Introduction

One might suspect that for arboreal animals like most primates, full functionality of the locomotor apparatus is an essential condition for survival. Nevertheless, the presence of healed fractures and of (congenital) malformations of the extremities reported from several wild primate populations suggest that affected individuals are capable of compensating impairments and thus to survive and even reproduce (Schultz 1939, 1956; Stokes & Byrne 2006; Arlet et al. 2009, Ferrari et al. 2010). Fractures (and perhaps other impairments) occurring at an early phase of life are suggested to be less fatal than those obtained during adulthood (Bulstrode et al. 1986). Here we report the case of a wild moustached tamarin, Saguinus mystax, who lacked one foot, probably from birth or shortly after birth on.

Methods

The observations reported here were made at the Estación Biológica Quebrada Blanco (EBQB) in north-eastern Peruvian Amazonia (4°21′S 73°09′W). Groups of S. mystax and its congener, the black-fronted tamarin, Saguinus nigrifrons,1*are observed at EBQB during behavioural and ecological projects by researchers, students and local field assistants or routinely in-between projects by local field assistants. Apart from project-specific data, we also record unusual and rare events in the life of the tamarins.

Results and discussion

On 12 September 2008, we detected that one juvenile in our S. mystax study group 2 (by that time consisting of two adult males, one adult female, and two juvenile males born around 20 September 2007) lacked the right foot. The juvenile's right leg ended in a round and naked stump with skin slightly lighter than the palms of the hands and the other foot (see video at  http://www.soziobio.uni-goettingen.de/Peru/patita.html). It is unknown whether the foot was already missing at birth, lost soon after birth or later through an accident or a bite. We suspect the first or second alternative: during observations of that group between 24 and 26 September 2007 we recorded one infant in an unusual position on the lower back of the carrier, and also falling from the back of the carrier, suggesting that the infant could not hold normally to the carrier and that the foot was already missing at that time.

At first glance, the locomotion of the juvenile male (Patita) was inconspicuous. Only when looking very scrupulously did it become evident that Patita was occasionally slipping from the substrate with his right leg. When travelling on large branches, Patita simply touched the substrate with the stump; when travelling on small branches, this could not be unequivocally observed. Patita used the same routes for travelling than the other group members, but sometimes seemed to be more hesitative when a large leap had to be made. During foraging, Patita employed all manoeuvres typical for S. mystax, i.e. rapid grasping, lunging and pouncing (see Peres 1993). He was also feeding and foraging on thin terminal branches, and clinging vertically to the trunks of Parkia trees to consume exudates. Patita was not seen falling from a tree more often than other tamarins with fully functional extremities. Only on very smooth surfaces did Patita have obvious problems with locomotion. When trying to reach the infructescences of Wettinia maynensis (Arecaceae), he was sliding down the basal part of the fronds rather than climbing down headfirst, and also climbed up with difficulty compared to other S. mystax.

Patita was regularly seen in rough-and-tumble and chase playing with his slightly larger twin brother, although we had the impression that his playing was less vigorous than in non-handicapped tamarins. He participated in carrying twins born in January 2009 only very sporadically, while his twin brother did almost two-thirds of all infant carrying. He was last seen with his natal group on 7 December 2009. On 9 December 2009, Patita participated in an in-tergroup encounter as member of a neighbouring group to which he obviously had immigrated.

While healed fractures of the extremities and congenital malformations of hands and feet are known from callitrichids in captivity and in the wild (Schultz 1956; French 1986; Ferrari et al. 2010), the complete lack of a cheiridium has not been reported. While we do not know the cause of the lack of a foot in Patita, our observations indicate that despite some handicap during locomotion on smooth surfaces, this tamarin compensated the impairment and performed most behaviour normally, as also reported for wild Mico argentatus with a congenital lack of the big toe (Ferrari et al. 2010). However, Patita was clearly handicapped with regard to infant carrying. This is not surprising, given the constraints that the heavy load represented by callitrichid infants imposes on locomotion and foraging, even in captive callitrichids (Schradin & Anzenberger 2001; Caperos et al. 2012).

Tamarin societies are highly cooperative, and S. mystax may modify their behaviour in response to an injured group member (Tirado Herrera & Heymann 2004). In the absence of quantitative data, we do not know whether Patita's group also had modified its behaviour, particularly in the critical transitional phase between infant dependency and locomotor independence. Primates with impairments are obviously also capable of surviving without any assistance from conspecifics (Struhsaker et al. 2011), but cooperation by group members would certainly be conducive to compensate impairments and thus to increase the likelihood of survival.