A role for thermoregulation in the <Emphasis Type="Italic">Polistes dominulus</Emphasis> invasion: a comparison of the thermoregulatory abilities of the invasive wasp <Emphasis Type="Italic">P. dominulus</Emphasis> and the native wasp <Emphasis Type="Italic">P. fuscatus</Emphasis>

Social insects are excellent invaders that have had negative impacts on native species and humans. Many invasive species move from warmer to cooler climates. For these species, thermal adaptations may both be important for their ability to invade and to limit their invasion range. The invasion of Polistes dominulus into North America provides an example of a primitively eusocial invader from a warmer climate. We studied the differences in thermoregulation between P. dominulus and the native P. fuscatus. We found that, during flight, thorax temperature in P. fuscatus was less affected by ambient temperature than thorax temperature of P. dominulus. We also found that P. dominulus and P. fuscatus showed different patterns of warming after removal from a cold environment. Unlike P. dominulus, live P. fuscatus never fully cooled down in a cold environment. P. fuscatus also reached their relative minimum flight temperatures earlier than P. dominulus, but P. dominulus maintained higher elevated temperatures for longer. These differences in thermoregulatory ability suggest that the lower winter survival of P. dominulus could be offset by a greater thermal tolerance during flight, while the lower thermal tolerance of P. fuscatus in flight is offset somewhat by better thermoregulatory ability.     

A ten-year comparative study of the population dynamics and parasitoidism in the native paper wasp Polistes fuscatus and the invasive P. dominulus

The invasive success of Polistes dominulus in North America has been attributed to its greater productivity relative to native Polistes. Liberation from parasites and parasitoids are thought to be major factors contributing to the high productivity of P. dominulus. We analyzed historical records of colony relative abundance and productivity of P. dominulus and the sympatric, native Polistes fuscatus from 1995 to 2010 using historical data from our Michigan Polistes study site. We also analyzed evidence of parasitoids from 294 P. fuscatus and 507 P. dominulus archived combs from 2001 to 2010. Additionally, we examined field and laboratory colonies from outside of our study site for parasites and parasitoids in 2009 and 2010. We documented one parasite and three parasitoids exploiting Polistes in our Michigan study sites. Our historical records document that P. dominulus initially displaced P. fuscatus rapidly, then slowed, and finally the two populations stabilized. Furthermore, the historical pattern of decreasing displacement of P. fuscatus by P. dominulus corresponded temporally with a significant decline in the productivity and a significant increase in Dibrachys cavus infestation of P. dominulus. Our evidence indicates that the parasitoid, D. cavus, is a major factor in stabilizing the populations of the sympatric P. dominulus and P. fuscatus.     

The comparative biology of two sympatric paper wasps in Michigan,the native <Emphasis Type="Italic">Polistes fuscatus</Emphasis> and the invasive <Emphasis Type="Italic">Polistes dominulus</Emphasis> (Hymenoptera,Vespidae)

Summary The biology of the invasive Polistes dominulus and the native P. fuscatus was compared at a field site in Rochester, Michigan over a two-year period. Colonies nesting semi-naturally in plywood nestboxes were studied using videography, extensive surveys, and colony-specific marking of gynes.Both single- and multiple-foundress colonies of P. dominulus were significantly more productive than comparable colonies of P. fuscatus. The disparity in productivity was significantly more pronounced in single-foundress colonies than in multiple-foundress colonies. P. dominulus had significantly shorter larval and pupal development times than P. fuscatus, which allowed P. dominulus to produce its first workers about a week earlier than P. fuscatus. P. dominulus had a number of additional advantages over P. fuscatus that contributed to its productivity including (1) significantly less parasitism by Strepsiptera, (2) significantly greater probability of renesting after predation by raccoons, (3) significantly lower usurpation pressures, and (4) possibly longer foraging days . The recovery of colony-marked foundresses indicated that gynes of P. dominulus suffered significantly greater mortality than gynes of P. fuscatus during winter diapause and that foundresses of both species were equally, strongly philopatric.P. dominulus is likely replacing P. fuscatus in many areas of southeastern Michigan via indirect or exploitative competition. The two species may be competing for nest sites.Received 7 July 2003; revised 10 October 2003; accepted 3 November 2003.     

