aye (Daubentonia madagascariensis) in Torotorofotsy, Madagascar

“…In addition to huge bat-like ears and ever-growing incisors, aye-ayes have highly modified hands with extremely elongated digits, claw-like nails, and a specialized metacarpophalangeal joint in the third ray (Napier & Napier, 1967;Oxnard, 1981;Petter & Petter, 1967;Soligo, 2005;Walker, 1974). This unusual morphology and highly derived foraging behavior are defining characteristics of aye-ayes and are known to influence multiple aspects of behavior, including locomotion (Curtis & Feistner, 1994;Hartstone-Rose et al, 2020;Kivell et al, 2010;Krakauer et al, 2002).…”