Seed choice by rodents: learning or inheritance?

The morphological and ecological variation of Arctostaphylos (Ericaceae) fruit: A link between plant ecology and animal foraging behavior

Persistent soil seed banks are characteristic of Arctostaphylos (Ericaceae) species in the Mediterranean-climate California Floristic Province. While most species are obligate seeders, regeneration of stands of all Arctostaphylos species ultimately depends on post-fire seedling recruitment. Arctostaphylos seed banks are created, in large part, by scatter-hoarding rodents. Variation in fruit morphology, therefore, is expected to impact the Arctostaphylos-rodent interaction. Seeds produce sufficient rewards (nutritious mature embryo) to entice rodents to disperse and ultimately bury seeds in the soil. Hard seed coats increase the time required to extract the embryo, encouraging rodents to choose storage over immediate predation, and nutlets are frequently empty. We assessed the variation of fruit nutlet fusion and seed viability among 38 Arctostaphylos taxa. Factors such as latitude, elevation, life history, ploidy, and phylogenetic position were also analyzed. Generalized mixed-effects models were used to determine the factors contributing to variation in fruit nutlet fusion and seed viability. Our results indicate that fruit volume and shape are the most important variables affecting nutlet fusion and seed viability. Additionally, other potential influences only show a weak correlation and are not predicted to significantly impact nutlet fusion or seed viability. These findings provide insights into evolved strategies used by plants to increase reproductive success via scatter-hoarding rodents. Our study benefits the conservation and restoration of Arctostaphylos stands by emphasizing the importance of animal-mediated dispersal and providing estimates of seed viability for different species. With the anticipated effects of climate change, such as departures from historic fire regimes, the preservation of the relationship between plants and animal foragers is crucial for the continued survival of Arctostaphylos and California's evergreen chaparral.

Inter-trophic Interaction of Gut Microbiota in a Tripartite System

Gut microbiota can be transmitted either environmentally or socially and vertically at intraspecific level; however, whether gut microbiota interact along trophic levels has been largely overlooked. Here, we characterized the gut bacterial communities of weevil larvae of Curculio arakawai that infest acorns of Mongolian oak (Quercus mongolica) as well as acorn-eating mammals, Siberian chipmunk (Tamias sibiricus), to test whether consumption of seed-borne larvae remodels the gut bacterial communities of T. sibiricus. Ingestion of weevil larvae of C. arakawai significantly altered the gut bacterial communities of T. sibiricus. Consequently, T. sibiricus fed larvae of C. arakawai showed higher capability to counter the negative effects of tannins, in terms of body weight maintenance, acorn consumption, N content in feces, urine pH, and blood ALT activity. Our results may first show that seed-borne insects as hidden players have a potential to alter the gut microbiota of seed predators in the tripartite system.

Preferencias de los granívoros con respecto al tamaño y la calidad de las bellotas en un bosque de Quercus en la zona centroseptentrional de México

Acorns of the species of the genus Quercus are highly appreciated by a diverse group of animals. It remains unclear whether the choice to move an acorn is related to the intrinsic characteristics of the fruit. In this work, we aimed to determine whether the size and quality of acorns (healthy or damaged) influenced their removal. We found that Q. affinis was the species with the largest acorns but the lowest removal rate, and Q. eduardii was the species with the smallest acorns but highest removal rates. Two groups of vertebrates carried out this removal, and this activity occurred at two clearly separate times. Jays Aphelocoma spp. carried out their activity during the day, and rodents Peromyscus spp. removed acorns at night. Size and quality only had a significant influence on the removal of Q. affinis. Our results suggest that absence of large animals could put the establishment of species with large acorns (such as Q. affinis) at risk.

