Male Grey squirrel (Sciurus carolinensis) reproductive cycles in Britain

A synchronized seasonal cycle of sexual regression and development is described for the male Grey squirrel in Britain. Sexual development is maximal in December and May and minimal in August. The seasonal cycle can be overriden, but this is exceptional. Comparisons with previous work and the suggestion that the synchronized seasonal sexual cycle is general for Grey squirrels in Britain, and probably America, are made.     

Bark-stripping by Grey squirrels (Sciurus carolinensis)

To investigate why Grey squirrels strip bark from young trees, squirrel populations and tree quality were studied at 30 English Midland woods. In young beech and sycamore woodlands, the area of bark which Grey squirrels stripped from each tree was strongly related to the phloem width (volume per unit area) of the tree, and not to the phloem sugar content. Variation in bark-stripping between woods was strongly related to average phloem widths in each plantation. The extent of bark-stripping also correlated with juvenile squirrel density, but only in years with relatively little squirrel breeding. There was a tendency for damage to recur in previously damaged areas, in a pattern suggesting that squirrels had learned the habit. The results indicate that bark-stripping can be initiated by young squirrels, perhaps through agonistic gnawing or exploratory feeding, or by older squirrels which have learned the habit, but that severe damage occurs only where the tree phloem is suitable.     

What future for British red squirrels?

The geographic range of red squirrels contracted sharply in Britain during the 1940s arid 1950s, as increasingly large areas were colonized by the congeneric North American grey squirrel. Red squirrels remain common only on offshore islands, and in the large conifer forests of northern England and Scotland. The initial replacement of red squirrels was in arras dominated by oak woodland, probably because acorn crops are exploited less efficiently by red squirrels than by grey squirrels. Dirt studies have shown that acorns are digested less efficiently by the red squirrel, which occurs in conifers through most of its Eurasian range, than by the introduced grey squirrel, which is primarily a native of deciduous woodland. The red squirrel will probably be replaced in deciduous and mixed woodland throughout mainland Britain, and may eventually persist only in large areas of conifers which arc far from oak trees. The conservation of red squirrels on islands is therefore particularly important for their survival, perhaps making it worthwhile to create new island populations where they do not at present exist.     

Competition between Eurasian red and introduced Eastern grey squirrels: the energetic significance of body-mass differences

Daily energy expenditure (DEE) was measured in sympatric populations of red and grey squirrels using the doubly labelled water technique. Grey squirrels had significantly higher DEEs than red squirrels. However, the difference between the species was not separable from the effects of body mass on DEE. The DEEs of both species were in accordance with published allometric predictions incorporating body mass and ambient temperature. The differences in energetic requirements and social dominance, both consequences of body size, may represent means by which grey squirrels exert more interspecific competition on red squirrels than do conspecifics, potentially driving populations below viable levels in some sites.     

Interspecific effects of grey squirrels (Sciurus carolinensis) on the space use and population demography of red squirrels (Sciurus vulgaris) in conifer plantations

Interspecific competition between red squirrels and grey squirrels was investigated by comparing the population demography, spacing behavior and habitat use of red squirrels in two large conifer plantations in northern England: one site had only red squirrels (the red-only site), in the other both red and grey squirrels occurred (the red–grey site). Despite more abundant food at the red–grey site, red squirrel densities (0.26 ha^–1 at the red–grey site, 0.29 ha^–1 at the red-only site), adult survival rates and the breeding rates of females were similar at both study sites. Grey squirrels at the red–grey site occurred at higher densities (0.92–1.1 ha^–1) than did the reds and tended to have higher breeding rates. In the presence of grey squirrels, the recruitment pattern of red squirrels changed and there was little recruitment of subadults. The juvenile recruitment rate in the red–grey site (13%) was much lower than in the red-only site (50%). Grey squirrels, in contrast, had higher juvenile recruitment rates at the red–grey site (41%). The core areas of the home ranges of red squirrels in the red–grey site were more strongly overlapped by grey squirrels than by conspecifics. Red squirrels did not select the habitat with the best tree seed crop (Scots pine) but preferred dense Sitka spruce plantations; they appeared to avoid the Scots pine area with its high grey squirrel density. Data on foot length and body condition indicated decreased body growth in young red squirrels when grey squirrels were present. Our data suggest that adult red squirrels suffered little from interspecific competition with grey squirrels and that the key factor is decreased juvenile recruitment in red squirrels.     