The cost of flight: a role in the <Emphasis Type="Italic">Polistes dominulus</Emphasis> invasion

Polistes dominulus is a primitively eusocial paper wasp from Mediterranean Europe that is invasive to North America. In Eastern North America, P. dominulus is in competition with P. fuscatus. One reason for the success of P. dominulus is that their colonies produce more reproductive offspring than P. fuscatus colonies. A partial explanation for this difference is that P. dominulus foundresses make more foraging trips in the pre-worker period, which likely helps them to rear workers more quickly. In comparing the species, we found that P. dominulus had a lower absolute flight metabolic rate, but that P. fuscatus had a lower mass-specific flight metabolic rate. In addition, in P. fuscatus, wingloading correlated with flight metabolic rate, but that this was not the case in P. dominulus. This suggests that P. fuscatus is not able to transport large loads inexpensively. Therefore, the lower overall cost of transport of P. dominulus may provide an advantage by allowing the foundresses to make more relatively efficient foraging trips. In addition, we compared time in flight by P. dominulus and P. fuscatus over a range of temperatures and found that while P. fuscatus flew well over a broad range of temperatures, P. dominulus had a relatively narrow range of optimal temperatures for flight (30–33°C). These differences may help explain both the success and the limitations of the P. dominulus invasion.     

The comparative biology of two sympatric paper wasps, the native Polistes fuscatus and the invasive Polistes dominulus (Hymenoptera, Vespidae)

Summary: Polistes dominulus (Christ), an old world paper wasp, was introduced accidentally into the eastern coast of the United States in the late 1970s and has been rapidly spreading westward, displacing the native P. fuscatus (F.). The biology of naturally nesting P. fuscatus and P. dominulus was compared at a field site in Rochester, Michigan. The basic methodology consisted of simultaneously videotaping spatially proximate, matched single-foundress colonies of P. fuscatus and P. dominulus (13 matched sets, 176.8 h of videography). In addition, extensive surveys and censuses were taken of colonies to record colony productivity, dates of nest initiation and first worker emergence, usurpation and parasitism.¶There was no evidence that P. dominulus is negatively impacting P. fuscatus through direct, agonistic encounters. However, P. dominulus is 4-5 times more productive than P. fuscatus, suggesting that P. dominulus is replacing P. fuscatus via exploitative competition. P. dominulus appears to have a number of advantages over P. fuscatus, including earlier production of workers, higher per capita foraging rates by queens and workers, higher queen survivorship, and lack of conspecific pressures. Nest site and/or prey availability may be limiting factors in the competition between the two species.     

Simulated Intraspecific Usurpations in Paper Wasps: Different Reproductive Tactics Affect Foreign Brood Destruction in Polistes fuscatus and Polistes dominulus

We simulated intraspecific usurpation in two species of paper wasps at a field site in Michigan to compare the species' treatment of foreign brood. Queens of Polistes fuscatus, a species that commonly uses intraspecific usurpation as an alternative reproductive tactic, destroyed significantly fewer large larvae and significantly more small larvae than queens of P. dominulus, a sympatric species that rarely usurps. The pattern of brood destruction exhibited by P. fuscatus was consistent with the previously published findings that P. fuscatus usurpers destroy reproductive‐destined brood (eggs and small larvae), but not worker‐destined brood (large larvae and pupae) that are subsequently used by a usurper to raise her own reproductives. The pattern of brood destruction displayed by P. dominulus differed from that of Polistes species that frequently engage in intraspecific usurpation. The brood destruction pattern in P. dominulus may have been shaped by nest adoption, a common alternative reproductive tactic in this species. If so, it is not clear why P. dominulus would destroy large, worker‐destined larvae.     

First detailed report of brood parasitoidism in the invasive population of the paper wasp Polistes dominulus (Hymenoptera, Vespidae) in North America

The European paper wasp Polistes dominulus (Christ) is a model system in the fields of behavioral ecology, ecological immunology, and invasion biology. Since its introduction to the US in 1978, its invasion success has been attributed, in part, to a lack of parasites or parasitoids infecting this population. This is despite the number of parasites which infest the native population and the generalist polistine parasites and parasitoids documented in sympatric North American species. Multiple studies have cited low parasite pressure as evidence that the invasive population of P. dominulus is benefiting from a post-invasion release from enemies. Here, we present the first well documented case of parasitoidism of the invasive population of P. dominulus in North America.     