Intrinsic and extrinsic drivers of intraspecific variation in seed dispersal are diverse and pervasive

There is growing realization that intraspecific variation in seed dispersal can have important ecological and evolutionary consequences. However, we do not have a good understanding of the drivers or causes of intraspecific variation in dispersal, how strong an effect these drivers have, and how widespread they are across dispersal modes. As a first step to developing a better understanding, we present a broad, but not exhaustive, review of what is known about the drivers of intraspecific variation in seed dispersal, and what remains uncertain. We start by decomposing 'drivers of intraspecific variation in seed dispersal' into intrinsic drivers (i.e. variation in traits of individual plants) and extrinsic drivers (i.e. variation in ecological context). For intrinsic traits, we further decompose intraspecific variation into variation among individuals and variation of trait values within individuals. We then review our understanding of the major intrinsic and extrinsic drivers of intraspecific variation in seed dispersal, with an emphasis on variation among individuals. Crop size is the best-supported and best-understood intrinsic driver of variation across dispersal modes; overall, more seeds are dispersed as more seeds are produced, even in cases where per seed dispersal rates decline. Fruit/seed size is the second most widely studied intrinsic driver, and is also relevant to a broad range of seed dispersal modes. Remaining intrinsic drivers are poorly understood, and range from effects that are probably widespread, such as plant height, to drivers that are most likely sporadic, such as fruit or seed colour polymorphism. Primary extrinsic drivers of variation in seed dispersal include local environmental conditions and habitat structure. Finally, we present a selection of outstanding questions as a starting point to advance our understanding of individual variation in seed dispersal.

Effects of insect infestation on rodent-mediated dispersal of Quercus aliena: results from field and enclosure experiments

Rodents influence plant establishment and regeneration by functioning as both seed predators and dispersers. However, these rodent-plant interactions can vary significantly due to various environmental conditions and the activity of other insect seed predators. Here, we use a combination of both field and enclosure (i.e. individual cage and semi-natural enclosure) experiments, to determine whether rodents can distinguish sound seeds from those infested with insects. We also demonstrate how such responses to insects are influenced by food abundance and other environmental factors. We presented rodents with 2 kinds of Quercus aliena seeds (sound and insect-infested seeds) in a subtropical forest in the Qinling Mountains, central China, from September to November of 2011 to 2013. The results showed that rodents preferred to hoard and eat sound seeds than infested seeds in the field and semi-natural enclosure, while they preferred to eat infested seeds over sound seeds in the individual cages. In addition, both hoarding and eating decisions were influenced by food abundance. Rodents hoarded more sound seeds in years of high food abundance while they consumed more acorns in years of food shortage. Compared with field results, rodents reduced scatter-hoarding behavior in semi-natural enclosures and ate more insect-infested seeds in smaller individual cages. These results further confirm that rodents distinguish infested seeds from non-infested seeds but demonstrate that this behavior varies with conditions (i.e. environment and food abundance). We suggest that such interactions will influence the dispersal and natural regeneration of seeds as well as predation rates on insect larvae.

Incorporating cache management behavior into seed dispersal: the effect of pericarp removal on acorn germination

Selecting seeds for long-term storage is a key factor for food hoarding animals. Siberian chipmunks (Tamias sibiricus) remove the pericarp and scatter hoard sound acorns of Quercus mongolica over those that are insect-infested to maximize returns from caches. We have no knowledge of whether these chipmunks remove the pericarp from acorns of other species of oaks and if this behavior benefits seedling establishment. In this study, we tested whether Siberian chipmunks engage in this behavior with acorns of three other Chinese oak species, Q. variabilis, Q. aliena and Q. serrata var. brevipetiolata, and how the dispersal and germination of these acorns are affected. Our results show that when chipmunks were provided with sound and infested acorns of Quercus variabilis, Q. aliena and Q. serrata var. brevipetiolata, the two types were equally harvested and dispersed. This preference suggests that Siberian chipmunks are incapable of distinguishing between sound and insect-infested acorns. However, Siberian chipmunks removed the pericarp from acorns of these three oak species prior to dispersing and caching them. Consequently, significantly more sound acorns were scatter hoarded and more infested acorns were immediately consumed. Additionally, indoor germination experiments showed that pericarp removal by chipmunks promoted acorn germination while artificial removal showed no significant effect. Our results show that pericarp removal allows Siberian chipmunks to effectively discriminate against insect-infested acorns and may represent an adaptive behavior for cache management. Because of the germination patterns of pericarp-removed acorns, we argue that the foraging behavior of Siberian chipmunks could have potential impacts on the dispersal and germination of acorns from various oak species.