The Mechanism of Replacement of Red Squirrels by Grey Squirrels: A Test of the Interference Competition Hypothesis

Introduced American grey squirrels have replaced native red squirrels in most of the range currently occupied in Britain and northern Italy. The mechanisms of the replacement are not yet fully understood. We restated the commonly cited Interference Competition Hypothesis (ICH) that grey squirrels interfere with the behaviour of red squirrels in three possible ways: 1. by direct aggressive interactions; 2. by interrupting red squirrel mating-chases; or 3. by forcing red squirrels to actively avoid areas intensively used by grey squirrels. We compared the activity pattern, behaviour and reproductive performance of red squirrels in two study areas in northern Italy, one with only red squirrels (control area C1), the other with both species (experimental area E1). The following predictions were tested: 1. the total time spent in both intraspecific and interspecific interactions by red squirrels increases in the experimental area; 2. most interspecific interactions are aggressive, with grey squirrels being the dominant species; 3. the proportion of breeding female red squirrels that are unsuccessful at weaning offspring increases in area E1; 4. grey squirrels take part and interfere with red squirrel mating-chases, and thereby decrease the reproductive output of red squirrel females; 5. the activity pattern of red squirrels in the mixed-species area is shifted with respect to that in the control area to the hours of the day during which grey squirrels show little activity; and 6. red squirrels will shift their home range (or at least their core-area) when grey squirrel densities increase to avoid interspecific core-area overlap. Our results supported only the first prediction of the ICH: they failed to support all the other predictions. Moreover, the increase in the percentage of active time red squirrels spent interacting with other squirrels in the experimental study area was very small (only 1–2 min/day). Red squirrels did not avoid the woodland patches most intensively used by grey squirrels and the interspecific core-area overlap was similar to red squirrel intraspecific core-area overlap. This suggested that red squirrels avoided spatial overlap with grey squirrels in a similar manner as with conspecifics and that an increase in grey squirrel numbers will augment the intensity of resource competition. We therefore conclude that our results do not lend support to the Interference Competition Hypothesis and that interference competition by grey squirrels cannot explain the large-scale replacement of red by grey squirrels that has occurred in Britain and in Piedmont.     

Seasonal changes in the testis and epididymis of the American Grey squirrel, Sciurus carolinensis

The histological changes in the testis and epididymis of the American Grey squirrel, Sciurus carolinensis (Gmelin) during development and the testicular cycle are described. It was observed that male Grey squirrels, as a population, did not undergo a pronounced seasonal reproductive cycle, however, individual males did experience periods of reproductive inactivity which were asynchronous and not related to seasonal changes in the environment. It is suggested that the individual sexual cycles are due to an endogenous rhythm of gonadotrophin secretion by the pituitaries of these animals.     

Effects of Parasitism and Morphology on Squirrelpox Virus Seroprevalence in Grey Squirrels (Sciurus carolinensis)