Rearing conditions influence quality signals but not individual identity signals in Polistes wasps

Given the diversity of animal signals, there has been recentinterest in categorizing signals into probable functions accordingto their properties. For example, models predict that signalsof quality should be costly and condition dependent, whereassignals of individual identity should be cheap and expressedindependently of condition. Here, we test these predictionsby comparing the condition dependence of signals of individualidentity and quality in Polistes wasps. Polistes fuscatus waspshave black and yellow patterns on the face and abdomen thatsignal individual identity, whereas Polistes dominulus waspshave black and yellow facial patterns that signal aspects ofquality related to dominance. We reared both species with andwithout supplemental food and examined the facial patterns ofthe resulting offspring. As predicted, food availability didnot influence the development of identity signals in P. fuscatus.In strong contrast, P. dominulus wasps reared with supplementalfood had facial patterns that signaled higher levels of qualitythan P. dominulus reared without supplemental food. Interestingly,the identity and quality signals have different condition dependence,despite being composed of similar pigments, suggesting thatsignal function has a stronger influence on signal propertiesthan pigmentation. Because body size is often correlated withquality signal elaboration, we also tested how food supplementationinfluenced offspring size. In both species, supplemented coloniesproduced smaller offspring than nonsupplemented colonies, suggestingthat queens may invest in producing fewer, larger offspringin stressful environments.     

Nest recognition in neighboring colonies: a comparison of two European species of Polistes wasps (P. dominulus and P. nimphus, Hymenoptera: Vespidae)

The ability of two species of Polistes wasps to distinguish their own from nearby nests was tested, following the procedure used by Espelie et al. Our experiments demonstrated that, in the laboratory, females of Polistes dominulus and Polistes nimphus preferentially selected their own nests rather than nearby nests. We also evaluated the role of odor cues in nest recognition by washing nests in hexane to remove the apolar solvent-soluble components of nest odor. Although P. nimphus females continued to discriminate nests even after washing, P. dominulus individuals failed to discriminate between their own and a foreign neighboring nest. In both species, wasps were able to recognize their own nests when nest extracts were subsequently reapplied to the nest surface. These results indicate that P. dominulus wasps recognize their nests through perception of nest odor. The ability of P. nimphus wasps to distinguish their own nests even after presumed removal of the nest odor is discussed. Received: January 27, 2000 / Accepted: May 22, 2000     

A test of the haploid susceptibility hypothesis using a species with naturally occurring variation in ploidy

The haploid susceptibility hypothesis (HSH) was proposed as an explanation for how behavioral roles in haplodiploid social systems evolved. It posits that haploid males are more susceptible to disease than diploid females due to decreased genetic variability at key disease resistance loci. The resulting decreased immunocompetence is hypothesized to have played a role in the evolution of social behavior by limiting the behavioral repertoire haploids perform. Here, we test this hypothesis in a study system that separates ploidy from behavioral sex roles: Polistes dominulus, a social wasp, has colonies with naturally occurring diploid males. We report results from two immune function assays—hemolymph phenoloxidase activity and encapsulation response—performed on haploid males, diploid males, and diploid females. Our data suggest that ploidy is not a significant contributor to immune function in P. dominulus; thus, our data do not support the HSH for the evolution of behavioral roles. Instead, our data indicate that time of emergence is the best predictor of immune function in Polistes. We speculate that seasonal trends result from seasonal differences in pathogens and parasites.     

Forager abundance and its relationship with colony activity level in three species of South American Pogonomyrmex harvester ants

The proportion of foragers in ant colonies is a fairly constant species-specific characteristic that could be determined by intrinsic or extrinsic factors. If intrinsic factors are relevant, species with similar life history characteristics (e.g., colony size and foraging strategies) would be expected to have a similar proportion of foragers in their colonies. Within the genus Pogonomyrmex, North American species can vary largely in their colony size, whereas only species with small colonies are known in South America. We studied the characteristics of the foraging subcaste in three sympatric South American species of Pogonomyrmex harvester ants, and compared it with the available information on other species of the same genus. We used two mark-recapture methods and colony excavations to estimate the number and proportion of foragers in the colonies of P. mendozanus, P. inermis, and P. rastratus, and to test the relationship between forager external activity levels and abundance per colony. Forager abundance in the three studied species was lower than in most North American species. The percentage of foragers in their colonies ranged 7–15 %, more similar to North American species with large colonies than to those with small colony size. Foraging activity was positively correlated with forager abundance in all three species, implying that colony allocation to number of foragers allows for higher food acquisition. Further comparative studies involving a wider range of traits in South and North American species would allow to unveil the role of environmental factors in shaping each species’ particular traits.     

