Christine R. Maher

Definitions of territoriality used in the study of variation in vertebrate spacing systems

Abstract Both species and populations within species display variation in spacing patterns, and the number of published reports documenting this variation is increasing rapidly. The potential for instructive comparisons is high, but the comparative approach requires common terminology. Vague or implicit definitions of spacing systems undermine the rigour of comparisons. Papers were reviewed that examined interspecific and intraspecific variation in spacing systems to determine the degree to which definitions currently produce a common terminology. This report describes and classifies the 48 conceptual and operational definitions of territoriality found in the literature. Only 12% of the papers gave an operational definition of territoriality. The single criterion of ‘defended area’ was used in 50% of the papers, and the other 50% used other definitions which often consisted of multiple criteria. These definitions were variants of three main themes: defended area, exclusive area, and site-specific dominance, and authors used up to three criteria to define territoriality. Conceptual and operational definitions differed because different questions were asked and because of logistical problems, such as collecting the same data on different species or populations. The taxonomic group being studied also seemed to determine which type of definition was chosen. Although defended area was used most often across four classes of vertebrates, researchers studying mammals chose spatial criteria, for example, amount of home range overlap, more often than researchers studying other taxa. Workers pursuing research in this area can enhance their contribution by using clear conceptual and operational definitions of territoriality, making them explicit at the outset.

A Review of Ecological Determinants of Territoriality within Vertebrate Species

Abstract We reviewed papers that compared intraspecific variation in territoriality vs. alternative forms of spatial or behavioral organization with three goals: (1) to discover which ecological variables act as determinants of territorial behavior and how they might act; (2) to extract and evaluate predictions and evidence for determinants of territoriality and (3) to suggest ways for future studies to build upon what the review revealed. Twenty ecological variables have been predicted, correlated with or experimentally demonstrated to relate to territoriality within vertebrate species. These variables include several characteristics of food: quantity, predictability, distribution, quality, renewal rate, type, density and assessibility. Other variables include nonfood resources, population density, habitat features, mates, space, refuges/spawning/home sites, predation pressure, host nests (for brood parasites) and energy availability. We suggest several reasons why food resources are cited most often, including their biological significance, ease of study and publishability of negative results. Certain groups of animals lend themselves to certain methods of study and, therefore, constrain the variables measured. Many variables are the subjects of apparently contradictory reports, i.e., some papers report that an increase in a given variable increases territoriality and others report that a decrease in the variable increases territoriality. After summarizing these reports we hypothesized U-shaped relationships between the ecological variables and behavior that could accommodate all these findings. However, these hypotheses cannot be tested rigorously by most current studies because of methodological limitations. We recommend a shift to quantification of intraspecifically varying spacing systems combined with simultaneous quantification of several ecological variables. Relative importance of different determinants of particular spacing systems can be revealed via multiple regression analysis. Hypothesized causal pathways, in which one ecological variable determines another variable that, in turn, determines territoriality, can be tested by path analysis.

Age-related changes in reproductive effort of male bison

SummaryThe behavior of male bison during the rut was studied, to test the prediction that reproductive effort should increase with age. Because bull bison do not show parental care, the major component of reproductive effort is competition with other bulls to obtain copulations with estrous females. Data were collected on activity budgets during the pre-rut and rut seasons, on interactions with other bulls, and on proximity to tended females during the rut. Participation in the rut started at ages 5–6. Older bulls showed greater percent time active and less percent time eating than younger bulls. The older bulls also had higher counts of dominance and fight-related interactions and lower counts for behavior associated with submission than did the younger bulls. Risk taken seemed to increase with age since bulls aged 6 to 12 participated more frequently in dangerous activities associated with fighting. Within the 6 to 12 year old group, activity patterns changed little with age. However, from the interaction information, peak ages of 8 to 11 years appeared. For the activities: back up, chase, clash, head nod, jump away, move away, run away, run toward, push heads, dominant, and risk, 11 and 12 year olds more closely resembled younger bulls. This suggests a reproductive effort curve that is flat at zero until about age 6, increases to age 8, flattens out again to around age 10, and then decreases somewhat. Reproductive effort in male bison thus generally conforms with life-history theory predictions.

