Population structure and reproduction in brown rats (Rattus norvegicus) from pig farms, Co. Kildare, Ireland

Recovery of small rodent populations after population collapse

In this review we summarise published knowledge regarding small mammal population recovery following sudden population collapse, regardless as to whether the collapse is caused by natural or man-made events. We determine recovery mechanisms, recovery time and recovery rate, and suggest how to adapt and optimise current methods to regulate small mammal population size, for pest management and/or conservation. It is vital that the principles underlying the recovery mechanisms are known for both pest control and conservation to align management methods to either maintain animal numbers at a permanent minimum level or increase population size. Collapses can be caused naturally, as in the declining phase of multi-annual fluctuations and after natural disasters, or by man-made events, such as pesticide application. In general, there are three ways population recovery can occur: (1) in situ survival and multiplication of a small remaining fraction of the population; (2) immigration; or (3) a combination of the two. The recovery mechanism strongly depends on life history strategy, social behaviour and density-dependent processes in population dynamics of the species in question. In addition, the kind of disturbance, its intensity and spatial scale, as well as environmental circumstances (e.g. the presence and distance of refuge areas) have to be taken into account. Recovery time can vary from a couple of days to several years depending on the reproductive potential of the species and the type of disturbances, regardless of whether the collapse is man made or natural. Ultimately, most populations rebound to levels equal to numbers before the collapse. Based on current knowledge, case-by-case decisions seem appropriate for small-scale conservation. For pest control, a large-scale approach seems necessary. Further investigations are required to make sound, species-specific recommendations.

Placentophagia in weanling female laboratory rats

Placentophagia is common in parturient mammals and offers physiological and behavioral advantages for mothers. In natural environments, weanlings are often present during the birth of younger siblings, but it is unknown if weanling rats are placentophagic or prefer placenta over other substances. To examine this, primiparous rats were remated during the postpartum estrus and their weanling daughters remained in the natal nest during their mother's next parturition. Continuous observation revealed that 58% of weanlings were placentophagic. To determine if this placentophagia occurs away from parturient mothers, weanling females still living in their natal nest were offered placenta, liver, or cake frosting in a novel chamber. They ingested more placenta and liver than frosting. Thus, many weanling female laboratory rats are placentophagic during the birth of younger siblings but do not selectively prefer placenta when tested outside their natal nest. Consequences of placentophagia by weanling female rats are unknown, but it may promote their alloparenting or later postpartum mothering.

15N signatures do not reflect body condition in Arctic ground squirrels

Studies using stable-isotope analysis documented an enrichment in δ15N values in nutritionally stressed animals. Investigators suggested that changes in δ15N values measured in urine, hair, and blood may be a good indicator of lean-tissue losses. During our investigations into the effects of population density on body condition and reproduction of female Arctic ground squirrels (Spermophilus parryii plesius) near Kluane Lake, Yukon, Canada, we examined the relations between body condition and δ15N values. Data obtained from 20 livetrapped female ground squirrels suggested that reproductive females from a population with moderate density and low food availability experienced a reduction in body condition, as indicated by mass loss and changes in blood urea nitrogen (BUN) and glucose concentrations. In contrast, those from a population that failed to reproduce successfully and had high density and low food availability experienced no nutritional stress. Similarly, those females from a high-density population with high food availability (i.e., supplemented food) that reproduced successfully suffered no noticeable nutritional stress. In contrast to our prediction, δ15N values did not show a decline with increasing body mass, and animals in poor and excellent body condition had similar δ15N values. In addition, female ground squirrels from the same group with access to similar types of food (natural or supplemented) and with similar body masses, BUN, and blood glucose concentrations showed a difference of up to 1.8‰ in δ15N values. Thus, our results suggest that the ecological process (i.e., diet selection) may have obscured the physiological one (i.e., recycling of nitrogen). Therefore, we recommend that field ecologists studying animal diets using stable-isotope analysis use alternative techniques when attempting to evaluate the body condition of their subjects.

Timing of reproduction in wild mink: the influence of spawning Pacific salmon

In many species of seasonally breeding mammals, reproduction occurs later at higher latitudes. Records of timing of reproduction in free-ranging American mink (Mustela vison) in North America and Europe suggest a similar trend. Observations on mink in southeast Alaska, however, revealed a deviation from this pattern, suggesting that factors other than latitude and associated day length may affect timing of breeding for this species in this area. I investigated timing of reproduction and body condition of wild, free-ranging mink on Chichagof Island, southeast Alaska, and hypothesized that seasonal food availability, especially abundant carcasses of spawning Pacific salmon (Onchorhynchus sp.), would determine timing of breeding in this population of mink. Blood progesterone levels, body condition, and testicle lengths were recorded for 24 adult mink, livetrapped from mid-March to late July in 1992 and 1993. Results suggest that these free-ranging mink mate during the later part of April to early May, and parturition occurs in late June to early July. Although male mink seemed to respond to photoperiodism in initiating reproduction, timing of reproduction in female mink was shifted so that lactation coincided with the availability of carcasses of Pacific salmon.