Wolbachia in the invasive European paper wasp Polistes dominulus

The European paper wasp Polistes dominulus has been expanding its North American range since its introduction in the 1970s. We screened P. dominulus from Italy and the northeastern U.S. for the presence of the intracellular reproductive symbiont Wolbachia. Infection rates among females varied from 16% to 87% among U.S. sites and from 33% to 71% in Italy. We also found infected haploid and diploid males, indicating that this is not a male-killing Wolbachia infection. Our data show that infected individuals from New York, Massachusetts, and Italy carry the same Wolbachia strain, and that some mtDNA haplotypes include both infected and uninfected individuals. We discuss possible implications of Wolbachia infection in this invasive social hymenopteran. Received 28 October 2005; revised 25 January 2006; accepted 17 February 2006.     

Predatory wasps learn to overcome the shelter defences of their larval prey

Larvae of Epargyreus clarus (Hesperiidae), the silver-spotted skipper, inhabit leaf-and-silk shelters that they construct on their leguminous host plants. In the field, Polistes spp. (Vespidae) wasps land on the shelters, quickly extracting and killing the larvae within. In marked contrast, wasps that emerge from field-collected colonies maintained in the laboratory visit and examine leaflets bearing sheltered caterpillars, but only rarely do they extract and kill the sheltered larvae. To examine whether learning is involved in the development of the ability of Polistes wasps to forage successfully on sheltered E. clarus larvae, we tested the responses of P. fuscatus and P. dominulus wasps to sheltered E. clarus larvae before and after their exposure to unsheltered larvae that were visible either on an opened host-leaf shelter (P. fuscatus and P. dominulus) or on a nonhost leaf in the absence of a shelter (P. fuscatus). After killing and processing an unsheltered larva that was visible on an opened leaf shelter, a majority of foragers subsequently extracted and killed larvae from closed shelters. Wasps that killed and processed an unsheltered larva on a nonhost leaf, on the other hand, generally did not later open shelters. Thus, it seems that experience with an exposed larva in the context of its shelter is necessary for a wasp to be able to prey on sheltered larvae. We conclude that the wasps must learn to associate the taste of the larva with shelter-related cues, such as presence of leaf damage and silk. In nature, this initial exposure may occur when the larva is visible in or near its shelter, perhaps when feeding or constructing a new shelter. Learning opportunities will thus depend on larval density. Our results show that invertebrate predators can learn to overcome their prey's defences, and are therefore able to make use of previously inaccessible prey.     

Beneath the bark: associations among Sirex noctilio development,bluestain fungi,and pine host species in North America

1. Invasive species with global distributions encounter unique environmental and biotic variables that can greatly affect the magnitude of their impact. The European woodwasp, Sirex noctilio Fabricius, is a prime example that has invaded climatically and ecologically distinct ecosystems across the Southern Hemisphere and, more recently, North America. 2. Northeastern North America presents a unique set of conditions, including pine host species, native parasitoids, a diverse assemblage of native co‐colonising insects, and fungal associates of these co‐colonisers. In North America, S. noctilio attacks both a naturalised ancestral host (Pinus sylvestris L.) from Europe and a naïve native host (P. resinosa). A large assemblage of insects and their associated bluestaining ophiostomatoid fungi colonise these pines. Competition between S. noctilio and this group is a hypothesised mechanism of biotic resistance in the invaded region of North America, possibly via superior resource capture abilities or alteration of host tissue by bluestain fungi. 3. Investigating these ecological interactions is challenging because they manifest deep in the xylem tissue. To overcome this, 30 experimentally stressed trees were systematically dissected with an electric log splitter to investigate the effects of bluestain fungi and tree factors on S. noctilio development and parasitism by native hymenopterans. 4. Body size and colonisation density were affected by pine species, with S. noctilio being 25% larger and densities three‐fold greater in P. sylvestris than P. resinosa. Survivorship was slightly negatively correlated with the proportional volume of bluestain infection. Interestingly, rhyssine (Rhyssa and Megarhyssa spp.) parasitism responded positively to greater S. noctilio density, but there was no density relationship with Ibalia leucospoides ensiger parasitism. Pine host species appears to play a strong role in S. noctilio development, which is important considering uninvaded regions of North America have a diversity of pine species that likely vary in their susceptibility to this woodborer.     

