Scatter-hoarding rodents and marsupials: convergent evolution on diverging continents

Neotropical rainforests host a rich community of fruit-eating animals, among them neotropical scatter-hoarding rodents that bury seeds in soil. These animals perform an important community-building process because the seeds germinate and establish seedlings away from the parent plant. In three recent studies, researchers have demonstrated a comparable role for paleotropical rodents in southeast Asia. With the discovery that a frugivorous kangaroo behaves similarly to its neotropical rodent counterpart, a new level has been reached in our understanding of the evolution of tropical forests.     

Mice prefer rare food

In the first experiment designed to test directly for frequency-dependent selection of food by a mammal, mice were presented with pairs of food in 9: 1 and 1:9 combinations. The foods used were crumbs of laboratory rat cake flavoured vanilla or peppermint and dyed green or brown: vanilla/green was presented with peppermint/brown, and vanilla/brown with peppermint/green. Vanilla was preferred to peppermint and green to brown: peppermint/brown was particularly unacceptable. Each type was preferred more when rare than when common. Preferences changed over the five days of the experiment, the preference for greens becoming less strong. Since the experimental trials were carried out in total darkness, the preferences must have been based on non-visual clues, indicating that the dyes were not odourless and tasteless to mice, as they are to man.     

Scatter-hoarding rodents use different foraging strategies for seeds from different plant species

Seed predation and dispersal by scatter-hoarding rodents are key processes that determine seed survival, and thus, plant regeneration within forests. For decades, there has been much debate on the important effects of seed size (one of the most important seed traits) on rodent foraging preference. Furthermore, the possible selective forces in the evolution of seed size may be influenced by primary selectivity and how rodents treat seeds after harvesting. In this study, different-sized seeds from four species (Pinus armandii, Pinus densata, Abies sp., and Viburnum sp.) harvested by scatter-hoarding rodents were studied in an alpine forest in Southwestern China for two consecutive years. Our results showed that seed size influenced rodent foraging preferences, with bigger seeds being preferred over smaller seeds, within and across species. Rodents only removed and cached the larger seeds of P. armandii, and ate the seeds of the other three species in situ. Rodents are purely seed predators for these three species. For the cached seeds of P. armandii, significantly positive correlations were observed between seed size and dispersal distance among both primary and secondary cached seeds in 2006, but not in 2005. Our results indicate that among many coexisting species with widely different-sized seeds, scatter-hoarding rodents played important roles in the seed dispersal of the big-seeded species alone. This caching behavior could offset the limited seed dispersal of large-seeded and wingless species (P. armandii), in comparison with that of small winged seed species (P. densata and Abies sp.) and frugivore-dispersed species (Viburnum sp.).     

More mice prefer rare food

In further experiments designed to examine frequency-dependent food selection by mice, we confirmed that there was an overwhelming preference for rare food and that this preference was not substantially altered over four days experience. The effect was less marked when rare and common food differed only in the dye applied to them than when they differed also in the flavour applied to them. There were no sustained differences between the results for day-time (high illuminance) and night-time (low illuminance).     

Great apes prefer cooked food

The cooking hypothesis proposes that a diet of cooked food was responsible for diverse morphological and behavioral changes in human evolution. However, it does not predict whether a preference for cooked food evolved before or after the control of fire. This question is important because the greater the preference shown by a raw-food-eating hominid for the properties present in cooked food, the more easily cooking should have been adopted following the control of fire. Here we use great apes to model food preferences by Paleolithic hominids. We conducted preference tests with various plant and animal foods to determine whether great apes prefer food items raw or cooked. We found that several populations of captive apes tended to prefer their food cooked, though with important exceptions. These results suggest that Paleolithic hominids would likewise have spontaneously preferred cooked food to raw, exapting a pre-existing preference for high-quality, easily chewed foods onto these cooked items. The results, therefore, challenge the hypothesis that the control of fire preceded cooking by a significant period.     

