A seasonal stable isotope survey of the food web associated to a peri-urban rocky shore

We characterized the structure and functioning of the food web associated to an intertidal rocky shore in the harbour of Brest (Western Brittany, France), through a seasonal δ¹³C and δ¹⁵N survey. The dominance of benthic-derived organic matter, and particularly resuspended sediment, in the local pool of suspended organic matter, is highlighted. This benthic-derived organic matter dominates the diet of filter-feeders, including a certain degree of selectivity displayed by various taxa belonging to this trophic group. The food web structure appeared very stable temporally, which suggested the dominance of mixed diets, rather than the consumption of pure sources. It was constituted of 4 trophic levels, including a large diversity of predators, likely favoured by the diversity of microhabitats that characterize this intertidal habitat. From our results, the trophic functioning of this peri-urban rocky shore community does not appear functionally simplified as previously reported for other anthropized and/or artificial rocky ecosystems. We suggest that the topological complexity of rocky habitats, including a diversity of microhabitats, might be responsible of the important animal diversity, in spite of the anthropogenic disturbances characterizing this peri-urban area.     

Microscale variations of food web functioning within a rocky shore invertebrate community

The assessment of relevant spatial scales at which ecological processes occur is of special importance for a thorough understanding of ecosystem functioning. In coastal ecosystems, the variability of trophic interactions has been studied at different spatial scales, but never at scales from centimetres to metres. In the present study, we investigated the link between habitat structure and small-scale variability of food web functioning on intertidal boulder field ecosystems. Two microhabitats, boulder-top and boulder-bottom, were considered, and the trophic ecology of invertebrate consumers was studied using stable isotope tracers. We found for two of the main suspension feeders of northern Atlantic rocky shores (the sponges Halichondria panicea and Hymeniacidon sanguinea) consistent ¹⁵N enrichment for individuals sampled under boulders, suggesting that these consumers relied on different trophic resource according to the microhabitat inhabited, at a centimetre scale. The high δ¹⁵N signatures found underneath boulders suggested higher use of highly decomposed organic matter in this microhabitat. The isotopic difference between the two microhabitats decreased in higher trophic level consumers, which likely foraged at a spatial scale including both microhabitats. Finally, our results reveal that in highly heterogeneous habitats such as boulder fields, trophic interactions are likely to vary strongly in space, which should be considered in future researches. The link between habitat physical structure and food web variability might also contribute to the high biological diversity characterizing heterogeneous ecosystems.     

The responsiveness of young rabbits to the mammary pheromone: developmental course in domestic and wild pups

Newborn rabbits rely on odour cues to localize and seize maternal nipples. In particular, they orally respond to the mammary pheromone (MP) emitted in rabbit milk. The activity of the MP was so far mainly investigated in newborns, and a drop in its releasing effect was shown in 23-day old pups. Here, we evaluated the activity of the MP in a large sample (n=1588) of domestic rabbit pups tested between birth and weaning. We also assessed whether wild-type pups respond to the MP, and whether the developmental course of their responsiveness matches the one of domestic pups. During the first postnatal week, the MP revealed to be a strong releasing stimulus in all pups, regardless of their genetic and environmental background. However, the saliency of the MP decreased with approaching weaning, and this decrease was clearly steeper in wild-type than in domestic pups. These results are discussed with regard to the concept of ontogenetic adaptation and the effects of artificial selection.     

Convergent changes in the maternal emission and pup reception of the rabbit mammary pheromone

Summary. Lactating rabbit females emit volatile odour cues that trigger specialized motor actions leading to sucking. But the activity of these cues may change with advancing lactation. Here, we tested this possibility in three experiments. In Exp. 1, we assessed whether 2-day-old pups respond differently to the odour of milk from females in early (day 2) as compared to late (day 23) lactation. In Exp. 2, a compound bearing pheromonal properties, the Mammary Pheromone (MP), was dosed in these milks to assess whether its concentration is correlated with behavioural activity. In Exp. 3, the responsiveness to a constant level of MP was compared in d2 versus d23-pups. Run on 240 pups, the assays showed that a) the milk activity declines between d2 and d23 of lactation; b) during this same period, the concentration of the MP decreases in milk; c) the MP itself becomes less active to elicit oral grasping in pre-weaning pups than in newborns. These results indicate that the MP is active during the period when pups are exclusively dependent on milk. The convergent changes in emission and reception of this pheromone may sequentially warrant that pups are first attracted to the mammae, and then that they progressively disinvest the mother as they begin to eat solid food and to be attracted by other conspecifics.     

A single key-odorant accounts for the pheromonal effect of rabbit milk: Further test of the mammary pheromone’s activity against a wide sample of volatiles from milk

Summary. In the rabbit, lactating females emit a volatile compound in milk, the mammary pheromone (MP), that triggers rooting for the nipple and its grasping in pups. Previous studies have shown that the MP seems to act selectively, in terms both of intensity and quality. Here, we aimed to add new evidence to these properties of the MP. Newborn rabbits (n=825) were submitted to an oral activation test allowing to measure their searching/grasping responses towards different stimuli. In Experiment 1 we assessed whether pups respond to the MP in an intensity-dependent manner. In Experiment 2 we assessed the activity of 20 volatiles previously identified in rabbit milk, other than the MP, which were never systematically tested for intensitydependence. The assays showed that a) neonatal responses are released by the MP only for a limited range of concentrations; b) the 20 other odorants from milk are inactive at any concentration. Thus, the MP appears to be the single volatile from rabbit milk that releases searching/grasping behaviour in pups.     

Odour-guided social behaviour in newborn and young cats: an analytical survey

Summary. This article surveys early chemosensory mechanisms regulating behaviour in neonatal and juvenile domestic cats (Felis sylvestris catus). It aims to retrieve material from the available literature which may promote a better understanding of the semiochemical system that supports female-to-kitten exchanges. Studies dealing with the development of olfaction from birth through weaning are presented in three sections. The first aims at locating developmental periods suitable for applying analytical procedures combining behaviour and chemistry. The second section offers a survey of observational and experimental work related to the role of olfaction in the social and feeding behaviour of kittens before weaning. The final section traces the current knowledge on the odour sources and substrates that may be involved in early mother-kitten transactions.     

Trophic significance of kelps in kelp communities in Brittany (France) inferred from isotopic comparisons

The relative contribution of kelps and other food sources to the diet of consumers in shallow coastal communities has been hotly debated in recent years. It has been suggested that considering proxies instead of phytoplankton isotopic signatures can lead to an overestimation of kelp contribution through isotopic mixing models. We analysed spatial patterns in carbon isotopic ratios of the dominant primary sources and consumers at two subtidal sites in Brittany (France) prior to the anticipated 2011 phytoplankton bloom to determine which of kelp and phytoplankton is the dominant food source in Laminaria hyperborea forests. We found simultaneous spatial variations for consumers, kelps and deposited particulate organic matter, suggesting that kelp-derived organic matter is a key component of associated food webs for at least part of the year.