RESUMO
Individual-level sibling interactions in the litter huddle have been studied extensively, especially in the domestic rabbit (Oryctolagus cuniculus). However, little is known about inter-litter differences in pup activity patterns during early postnatal life, in particular regarding the drivers of such variation. In our study on 2-3-day-old rabbit pups, we predicted lower locomotor activity in litters with lower mean body masses on the day of birth (starting body mass) and with lower daily milk intake per pup, possibly constituting a behavioral strategy of pups to cope with associated energetic constraints. For an automatized assessment of pup locomotor activity in the litter huddle, we successfully developed and validated a method based on the quantification of dissimilarities between consecutive frames of video footage. Using this method, we could confirm a U-shaped time course of litter-level locomotor activity, with maximum values shortly before and after the once-daily nursing typical for the rabbit. As predicted, between-litter variation in mean starting body mass and in daily milk intake affected the degree of locomotor activity in the litter huddle, in an interactive way. That is, in litters with heavier starting body masses, pup locomotor activity was greater in pups with an initially higher milk intake, suggesting that only pups with better body condition and a higher energy intake could afford higher levels of activity. This interaction was exclusively apparent during the middle phase of the 24 h inter-nursing interval, when litter activity was low. Shortly before nursing, when pups show higher levels of locomotor behavior in anticipation of the mother's arrival, and shortly after nursing when the pups were more active possibly due to adjustments of their positions in the huddle, activity levels were decoupled from pups' starting body mass and previous milk intake. Our findings highlight the importance of pup body mass and daily energy intake, two parameters known to be related to maternal characteristics, in shaping inter-litter differences in pup locomotor activity.
Assuntos
Comportamento Animal , Ingestão de Energia , Animais , Coelhos , Gravidez , Feminino , Animais Recém-Nascidos , Locomoção , Tamanho da Ninhada de Vivíparos , Peso CorporalRESUMO
Judgment bias tests have become an important tool in the assessment of animals' affective states. Subjects are first trained to discriminate between two cues associated with a positive and a less-positive outcome. After successful training, they are confronted with an ambiguous cue, and responses are used for judgment bias assessment. In spatial settings, ambiguous cue presentation is typically linked with novelty, i.e. to yet unexplored areas or areas to which the animal has a low degree of habituation. We hypothesized that in such settings, responses to ambiguity might be biased by the animals' perception of novelty. We conducted judgment bias tests in mound-building mice phenotyped for their exploration tendency. After subjects had learned to distinguish between the positively and less-positively rewarded arms of a maze, a new ambiguous middle-arm was introduced. During the first test trial, more exploratory, less neophobic individuals displayed higher bidirectional locomotion in the ambiguous arm, indicating intensive exploration. Although this resulted in longer latencies to the reward in more exploratory animals, we conclude that this did not reflect a 'more pessimistic judgment of ambiguity'. Indeed, during the following two trials, with increasing habituation to the ambiguous arm, the direction of the association was inversed compared to the first trial, as more exploratory individuals showed relatively shorter approach latencies. We suggest that in spatial test settings associating the ambiguous cue to novel areas, results can be confounded by subjects' personality-dependent motivational conflict between exploration and reaching the reward. Findings obtained under such conditions should be interpreted with care.