Intraspecific variation in space use, group size, and mating systems of caviomorph rodents

Abstract Intraspecific variation in social systems is widely recognized across many taxa, and specific models, including polygamy potential, resource defense, and resource dispersion, have been developed to explain the relationship between ecological variation and social organization. Although mammals from temperate North America and Eurasia have provided many insights into this relationship, rodents from the Neotropics and temperate South America have largely been ignored. In this review we focus on reports documenting intraspecific variation in spacing systems, group size, and mating systems of caviomorphs. This large group of New World hystricognath rodents occupies a diverse array of habitats; thus, members of the same species potentially exhibit different social systems in response to different ecological conditions. Spatial patterns vary in response to a diverse array of factors, including predation, food availability, population density, and soil characteristics. Changes in group size typically correlate with changes in resource availability, particularly food. Mating systems generally reflect the ability of males to control access to females, which may depend on population density or food distribution. In general, social organization in caviomorphs fits predictions of resource-based models; however, most studies have been purely observational, involving small numbers of animals over short time periods and reporting qualitative rather than quantitative levels of ecological correlates. In future studies the use of molecular techniques and controlled, experimental manipulations can increase our understanding of intraspecific variation in caviomorph social systems. This understudied group of rodents offers excellent opportunities to provide insights into the influence of ecological conditions on behavior such as social systems.

Activity Budgets and Mating System of Male Pronghorn Antelope at Sheldon National Wildlife Refuge, Nevada

A population of pronghorn antelope ( Antilocapra americana ) at Sheldon National Wildlife Refuge in northwestern Nevada was observed during summer 1988 to determine activity budgets and social organization of males. Males spent more time running and walking and less time feeding during a 3-week period beginning in late August than they did earlier in the summer. Before late August, males scarcely interacted with each other or with females; during late summer, however, interaction rates increased. Males interacted significantly more often with females than with males. Five copulations were seen during late summer. Males were never observed defending an area. Instead, pronghorn antelope appeared to occupy undefended home ranges, and males defended harems of females.

Relationship between blood mercury levels and components of male song in Nelson's sparrows (Ammodramus nelsoni)

Mercury (Hg) adversely affects the health and behavior of exposed wildlife; however, behavioral effects remain largely unknown. Changes in avian singing behavior may affect a male’s fitness because song reveals male quality and thus influences female mate choice and male territory-holding ability. Nelson’s sparrows (Ammodramus nelsoni) live exclusively on salt marshes and risk high levels of Hg exposure and bioaccumulation. We recorded songs of male Nelson’s sparrows at two locations with different Hg exposure to determine if total blood Hg concentration was related to song characteristics, as previously reported for other species. Males with higher blood Hg levels sang at higher maximum tonal frequency, but blood Hg and site location did not influence low tonal frequency and bout duration, contrary to predictions based on other species. Within the contaminated site, Hg levels were related to bouts per minute and gap duration, such that males at that site sang faster songs. Hg influences hormones and alters brain development, raising questions about specific effects on the brains and singing behavior of male Nelson’s sparrows.

Intrasexual Territoriality in Woodchucks (Marmota Monax)

Abstract Species demonstrating intraspecific variation in social systems can be powerful models for understanding evolution of those systems. As a group, marmots exhibit several types of spacing systems, usually involving some degree of territoriality. Researchers have described populations of 1 species, the woodchuck, Marmota monax, as territorial and as nonterritorial, and such variation has been linked to ecological conditions. I used direct observations of individually recognized animals to describe the spacing system of a high-density population of Marmota monax in southern Maine. This population exhibited intrasexual territoriality. Home range overlap generally was higher between males and females than between same-sex individuals, and woodchucks tended to approach more members of the same sex more quickly compared with members of the opposite sex. Time spent scent marking varied across the active season but did not vary by sex. Males had larger home ranges than females, and home range sizes varied over time, perhaps in response to resources. Amount of overlap also was greater in some years than others, and such changes may be related to kinship. Philopatry and timing of dispersal also vary in this species and have implications for the evolution of sociality.