Invasive species have been cited as major causes of population extinctions in several animal and plant classes worldwide. The North American grey squirrel (Sciurus carolinensis) has a major detrimental effect on native red squirrel (Sciurus vulgaris) populations across Britain and Ireland, in part because it can be a reservoir host for the deadly squirrelpox virus (SQPV). Whilst various researchers have investigated the epizootiology of SQPV disease in grey squirrels and have modelled the consequent effects on red squirrel populations, less work has examined morphological and physiological characteristics that might make individual grey squirrels more susceptible to contracting SQPV. The current study investigated the putative relationships between morphology, parasitism, and SQPV exposure in grey squirrels. We found geographical, sex, and morphological differences in SQPV seroprevalence. In particular, larger animals, those with wide zygomatic arch widths (ZAW), males with large testes, and individuals with concurrent nematode and/or coccidial infections had an increased seroprevalence of SQPV. In addition, males with larger spleens, particularly those with narrow ZAW, were more likely to be exposed to SQPV. Overall these results show that there is variation in SQPV seroprevalence in grey squirrels and that, consequently, certain individual, or populations of, grey squirrels might be more responsible for transmitting SQPV to native red squirrel populations.     

Heavy metal concentrations in kidneys of urban gray squirrels

Concentrations of lead and zinc in the kidneys of 180 urban gray squirrels were determined by spectrophotometry and found similar for all age groups; however, concentrations of cadmium increased up to two years of age. Values for 12 rural squirrels were significantly lower than those of the urban animals. There were no differences in mean concentrations of the metals when urban squirrels were grouped by the land usage pattern of the sites in which they were captured. Grouping squirrels by human socioeconomic strata for the city revealed that squirrels in low socioeconomic areas have significantly higher levels of lead than animals residing in middle or high socioeconomic areas.     

Landscape scale impacts of culling upon a European grey squirrel population: can trapping reduce population size and decrease the threat of squirrelpox virus infection for the native red squirrel?

The control of grey squirrels (Sciurus carolinensis) is widely undertaken as a conservation measure to protect red squirrel (Sciurus vulgaris) populations in the UK. However, inconsistencies and omissions in data collection, as well as fluctuating financial resourcing of control efforts, have meant that it has to date proved difficult to quantify the impact of any regional control initiative upon populations. Here we have scrutinized a 13 years period (1998–2010) within an ongoing grey squirrel control project that reflects the resource challenges typically faced by red squirrel conservation programmes. We present evidence that despite variation in grey squirrel control intensity, the abundance of grey squirrels ultimately decreased significantly. Trapping success was significantly higher in spring and summer months and a greater abundance of grey squirrels was found in deciduous woodland and hazel dominated scrub relative to other habitats; two findings that reinforce existing guidance within national control best practice. Grey squirrels carry an infection that causes epidemic pathogenic disease if spread to the native red squirrel. We observed that the proportion of seropositive grey squirrels decreased constantly from 2003 to 2010 when only 4 % of sampled animals were seropositive. This discovery indicates that culling can in parallel remove both the competitive and disease threat posed to red by grey squirrels. The historical paucity of scientific data on the effectiveness of grey squirrel control as a tactic in UK red squirrel conservation means that the findings of this study will significantly advance conservation best practice and inform the development of future national strategy.     

Conservation of genetic uniqueness in remaining populations of red squirrels (Sciurus vulgaris L.) in the South of England

The Eurasian red squirrel (Sciurus vulgaris) is an emblematic species for conservation, and its decline in the British Isles exemplifies the impact that alien introductions can have on native ecosystems. Indeed, red squirrels in this region have declined dramatically over the last 60 years due to the spread of squirrelpox virus following the introduction of the gray squirrel (Sciurus carolinensis). Currently, red squirrel populations in Britain are fragmented and need to be closely monitored in order to assess their viability and the effectiveness of conservation efforts. The situation is even more dramatic in the South of England, where S. vulgaris survives only on islands (Brownsea Island, Furzey Island, and the Isle of Wight). Using the D‐loop, we investigated the genetic diversity and putative ancestry of the squirrels from Southern England and compared them to a European dataset composed of 1,016 samples from 54 populations. We found that our three populations were more closely related to other squirrels from the British Isles than squirrels from Europe, showed low genetic diversity, and also harbored several private haplotypes. Our study demonstrates how genetically unique the Southern English populations are in comparison with squirrels from the continental European range. We report the presence of four private haplotypes, suggesting that these populations may potentially harbor distinct genetic lineages. Our results emphasize the importance of preserving these isolated red squirrel populations for the conservation of the species.     