Incorporating insect infestation into rodent seed dispersal: better if the larva is still inside

Many nutritious seeds are commonly attacked by insects which feed on the seed reserves. However, studies have not fully explored the ecological implications of insect infestation in animal seed dispersal and subsequent plant regeneration. Our question is whether the fact that an infested seed still contains the larva or not might increase/decrease the probability of being successfully dispersed by animals. This study examines the effects of weevil-infested seeds on the natural regeneration of a rodent-dispersed oak species. Rodents showed a high ability to discriminate between sound and infested seeds, even when the larva was still inside. As a result, rodents caused differential seed dispersal for sound and infested seeds by modifying multiple aspects of the dispersal process. We found that, for the same seed weight, infested acorns with a larva still inside can contribute to natural regeneration (0.7 % of seedlings in next summer), although in comparison to sound acorns they suffered higher predation rates by rodents (both partial and complete), were removed later from the ground (less preferred), cached less frequently, and dispersed to shorter distances, which reduced their potential to colonize new environments. However, infested seeds with exit holes are notably less preferred by rodents and, when dispersed, they are mostly deposited on the litter (uncached) with shorter dispersal distances and lower emergence success. Thus, the probability that larval-holed acorns will produce viable seedlings is extremely low (null in this study). Whether infested seeds still contain a larva or not clearly determines the probability of being successfully dispersed. Premature seed drop prolongs the presence of the larva inside the acorn after seed drop, and could be a possible mechanism to allow dispersal of infested seeds.

How stress alters memory in 'smart' snails

Cognitive ability varies within species, but whether this variation alters the manner in which memory formation is affected by environmental stress is unclear. The great pond snail, Lymnaea stagnalis, is commonly used as model species in studies of learning and memory. The majority of those studies used a single laboratory strain (i.e. the Dutch strain) originating from a wild population in the Netherlands. However, our recent work has identified natural populations that demonstrate significantly enhanced long-term memory (LTM) formation relative to the Dutch strain following operant conditioning of aerial respiratory behaviour. Here we assess how two populations with enhanced memory formation (i.e. ‘smart’ snails), one from Canada (Trans Canada 1: TC1) and one from the U.K. (Chilton Moor: CM) respond to ecologically relevant stressors. In control conditions the Dutch strain forms memory lasting 1–3 h following a single 0.5 h training session in our standard calcium pond water (80 mg/l [Ca²⁺]), whereas the TC1 and CM populations formed LTM lasting 5+ days following this training regime. Exposure to low environmental calcium pond water (20 mg/l [Ca²⁺]), which blocks LTM in the Dutch strain, reduced LTM retention to 24 h in the TC1 and CM populations. Crowding (20 snails in 100 ml) immediately prior to training blocks LTM in the Dutch strain, and also did so in TC1 and CM populations. Therefore, snails with enhanced cognitive ability respond to these ecologically relevant stressors in a similar manner to the Dutch strain, but are more robust at forming LTM in a low calcium environment. Despite the two populations (CM and TC1) originating from different continents, LTM formation was indistinguishable in both control and stressed conditions. This indicates that the underlying mechanisms controlling cognitive differences among populations may be highly conserved in L. stagnalis.

Food preferences and mound-building behaviour of the mound-building mice Mus spicilegus

Optimal foraging strategies and food choice are influenced by various factors, e.g. availability, size and caloric content of the food type and predation risk. However, food choice criteria may change when food is not eaten immediately but has to be carried to a storage site for later use. For example, handling time in terms of harvesting and transport time should be optimized, particularly when the risk of predation is high. Thus, it is not clear whether food selected by hoarding animals reflects their food preference due to intrinsic features of the food type, e.g. size, caloric or lipid content, or whether the food type selected is a compromise that also considers the handling time required for harvesting and transport. We investigate this question in relation to food hoarding behaviour in mound-building mice. In autumn, mound-building mice Mus spicilegus collect seeds and other plant material and cover it with soil. Such above-ground storage is quite unusual for rodents. Here, we investigated whether there is a relationship between the seed species preferred as building materials and those preferred for food. We conducted a seed preference test using three most collected weed species for mound building. Controlling factors like food availability or predation risk, mice prefer Setaria spp. as food, although Amaranthus spp. and Chenopodium spp. were preferentially harvested and stored. By including the availability of the three species, our experimental results were confirmed, namely, a clear preference for Setaria spp. Also, handling time and seed size revealed to influence plant choice.