Abstract
Suckling of a non-filial calf, or allosuckling, is an extreme case of allomaternal care in mammals. There have been many hypotheses postulated in an attempt to explain this behaviour; however, the supporting evidence differs, together with the conclusions drawn from the investigated variables. Previously, suckling bout rejection was analysed, and the milk theft and reciprocity hypotheses were both determined as the most appropriate explanation of allosuckling in giraffe. In this study, seven hypotheses were tested using different behavioural parameters, namely suckling bout frequency, suckling bout duration, and time spent suckling. It is well-documented that these parameters are associated with various aspects in ungulate biology; for example, suckling rejection typically reflects milk intake and parent–offspring conflict, whereas the suckling bout duration and frequency is associated with social behaviours (affiliation, bonding, social stress). In total, 22 nursing females and 47 suckling calves were observed, in four Czech zoos during a five-year period. The correlation of the observed parameters between the reciprocal female-calf dyads was found to be in line with the reciprocity hypothesis. In addition, non-filial calves tried to steal the milk from non-maternal females, supporting the milk theft hypothesis. Thus, the results support both the reciprocity and milk-theft hypotheses as the most plausible explanation of allosuckling in giraffe, and illustrates the importance of using suckling bout duration and frequency, and the time spent suckling, as behavioural parameters that may aid in explaining the extremity of maternal investment, such as allosuckling.
Highlights
Suckling of a non-filial calf, or allosuckling, is an extreme case of allomaternal care in mammals
The suckling bout duration was affected by the position of the suckling calf (F2,717 = 5.56; P = 0.004),with the bout lasting longer when the calf adopted an antiparallel position, compared to if they were in a parallel position (t717 = 3.22; P = 0.004) or in a position behind the female (t717 = 2.49; P = 0.034)
The suckling bout duration was affected by the number of suckling calves within the bout (F1,717 = 75.37; P < 0.001), with the duration of the suckling bout decreasing with the increasing number of calves suckling simultaneously
Summary
Suckling of a non-filial calf, or allosuckling, is an extreme case of allomaternal care in mammals. The results support both the reciprocity and milk-theft hypotheses as the most plausible explanation of allosuckling in giraffe, and illustrates the importance of using suckling bout duration and frequency, and the time spent suckling, as behavioural parameters that may aid in explaining the extremity of maternal investment, such as allosuckling. Other behavioural parameters are frequently used for the expression of maternal care, and they reflect the offspring’s demand rather than the rate of milk transfer[21,22,23] These parameters are: suckling bout duration, and/or suckling bout frequency[8,24,25]. It was reported that in extreme cases, like allosuckling, these parameters could reflect parent–offspring conflict, as the bout duration may be connected with a shortage of milk, as well as maternal care for filial c alves[21,22,26,27]. The suckling bout duration of the filial calf will not be longer than those of non-filial ones The bouts will not last longer when an antiparallel position was adopted compared to two other positions Female sniffing the calf during the bout will not affect the suckling duration (shorter for non-filial and longer for filial)
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