啮齿动物的分散贮食行为

食物贮藏是许多动物重要的适应性行为,分散贮藏的食物以植物种子为主。每个贮藏点贮藏数量不等的食物项目。啮齿动物分散贮藏食物之后,可降低食物被其他个体获取的机率,提高对食物资源的控制能力,最终有利于自身的生存和繁殖成功。植物种子被贮藏之后,可减少非贮食鼠类对种子的取食。同时,合适的微生境和埋藏有利于种子萌发、幼苗建成和植物的更新;使植物的分布区得以扩展。探讨啮齿动物的分散贮食行为,能够更好地理解食物贮藏在啮齿动物生活史中的作用,进一步认识鼠类和植物的相互关系以及不同啮齿动物在群落形成中的潜在作用。本综述了啮齿动物分散贮食的研究进展,并提出今后工作中的几点建议。     

Time-course of astringent sensations

Qualitative and quantitative perceptual reactions to astringentmaterials were examined for three diverse chemical substances(alum, tannic acid and tartaric acid) at several concentrationsproducing moderate to strong levels of perceived sensation.Group discussions were held to determine language appropriateto describe the sensations arising from solutions of the threecompounds and a composite ballot of six rating scales (astringency,mouth drying, puckery feeling, mouth roughing, bitterness andsourness) was developed. For both experiments, two concentrationsof each compound were rated on the six attributes for five tosix minutes, a discrete-point time-intensity scaling procedure.All ratings showed roughly exponential decays over time. Theintensity ratings for each attribute were found to depend onboth the particular astringent substance and concentration tested.The results from experiment 2 suggested that the four tactileattributes of drying, puckery feeling, roughing, and overallastringency may not be totally interchangeable and that theremay be multiple sub-qualities in the sensory reactions groupedas astringency. It is recommended that future structure-activitystudies make use of time-intensity procedures with multiplerated attributes, using 1 g/1 alum as a reference material,since it is relatively low in perceived bitterness and sourness,but produces pronounced drying, roughing, puckery/drawing sensations.     

Interactions of astringent substances

Two-component mixtures of astringent materials were rated forperceived intensity of astringent and taste attributes overtime. Components included alum (a complex salt), gallic acid(the monomeric component of hydrolyzable tannins), catechin(the monomeric component of condensed tannins) and citric acid.Mixtures of alum and gallic acid showed mixture suppression,in that the 50/50 mixture was less intense than either componentin astringency, drying, roughing and puckery/drawing sensations.Suppression was seen at concentration levels producing moderateto strong astringency but was absent or less pronounced at lowerconcentration levels. A similar pattern held for citric acid,although the suppressive effects were less pronounced. Catechinand gallic acid mixtures were additive. Sensory interactionsbetween astringent materials appears to depend on the substancesinvolved and their concentrations (or intensity levels).     

Bioenergetics and food preferences in sympatric southern Chilean rodents

Summary Ecological divergences in two congeneric sympatric woodland rodents, Akodon olivaceus brachiotis and Akodon longipilis apta (Family Cricetidae) was demonstrated from determination of energetic bud gets and food preferences. It was found that each species posseses a different bioenergetic strategy. The heavier (A. longipilis) species has a larger daily energetic budget as compared to the lighter one (A. olivaceus). This phenomenon can be partially explained by their differential selection of seeds.Finally, it is hypothesized that these ecological differences could be important in enabling the coexistence for these species.     

Land molluscs as food of Malayan rodents and insectivores

Laboratory and field experiments demonstrate that, contrary to previously published stomach analysis data, rats selectively consume land molluscs. Rattus jalorensis and R. argentiventer prefer molluscs as food, while R. exulans and R. r. diardi are less inclined to do so. Macrochlamys resplendens , a land snail, is most favoured, with the slug Micropar-mariort malayanus next in preference. The heavily-shelled Achatina fulica is only occasionally eaten and Quantula striata is ignored. Examination of gastro-intestinal contents of wild rats caught from various habitats in Malaya show that when present in large numbers molluscs form an important part of the natural food of rats, yet laboratory evidence demonstrates that as many as 60% of mollusc-eating rats may fail to show evidence of shells on examination within 12 hours of feeding–an indication that stomach analyses alone may be of limited value.
The Malayan insectivores Echinosorex gymnurus and Suncus murinus often feed on snails and slugs, though they appear not to be suitable hosts for infection with the rat lungworm Angiostrongylus cantonensis.     

