Tamar Dayan

BODY MASS OF LATE QUATERNARY MAMMALS

The purpose of this data set was to compile body mass information for all mammals on Earth so that we could investigate the patterns of body mass seen across geographic and taxonomic space and evolutionary time. We were interested in the heritability of body size across taxonomic groups (How conserved is body mass within a genus, family, and order?), in the overall pattern of body mass across continents (Do the moments and other descriptive statistics remain the same across geographic space?), and over evolutionary time (How quickly did body mass patterns iterate on the patterns seen today? Were the Pleistocene extinctions size specific on each continent, and did these events coincide with the arrival of man?). These data are also part of a larger project that seeks to integrate body mass patterns across very diverse taxa (NCEAS Working Group on Body Size in Ecology and Paleoecology: linking pattern and process across space, time, and taxonomic scales). We began with the updated version of D. E. Wilson an...

Character Displacement, Sexual Dimprphism, and Morphological Variation among British and Irish Mustelids

Five native terrestrial mustelids are found in Great Britain. Only three of these occur in Ireland. Farmed American mink have recently established feral populations on both islands. We studied inter- and intraspecific size relationships, sexual size dimor- phism, and morphological variation among these mustelids. We viewed each sex as a separate morphospecies, skull length as a measure of body size, and the upper canine tooth as the organ used to kill prey. Geographic variation was low in both islands, so we considered the mustelid population of each island a single unit. Community-wide character displacement (evidenced by equal size ratios) was found among British mustelids for canine diameter. For skull length it was seen only when the largely vermivorous badger was excluded. When we added feral mink the regular pattern disappeared, but when we substituted the mink for the polecat, which is now restricted to parts of Wales and adjacent England, community-wide character displacement was man- ifest. For Irish mustelids size ratios were not equal, but the pattern for canines was more regular than for skull lengths. Adding the local feral mink did not result in a regular pattern, but addition of the mink and exclusion of the badger yielded equal ratios for skull length but not for canines. These patterns plus published empirical data support a hypothesis of prey size partitioning. The significant differences in size between some of the British and Irish populations of the same morphospecies suggest the possibility of ecological release among Irish mustelids, whose populations originally derive from British ones. In particular, canine sexual size dimorphism is greater for Irish pine martens, stoats, and mink, as would be expected if there were fewer competitors. For the marten and the stoat, Irish females have evolved to be strikingly smaller than their British counterparts, in each case approximating the size of the male of a missing species (polecat for the marten, weasel for the stoat). For skull length there is no consistent pattern. Finally, morphological variation is greater in Ireland for five of six morphospecies, as predicted by the niche-variation hypothesis.

Similarity of Mammalian Body Size across the Taxonomic Hierarchy and across Space and Time

Although it is commonly assumed that closely related animals are similar in body size, the degree of similarity has not been examined across the taxonomic hierarchy. Moreover, little is known about the variation or consistency of body size patterns across geographic space or evolutionary time. Here, we draw from a data set of terrestrial, nonvolant mammals to quantify and compare patterns across the body size spectrum, the taxonomic hierarchy, continental space, and evolutionary time. We employ a variety of statistical techniques including “sib‐sib” regression, phylogenetic autocorrelation, and nested ANOVA. We find an extremely high resemblance (heritability) of size among congeneric species for mammals over ∼18 g; the result is consistent across the size spectrum. However, there is no significant relationship among the body sizes of congeneric species for mammals under ∼18 g. We suspect that life‐history and ecological parameters are so tightly constrained by allometry at diminutive size that animals can only adapt to novel ecological conditions by modifying body size. The overall distributions of size for each continental fauna and for the most diverse orders are quantitatively similar for North America, South America, and Africa, despite virtually no overlap in species composition. Differences in ordinal composition appear to account for quantitative differences between continents. For most mammalian orders, body size is highly conserved, although there is extensive overlap at all levels of the taxonomic hierarchy. The body size distribution for terrestrial mammals apparently was established early in the Tertiary, and it has remained remarkably constant over the past 50 Ma and across the major continents. Lineages have diversified in size to exploit environmental opportunities but only within limits set by allometric, ecological, and evolutionary constraints.