Antibodies to Pseudogymnoascus destructans are not sufficient for protection against white‐nose syndrome

White‐nose syndrome (WNS) is a fungal disease caused by Pseudogymnoascus destructans (Pd) that affects bats during hibernation. Although millions of bats have died from WNS in North America, mass mortality has not been observed among European bats infected by the fungus, leading to the suggestion that bats in Europe are immune. We tested the hypothesis that an antibody‐mediated immune response can provide protection against WNS by quantifying antibodies reactive to Pd in blood samples from seven species of free‐ranging bats in North America and two free‐ranging species in Europe. We also quantified antibodies in blood samples from little brown myotis (Myotis lucifugus) that were part of a captive colony that we injected with live Pd spores mixed with adjuvant, as well as individuals surviving a captive Pd infection trial. Seroprevalence of antibodies against Pd, as well as antibody titers, was greater among little brown myotis than among four other species of cave‐hibernating bats in North America, including species with markedly lower WNS mortality rates. Among little brown myotis, the greatest titers occurred in populations occupying regions with longer histories of WNS, where bats lacked secondary symptoms of WNS. We detected antibodies cross‐reactive with Pd among little brown myotis naïve to the fungus. We observed high titers among captive little brown myotis injected with Pd. We did not detect antibodies against Pd in Pd‐infected European bats during winter, and titers during the active season were lower than among little brown myotis. These results show that antibody‐mediated immunity cannot explain survival of European bats infected with Pd and that little brown myotis respond differently to Pd than species with higher WNS survival rates. Although it appears that some species of bats in North America may be developing resistance to WNS, an antibody‐mediated immune response does not provide an explanation for these remnant populations.     

Diet and seed removal rates by the harvester ants Pogonomyrmex rastratus and Pogonomyrmex pronotalis in the central Monte desert, Argentina

Granivorous animals, through seed consumption, may have an important influence on plant abundance, distribution and species composition in desert ecosystems. The aims of this study are twofold: to quantify the diet of Pogonomyrmex rastratus (Mayr) and pogonomyrmex pronotalis (Santschi), and to estimate seed removal per colony of both species throughout their activity season (October-April) in the central Monte desert, Argentina. Both species rely heavily upon seeds, which account for 87–94 % of the items carried to the nests. Their diets are similar, consisting mainly of grass seeds, which represent more than 93 % of the seeds. Among them, three species predominate: Aristida spp., Trichloris crinita and Pappophorum spp. Seasonal variations as well as seed species richness in the diet are also similar between species. However, their food-handling behaviour differs: most caryopses carried by P. pronotalis bear bracts whereas most caryopses carried by P. rastratus lack them. Seed removal per colony by P. rastratus (6 × 10⁴ seeds/colony) and by P. pronotalis (5 × 10⁴ seeds/colony) throughout the season is similar to the one reported for P. occidentalis in North America. However, seed removal per hectare, which could be estimated for P. rastratus (8.3 × 10⁵ seeds/ha), is lower than removal rates reported for the North American species P. barbatus, P. desertorum, P. rugosus and P. californicus, probably because P. rastratus has lower activity levels and smaller colonies than the North American studied species. Received 6 April 2005; revised 1 September 2005; accepted 2 September 2005.     

Parasite spillover: indirect effects of invasive Burmese pythons

Identification of the origin of parasites of nonindigenous species (NIS) can be complex. NIS may introduce parasites from their native range and acquire parasites from within their invaded range. Determination of whether parasites are non‐native or native can be complicated when parasite genera occur within both the NIS’ native range and its introduced range. We explored potential for spillover and spillback of lung parasites infecting Burmese pythons (Python bivittatus) in their invasive range (Florida). We collected 498 indigenous snakes of 26 species and 805 Burmese pythons during 2004–2016 and examined them for lung parasites. We used morphology to identify three genera of pentastome parasites, Raillietiella, a cosmopolitan form, and Porocephalus and Kiricephalus, both New World forms. We sequenced these parasites at one mitochondrial and one nuclear locus and showed that each genus is represented by a single species, R. orientalis, P. crotali, and K. coarctatus. Pythons are host to R. orientalis and P. crotali, but not K. coarctatus; native snakes are host to all three species. Sequence data show that pythons introduced R. orientalis to North America, where this parasite now infects native snakes. Additionally, our data suggest that pythons are competent hosts to P. crotali, a widespread parasite native to North and South America that was previously hypothesized to infect only viperid snakes. Our results indicate invasive Burmese pythons have affected parasite‐host dynamics of native snakes in ways that are consistent with parasite spillover and demonstrate the potential for indirect effects during invasions. Additionally, we show that pythons have acquired a parasite native to their introduced range, which is the initial condition necessary for parasite spillback.     