Fine root production of astringent phenolics

Summary Chemical quality of fine roots (<1 mm diameter) was determined over a gradient of species composition in the Mixed Mesophytic Forest Region. Ash-free nitrogen, calcium, and phosphorus concentrations of roots declined by 49, 41, and 72%, respectively, over a gradient of increasing soil acidity (pH 5.3 to 4.7). Lignin concentration was unrelated to either the vegetation gradient or any of the soil changes it encompassed; however, astringent phenolics increased by 275% over the same gradient. Trends in the chemical constituency of fine roots suggest that the production of phenolics in below-ground plant parts is increased on nutrient-poor sites. This response is best related to changes in species composition, especially increasing importancy of Quercus spp.This investigation (No. 88-8-36) is connected with a project of the Kentucky Agricultural Experiment Station and is published with the approval of the Director     

Tropical rodents change rapidly germinating seeds into long-term food supplies

Seed‐hoarding vertebrates may survive yearly periods of food scarcity by storing seeds during the preceding fruiting season. It is poorly understood why rodents creating long‐term reserves, especially those in the tropics, incorporate seeds from plant species that germinate rapidly and hence seem unsuitable for long‐term storage. We carried out a series of experiments to understand why red acouchies (Myoprocta exilis) scatter‐hoard Carapa procera (Meliaceae) seeds, which lack dormancy and germinate rapidly. Hypotheses tested were: (1) even germinated seeds are still significant long‐term energy reserves for acouchies, (2) acouchies use the seeds only as short‐term supplies, and (3) acouchies manipulate the seeds to slow down germination. The first two hypotheses were not supported; seed reserves were rapidly depleted during experimental planting, and acouchies did use seeds for long‐term storage. We did find support for the third hypothesis. Acouchies intervened in germination by removing the protruding radicle and epicotyl after which they re‐cached seeds. Pruning stopped further seedling development, yet the pruned seeds did not decay. The cotyledons apparently stayed alive and physiologically active as “zombie seeds” that only formed undifferentiated calli. Pruned seeds were suitable for long‐term storage, with negligible loss of endosperm over time. Pruning was most effective after sprouting of the epicotyl, and germinating seeds were most susceptible to pruning during two weeks upon emergence. Acouchies actively managed their food supplies and must continuously survey for germinating seeds to timely intervene in seed reserve depletion, within the brief period of seedling emergence. We suggest that the trees use the rodents to achieve seed dispersal, and gain from imperfect intervention in germination by the rodents. Because scatter‐hoarding rodents and large‐seeded plant species with similar germination co‐occur in tropical forests world‐wide, it is plausible that the phenomenon of seed pruning to preserve seeds is more general than currently appreciated.     

Chickens prefer beautiful humans

We trained chickens to react to an average human female face but not to an average male face (or vice versa). In a subsequent test, the animals showed preferences for faces consistent with human sexual preferences (obtained from university students). This suggests that human preferences arise from general properties of nervous systems, rather than from face-specific adaptations. We discuss this result in the light of current debate on the meaning of sexual signals and suggest further tests of existing hypotheses about the origin of sexual preferences.     

A simple model for the interplay of predators, rodents and food

This paper presents a simple mathematical model for multiannual population cycles, in particular the periods, for the triple of small rodents and their predators and food. The parameters used are average birth rates of rodents and predators. The period lengths fit observations of lemmings and voles rather well and the model explains why the observed periods cluster around 4 years and around 10 years.