The Evolution of Maximum Body Size of Terrestrial Mammals

How Mammals Grew in Size Mammals diversified greatly after the end-Cretaceous extinction, which eliminated the dominant land animals (dinosaurs). Smith et al. (p. 1216) examined how the maximum size of mammals increased during their radiation in each continent. Overall, mammal size increased rapidly, then leveled off after about 25 million years. This pattern holds true on most of the continents—even though data are sparse for South America—and implies that mammals grew to fill available niches before other environmental and biological limits took hold. Maximum mammal size increased at the beginning of the Cenozoic, then leveled off after about 25 million years. The extinction of dinosaurs at the Cretaceous/Paleogene (K/Pg) boundary was the seminal event that opened the door for the subsequent diversification of terrestrial mammals. Our compilation of maximum body size at the ordinal level by sub-epoch shows a near-exponential increase after the K/Pg. On each continent, the maximum size of mammals leveled off after 40 million years ago and thereafter remained approximately constant. There was remarkable congruence in the rate, trajectory, and upper limit across continents, orders, and trophic guilds, despite differences in geological and climatic history, turnover of lineages, and ecological variation. Our analysis suggests that although the primary driver for the evolution of giant mammals was diversification to fill ecological niches, environmental temperature and land area may have ultimately constrained the maximum size achieved.

CHARACTER DISPLACEMENT AND RELEASE IN THE SMALL INDIAN MONGOOSE, HERPESTES JAVANICUS

In western parts of its native range, the small Indian mongoose (Herpestes javanicus) is sympatric with one or both of two slightly larger congeners. In the eastern part of its range, these species are absent. The small Indian mongoose was introduced, about a century ago, to the West Indies, the Hawaiian islands, Mauritius, the Fijian islands, and Okinawa. All introductions except possibly that to Mauritius were from the region of Calcutta and Bangladesh, where it is sympatric with both congeners. No other mongoose is present on any of these islands. In each instance, the introduced population derived from a small propagule. We examined size variation in the maximum diameter of the upper canine tooth (the prey-killing organ) and skull length. In the eastern (allopatric) part of its native range, males of the small Indian mongoose are much larger in both traits than in the western (sympatric) regions, approaching the size of the smaller of its absent two congeners, Herpestes edwardsii. Females from the allopatric part of the range are also larger than those of the source region. The species is more sexually dimorphic in the region of allopatry. On all islands to which it has been introduced, in 100-200 generations the small Indian mongoose has increased in male size and in sexual dimorphism; changes in female size have been slight and inconsistent. In general, sizes of island individuals are approx- imately intermediate in size between those in the region of origin (where they are sympatric and small) and those in the region of allopatry. Sexual dimorphism is greatest for canine diameter. Thus, H. javanicus shows variation consistent with ecological release from com- petition with its congeners. There is no evidence on whether this variation is genetic, nor on what dietary items might be partitioned between species and between sexes. However, morphological variation is consistently smaller for both traits and both sexes on the islands of introduction than in any part of the native range, consistent with idea of a genetic bottleneck imposed by the small propagule size. Neither of the two congeneric mongooses shows morphological variation consistent with ecological release from competition with H. javanicus in the southern part of their ranges, where the latter species is absent.

Body Size of Insular Carnivores: Little Support for the Island Rule

Large mammals are thought to evolve to be smaller on islands, whereas small mammals grow larger. A negative correlation between relative size of island individuals and body mass is termed the “island rule.” Several mechanisms—mainly competitive release, resource limitation, dispersal ability, and lighter predation pressure on islands, as well as a general physiological advantage of modal size—have been advanced to explain this pattern. We measured skulls and teeth of terrestrial members of the order Carnivora in order to analyze patterns of body size evolution between insular populations and their near mainland conspecifics. No correlations were found between the size ratios of insular/mainland carnivore species and body mass. Only little support for the island rule is found when individual populations rather than species are considered. Our data are at odds with those advanced in support of theories of optimal body size. Carnivore size is subjected to a host of selective pressures that do not vary uniformly from place to place. Mass alone cannot account for the patterns in body size of insular carnivores.

Seasonal Thermogenic Acclimation of Diurnally and Nocturnally Active Desert Spiny Mice

Diurnally active golden spiny mice (Acomys russatus) and nocturnal common spiny mice (Acomys cahirinus) coexist in hot rocky deserts of Israel. Diurnal and nocturnal activities expose these species to different climatic conditions. Nonshivering thermogenesis (NST) capacity of individuals of both species immediately upon removal from the field exhibited seasonal changes, with no significant interspecific difference. Colony‐reared mice of either species transferred in the laboratory from long to short photoperiod increased NST capacity, though to a lesser extent than observed in the seasonal acclimatization. The underlying biochemical mechanisms of short photoperiod acclimation differed between the species. In both Cytochrome‐c oxidase (Cox) activity was higher in short as compared to long photoperiod. In short‐photoperiod‐acclimated A. cahirinus uncoupling protein (UCP) content in brown adipose tissue (BAT) was significantly higher than in long photoperiod, while in A. russatus there was no significant change. In A. russatus there was a significant increase in lipoprotein lipase (LPL) activity in BAT in short‐photoperiod‐acclimated individuals, while in A. cahirinus LPL activity was high under both acclimations. The low LPL activity in brown adipose tissue of desert‐adapted A. russatus may facilitate lipid uptake in white adipose tissue, an advantage in desert conditions where food is scarce and irregularly distributed in space and time.