Population genetic structure of a common host predicts the spread of white‐nose syndrome,an emerging infectious disease in bats

Landscape complexity influences patterns of animal dispersal, which in turn may affect both gene flow and the spread of pathogens. White‐nose syndrome (WNS) is an introduced fungal disease that has spread rapidly throughout eastern North America, causing massive mortality in bat populations. We tested for a relationship between the population genetic structure of the most common host, the little brown myotis (Myotis lucifugus), and the geographic spread of WNS to date by evaluating logistic regression models of WNS risk among hibernating colonies in eastern North America. We hypothesized that risk of WNS to susceptible host colonies should increase with both geographic proximity and genetic similarity, reflecting historical connectivity, to infected colonies. Consistent with this hypothesis, inclusion of genetic distance between infected and susceptible colonies significantly improved models of disease spread, capturing heterogeneity in the spatial expansion of WNS despite low levels of genetic differentiation among eastern populations. Expanding our genetic analysis to the continental range of little brown myotis reveals strongly contrasting patterns of population structure between eastern and western North America. Genetic structure increases markedly moving westward into the northern Great Plains, beyond the current distribution of WNS. In western North America, genetic differentiation of geographically proximate populations often exceeds levels observed across the entire eastern region, suggesting infrequent and/or locally restricted dispersal, and thus relatively limited opportunities for pathogen introduction in western North America. Taken together, our analyses suggest a possibly slower future rate of spread of the WNS pathogen, at least as mediated by little brown myotis.     

Selection for individual recognition and the evolution of polymorphic identity signals in Polistes paper wasps

Individual recognition (IR) requires individuals to uniquely identify their social partners based on phenotypic variation. Because IR is so specific, distinctive phenotypes that stand out from the crowd facilitate efficient recognition. Over time, the benefits of unique appearances are predicted to produce a correlation between IR and phenotypic variation. Here, we test whether there is an association between elevated phenotypic polymorphism and IR in paper wasps. Previous work has shown that Polistes fuscatus use variable colour patterns for IR. We test whether two less variable wasp species, Polistes dominulus and Polistes metricus, are capable of IR. As predicted, neither species is capable of IR, suggesting that highly variable colour patterns are confined to Polistes species with IR. This association suggests that elevated phenotypic variation in taxa with IR may be the result of selection for identity signals rather than neutral processes. Given that IR is widespread among social taxa, selection for identity signalling may be an underappreciated mechanism for the origin and maintenance of polymorphism.     

A New Small-Bodied Species of <Emphasis Type="Italic">Palaeonictis</Emphasis> (Creodonta,Oxyaenidae) from the Paleocene-Eocene Thermal Maximum

Oxyaenid creodonts are extinct carnivorous mammals known from the Paleogene of North America, Europe, and Asia. The genus Palaeonictis is represented by three species that together span the late Paleocene to early Eocene of North America, and at least one species from the early Eocene of Europe. Previously, only a single trigonid of Palaeonictis was known from the interval encompassing the Paleocene-Eocene Thermal Maximum (PETM) in North America. We describe Palaeonictis wingi sp. nov. from the PETM in the Cabin Fork drainage, southeastern Bighorn Basin, Wyoming, based on associated right and left dentaries with P₂-M₂. Palaeonictis wingi sp. nov. is substantially smaller than the other North American congeners, making it similar in size to P. gigantea from the earliest Eocene of Europe and the previously described PETM specimen. We suggest that a form similar to the large-bodied late Paleocene P. peloria from North America gave rise to two smaller species in the earliest Eocene of North America (P. wingi) and Europe (P. gigantea). Palaeonictis wingi may have given rise to P. occidentalis following the PETM in North America. Dispersal of Palaeonictis to Europe coincided with rapid global warming of 5–10°C and related geographic range shifts in plants and other animals during the PETM. It has been suggested that certain mammalian lineages decreased in body size during the PETM, possibly in response to elevated temperature and/or higher CO₂ levels. Results from a dietary analysis of Palaeonictis indicate that it was an omnivore that primarily consumed meat. This suggests that the decreased nutritious quality of vegetation caused by increased CO₂ levels was not the direct contributing factor that caused body size reduction of this lineage during the PETM. Other selective pressures such as temperature, aridity, and prey size may have also contributed to the smaller body size of carnivorous mammals during this interval, although the presence of smaller species could also be explained by latitudinal range shifts of mammals during the PETM.