Quantitative variation in ecological and hormonal variables correlates with spatial organization of pronghorn (Antilocapra americana) males

Abstract Whereas variation in pronghorn (Antilocapra americana) spatial organization is well documented, underlying ecological or physiological explanations are not well understood. This study quantitatively describes spacing systems of pronghorn males and correlates of their spatial organization. I collected behavioral data from two populations in South Dakota (Wind Cave) and Montana (Bar Diamond) to determine if males differed in space use, response to intruders, and behavior patterns indicative of area defense. I measured sex ratio and population density, and I examined characteristics of food resources, including forb species diversity, richness, coverage, biomass, and nitrogen content, and how they changed during the growing season. I also collected and analyzed fecal samples to determine if males differed in testosterone concentrations. Pronghorn males at Wind Cave were more territorial than males at Bar Diamond, although males at Bar Diamond became more territorial during the second year. The forb community at Wind Cave was more diverse, contained greater amounts of forbs later in summer, and had a higher nitrogen content later in summer. Population density was lower at Wind Cave, although density dropped at Bar Diamond during the second year, and sex ratios were skewed toward males at Bar Diamond. Finally, males at Wind Cave had higher testosterone concentrations than did Bar Diamond males, although differences were not statistically significant. With lower population density and higher forb abundance and quality, food resources were more economically defensible at Wind Cave, and males were more territorial there. Analyses using these and other pronghorn populations revealed that population density and sex ratio correlated weakly with spatial organization, whereas precipitation correlated most strongly, which suggests plant productivity has a powerful role in determining pronghorn territoriality.

Effects of relatedness on social interaction rates in a solitary marmot

Kinship often influences social interactions, such that animals behave more amicably towards close kin than towards distant kin or unrelated individuals. However, competition also may affect interactions among kin and the degree of sociality in a population. I used microsatellite DNA markers and behavioural observations to examine effects of kinship on social interactions among woodchucks, Marmota monax, which generally are considered asocial or aggressive. Rates of agonistic interactions did not change with relatedness, but rates of amicable interactions increased with increasing relatedness. I observed more amicable interactions than expected between mother–offspring and littermate sibling dyads, but I observed fewer amicable interactions between nonlittermate siblings and more distant kin. Instances of physical aggression were less frequent than expected in mother–offspring, nonlittermate siblings and distant kin dyads, but more frequent than expected among littermate siblings. Interactions between mother–offspring dyads also changed with offspring age, with mothers behaving more amicably towards younger offspring and more agonistically towards older offspring, especially females. Among littermate siblings, rates of amicable interactions were highest between juveniles and yearlings, but then decreased between adult siblings. Social interactions reflected a balance between cooperation and competition. Although woodchucks in this population lived near kin and biased interactions based on kinship, many interactions were agonistic, and animals did not show greater degrees of sociality. Ecological factors, such as predation risk, resource distribution and climate, may favour solitary living in this population.

Horn growth in male pronghorns (Antilocapra americana): selection for precocial maturation in stochastic environments

PronghornsAntilocapra americana (Ord, 1818), the sole member of a family unique to North America, grow rapidly and reproduce at an early age. Recent studies have found male pronghorns can grow large horns by 2 to 3 yrs of age. This pattern contrasts with many other ungulates, and it has profound implications for life history strategies. We examine 5 hypotheses that might explain precocial horn growth: (1) sampling bias, (2) nutrition, (3) phylogenetic inertia, (4) reproductive benefits conveyed by rapid horn growth alone, and (5) rapid horn growth as part of a suite of characteristics acquired due to precocial maturation. Hypotheses 1, 2 and 3 do not imply any natural selection, whereas hypotheses 4 and 5 do. We reject hypotheses 1, 2 and 4, and we cannot evaluate hypothesis 3. We conclude that hypothesis 5 most likely explains precocial maturation in male pronghorns, and mortality related to frequent, severe weather events may drive this pattern. We suggest several experiments to further examine relationships between age, size, and horn growth.