The dietary basis for temporal partitioning: food habits of coexisting Acomys species

Abstract Two rodent species of the genus Acomys coexist on rocky terrain in the southern deserts of Israel. The common spiny mouse (A. cahirinus) is nocturnally active whereas the golden spiny mouse (A. russatus) is diurnally active. An early removal study suggested that competition accounts for this pattern of temporal partitioning: the golden spiny mouse is forced into diurnal activity by its congener. Theoretically, temporal segregation should facilitate coexistence if the shared limiting resources differ at different times (primarily among predators whose prey populations have activity rhythms), or if they are renewed within the period of the temporal segregation. We studied food preferences of the two Acomys species in a controlled cafeteria experiment in order to assess resource overlap and the potential for competition for food between the two species. We found no significant difference in food preferences between species. The dietary items preferred by both were arthropods. We also carried out a seasonal study of the percentage and identity of arthropods taken in the field by individuals of the two species. Individuals of both species took on annual average a high percentage of arthropods in their diets. Seasonal diet shifts reflect seasonal abundance of arthropods at Ein Gedi during day and night. Diurnal activity may also reduce interspecific interference competition between A. russatus and A. cahirinus. However, the strong interspecific dietary overlap in food preference, the heavy reliance on arthropods in spiny mouse diets, and the seasonal and circadian differences in arthropod consumption suggest that prey partitioning may be a viable mechanism of coexistence in this system.

Morphological Relationships Among Coexisting Heteromyids: An Incisive Dental Character

We studied morphological relationships among coexisting heteromyid rodents from two North American deserts. Because limb morphology affects foraging behavior, we divided species into bipedal and quadrupedal guilds. The trait for which interspecific size ratios tended most toward equality both within each guild and among combined guilds is the width (cutting edge) of the upper incisor. Sizes of this trait were interspersed regularly between bipedal and quadrupedal guild members. Size ratios for pouch volume tended toward equality among Great Basin heteromyids both within each guild and among combined guilds. For no other traits did size ratios approach equality. Incisors of heteromyids are used for husking, some of which occurs above ground where predation risk is high. Therefore, husking speed may be critical. For each species there may be an optimal size for a seed that is too large to be pouched unhusked but that can be husked efficiently enough to outweigh the predation risk, and this seed size class may be the object of evolutionary specialization. A strong correlation between pouch volume and incisor width supports this hypothesis. Specializing on different seed sizes may result from competition.

On the role of phylogeny in determining activity patterns of rodents

Evolutionary plasticity is limited, to a certain extent, by phylogenetic constraints. We asked whether the diel activity patterns of animals reflect their phylogenies by analyzing daily activity patterns in the order Rodentia. We carried out a literature survey of activity patterns of 700 species, placing each in an activity time category: diurnal, nocturnal, or active at both periods (a-rhythmic). The proportion of rodents active at these categories in the entire order, was compared to the activity patterns of species of different families for which we had data for over ten species each: Dipodidae, Echimyidae, Geomyidae, Heteromyidae, Muridae, and Sciuridae. Activity times of rodents from different habitat types were also compared to the ordinal activity time pattern. We also calculated the probability that two random species (from a particular subgroup: family, habitat, etc.) will be active in the same period of the day and compared it to this probability with species drawn from the entire order. Activity patterns at the family level were significantly different from the ordinal pattern, emphasizing the strong relationship between intra-family taxonomic affiliation and daily activity patterns. Large families (Muridae and Sciuridae) analyzed by subfamilies and tribes showed a similar but stronger pattern than that of the family level. Thus it is clear that phylogeny constrains the evolution of activity patterns in rodents, and may limit their ability to use the time niche axis for ecological separation. Rodents living in cold habitats differed significantly from the ordinal pattern, showing more diurnal and a-rhythmic activity patterns, possibly due to physiological constraints. Ground-dwelling rodents differed significantly, showing a high tendency towards a-rhythmic activity, perhaps reflecting their specialized habitat.