A grey future for Europe: Sciurus carolinensis is replacing native red squirrels in Italy

Introduced mammals can cause extinction of native species due to replacement competition, disease, predation or hybridization. We studied the colonization of Piedmont (NW-Italy) by American grey squirrel (Sciurus carolinensis) and its effect on the native red squirrel (Sciurus vulgaris). Presence/absence data (2 × 2 km²), of both species were (re)constructed using questionnaires, literature, existing databases, unpublished information, and direct monitoring with hair-tubes. In 1970 red squirrels were still widespread and greys were restricted to forests near the introduction site. By 1990, grey squirrels had increased their range to 220 km², which coincided with the disappearance of native squirrels from 33 squares inside this range. The invasive species continued its spread occupying an area of 2,016 km² in 2010; within this area red squirrels went extinct in 88 squares. Overall, from 1970 to 2010 red squirrel went extinct in 62 % of 2 × 2 km² (ca. 1,689 km²), and were replaced by grey squirrels. The spread of the alien species was slow in the first 20 years, but doubled in the successive two decades. Nevertheless spread was slower than in Ireland and England. Grey squirrel adapt to climate and habitats in both North and South Europe, causing extinction of the native red squirrel. A EU LIFE co-funded project with the aim to control the grey squirrel in North Italy and recent trade-restrictions and trade-ban are a first step in reducing the risk of grey squirrels invading other countries, but their effectiveness will have to be evaluated.     

Neonatal androgenization in ground squirrels: influence on sex differences in body mass and luteinizing hormone levels

Female squirrels were injected at birth with 50 or 1000 micrograms testosterone propionate (TP); control males and females were treated with oil vehicle. Squirrels were gonadectomized at 47 days of age. Body mass was recorded weekly and plasma luteinizing hormone (LH) was determined once monthly over the next year. Marked annual cycles in body mass were manifested by 30 out of 31 squirrels. Peak body mass and peak-to-trough differences were greater for control male and TP-female squirrels than for control female squirrels. Trough body weights did not differ among the groups. Luteinizing hormone was detectable in all male and most androgenized females but not in any control female squirrels during the first 4 mo after gonadectomy. Peak LH values were significantly greater for control male than for control female squirrels and were not influenced by neonatal androgenization in females. Testosterone propionate treatment also did not affect sex differences in timing of LH peaks or the total number of months in which LH was detectable. We conclude that testicular hormones secreted during the early postnatal period induce sex differences in the circannual pattern of weight change and some aspects of LH secretion. Complete masculinization, however, either requires more extensive action of gonadal hormones, perhaps both pre- and postnatally, or occurs through some androgen-independent mechanism.     

Diabetes mellitus in the 13-lined ground squirrel (Citellus tridecemlineatus).

Spontaneous diabetes mellitus was diagnosed in six of 126, 13-lined ground squirrels, Citellus tridecemlineatus. Serum glucose values were significantly higher in the diabetic ground squirrels than in the non-diabetic ground squirrels, while serum insulin values of fasted diabetic squirrels were significantly lower than fasted nondiabetic ground squirrels. In addition, the classic diabetic signs of poly-dipsia, polyuria, glycosuria, ketonuria, polyphasia, and weight loss were present. The proportion of islet tissue to total pancreatic area in diabetic ground squirrels was less than 25% of that in the nondiabetic ground squirrels. Both the number and size of the islets of Langerhans in diabetic ground squirrels were less than those in nondiabetic ground squirrels.     

Nomenclature for seal research in Britain

A wide terminology is now in use in the study of seals. Terms which are useful for research on Grey seals (Halichoerus grypus and Common seals (Phoca vitulina)in Great Britain are redefined.     

Reintroducing Red Squirrels into Regent's Park

In late 1984 10 young Red Squirrels ( Sciurus vulgaris ) trapped in Scotland were released into Regent's Park and monitored by radio-telemetry. We wanted to investigate the reasons why the introduced Grey Squirrel has replaced the Red over much of Britain; disease, aggression and ecological competition have all been suggested as the cause. The relevant guidelines governing policy on reintroductions were followed. A few months after release, the animals appear well established. They benefit from a specially-designed hopper which allows them to feed from it but prevents Grey Squirrels doing so.     

Persistence of weight gain and hibernation onset in juvenile thirteen-lined ground squirrels (Spermophilus tridecemlineatus) in spite of long-term administration of naloxone

Sexual differences in body weight of juvenile thirteen-lined ground squirrels (Spermophilus tridecemlineatus) were significant (P less than 0.05) at all weeks of age except weeks 0-4, 6, 7, 9, 20 and 24. Hibernation onset between sexes did not differ significantly. Naloxone administration did not alter weight gain nor onset of hibernation when compared to saline controls.     

Evaluation of the eye lens method for age determination in roe deer<Emphasis Type="Italic">Capreolus capreolus</Emphasis>

Eye lens weight curves were obtained from a sample of 522 male and 171 female free-living roe deerCapreolus capreolus (Linnaeus, 1758), collected during the regular annual harvest 1998/99–2005/06 (May–January) in 12 different field hunting grounds in Vojvodina (North part of Serbia). We found no significant differences in eye lens weight between males and females within the same age class (except age class 1). Thus, the eye lens weight data for both sexes within the same age class were pooled for further analyses. The rate of increase in eye lens weight declined at a relatively early age, but lenses continued to increase in weight through age class 9. However, there was a considerable overlap in ranges of eye lens weights within all age classes, which restricted the usefulness of this method. The estimate precision of the eye lens method in determining age was evaluated by the comparison of “known age” (cementum layer technique) and predicted age (eye lens weight technique). We examined model validity on 78 roe deer not used previously to construct the regression line. The lack of correspondence between the two techniques was evident among individuals within older age classes. The percentage of correctly determined age rapidly declined already from age class 4 (<30%).     

Where's my dinner? Adult neurogenesis in free‐living food‐storing rodents

Postnatal hippocampal neurogenesis in wild mammals may play an essential role in spatial memory. We compared two species that differ in their reliance on memory to locate stored food. Yellow-pine chipmunks use a single cache to store winter food; eastern gray squirrels use multiple storage sites. Gray squirrels had three times the density of proliferating cells in the dentate gyrus (determined by Ki-67 immunostaining) than that found in chipmunks, but similar density of young neurons (determined by doublecortin immunostaining). Three explanations may account for these results. First, the larger population of young cells in squirrels may increase the flexibility of the spatial memory system by providing a larger pool of cells from which new neurons can be recruited. Second, squirrels may have a more rapid cell turnover rate. Third, many young cells in the squirrels may mature into glia rather than neurons. The densities of young neurons were higher in juveniles than in adults of both species. The relationship between adult age and cell density was more complex than that has been found in captive populations. In adult squirrels, the density of proliferating cells decreased exponentially with age, whereas in adult chipmunks the density of young neurons decreased exponentially with age.     

社鼠的年龄鉴定与种群年龄组成

年龄组成是种群的重要特征之一。分析年龄组成中不同时期各年龄组数量的比例,有助于了解种群数量变动的规律。寿振黄等(1959)以臼齿齿根的分岐与否和前后根的长度作为红背(鼠平)的年龄标准。罗泽珣(1963)和诸葛阳等(1959、1978)分别用臼齿磨损程度和体重来划分黑线姬鼠的年龄。国外有以晶体干重为依据分析年龄组成,如Lord(1959),Adamczewska-Andrzejewska(1972),Geurley(1975),Yabe(1979)等分别对白尾兔(Cottontail rabbit)、黑线姬鼠(Apodemus agrarius)2种田鼠(Microtus pinetorum和Microtus montanus)、和褐家鼠(Rattus norvegicus)进行了不少工作。