Edward H. Miller

Vocal signalling of male southern elephant seals is honest but imprecise

In the most common models of communication, it is assumed that animals provide reliable information about phenotype, and hence can settle competitive contests without physical interactions like fights. This assumption has rarely been tested for wild mammals. Recent studies of mammals have revealed relationships of vocal attributes to age and body size. Here, we analyse relationships of frequency attributes of agonistic vocalizations to phenotype (age, body size, proboscis size and agonistic behaviour) in males of the southern elephant seal, Mirounga leonina, a species with intense male competition for access to females, and in which vocalizations are used frequently to settle male–male contests. We analysed formant structure and vocal tract size, and found that nasal and oral components of the vocal tract contribute separately to vocal formants; hence, the males proboscis serves to elongate the vocal tract. We also found that formants in the upper part of the frequency spectrum (fourth and fifth in particular) and formant dispersion convey significant information about age, size and resource holding potential at large, and, therefore, can be honest signals of a vocalizers phenotype. Explained variance was statistically significant in our study and in similar studies but was not high, so formant structure cannot serve as the sole basis of acoustic assessment. Other possible sources of information exchanged in elephant seal contests are nonvocal acoustic signals (e.g. vibrations) and optical displays.

Communication in pinnipeds, with special reference to non-acoustic signalling

Communication is complex and variable in long-lived and adaptable species. In such species, communication is shaped by learning, personal histories, variable social and ecological circumstances, and bonds between individuals. It is also often extremely subtle, especially in highly social species, in which individuals continually appraise and respond to the general social setting and to features of others around them: movements, activity levels, distances, postures, sounds, smells, and appearance. Much communication therefore takes place through unspecialized and undirected signals - sources of information that have not specially evolved or developed to be informative. In pinnipeds, the processes of communication are complicated further because all species communicate in both air and water. These media have different effects on sensory and motor abilities, and on physical aspects of signal transmission (Hopkins, 1987; Rogers and Cox, 1987; see Chapter 7).

Bacular and testicular growth, allometry, and variation in the harp seal (Pagophilus groenlandicus)

We investigated quantitative relationships of bacular size to age, length of body, and testicular size in the harp seal ( Pagophilus groenlandicus ), based on collections made in the northwestern Atlantic from 1985 to 1992. Bacular growth was faster than growth in length of body except in old seals (>9 years of age), in which relative growth was isometric. A pubertal spurt in growth in bacular size occurred between 3 and 4 years of age when length increased by 48.3%, mass by 331%, and density by 185%. Concurrently, length of body showed a spurt in growth of 7.2%. Bacular length reached 90% of its asymptotic size of 17.4 cm at ca. 9 years of age, and bacular mass reached that point relative to its asymptote (49.6 g) later at ca. 20 years. In contrast, length of body reached 90% of asymptotic size (176 cm) at only 5–6 years of age. In old seals, bacular length was correlated positively with length of body and age independently. It averaged 9.9% of length of body in old seals. Bacular size was variable: CV = 8.3% for bacular length and 32.8% for bacular mass in old seals. Testicular size peaked in February and March. For young seals (≤9 years old), growth in bacular length was isometric, and growth in bacular mass was positively allo-metric relative to testicular length and mass. In contrast, testicular length and mass of breeding old seals were not correlated significantly with bacular size, length of body, or age. In breeding old seals, testes averaged 10.1 cm in length (range = 7.4–13.2 cm, CV = 10.9%) and 128 g in mass (range = 60.6–204 g, CV = 24.9%), and testicular length averaged 5.7% of length of body. Variation in bacular and testicular size suggests that males have variable reproductive strategies. Comparative analyses on phocid species with known mating systems are needed to test this interpretation.

Growth and variation in the bacula of polar bears (Ursus maritimus) in the Canadian Arctic

Structure and growth of the baculum (os penis) in arctoid carnivores have been well described for many species. This study presents the first extensive analysis of bacular growth and variation for bears (Ursidae), based on 871 bacula of polar bears Ursus maritimus (858 of known age) that were shot in the Canadian Arctic from 1994 to 1997. Bacular length, maximal diameter and mass increased from 109 mm, 5.93 mm and 2.68 g respectively in 1-year-old bears (n = 34) to 190 (maximum 222) mm, 18.7 (24.4) mm, and 20.4 (31.9) g respectively in bears ≥ 10 years of age (n > 200). Bacular length is ∼ 8% of body length in adults. Bacula were moderately variable in size: coefficient of variation for length, diameter, and mass 1/3 were 5.2, 10.0 and 5.4% respectively (16.3% for mass). No geographic variation in size was apparent. Bacula reached asymptotic size at 8–9 years of age. At physiological sexual maturity (∼ 6 years of age), bacula were at 89, 73 and 62% of asymptotic length, diameter and mass respectively (compared with body length at ∼ 95%). Interpretation of these findings must await behavioural and physiological information on courtship and copulation in the polar bear, and comparative data on other ursids.

Prey Selection by Marine-coastal River Otters (Lontra canadensis) in Newfoundland, Canada

Abstract Previous studies have suggested that diets of river otters (Lontra canadensis) vary in response to seasonal shifts in prey availability, and that they select slowly moving fish of moderate size. To test these assumptions for marine-coastal river otters in Newfoundland, Canada, we reconstructed diets and estimated body length of important fish prey through analysis of otoliths and other hard parts recovered from scats collected in Bonavista and Placentia bays. Diet of otters in Bonavista Bay also was compared with the species and size composition of the nearshore fish community, as determined by concurrent beach-seine sampling. Diets were qualitatively similar but quantitatively different between bays. Otters consumed proportionally more cunner (Tautogolabrus adspersus) and stickleback (Gasterosteidae) in Placentia Bay, and more sculpin (Cottidae) and cod (Gadus) in Bonavista Bay. Flounder (Pseudopleuronectes americanus) was important in both bays, based on biomass, because individual fish in the diet were large. Fish-community composition in Bonavista Bay varied seasonally, a pattern that was not reflected in the diet, suggesting active selection of fish prey by otters. Slow-swimming fish (sculpin, flounder, and ocean pout [Macrozoarces americanus]/rock gunnel [Pholis gunnellus]) were overrepresented in the diet, and fast-swimming species (cod and hake [Urophycis]) were underrepresented. Otters also selected larger individuals within taxa. The lower limits at which size classes were incorporated into diet varied across fish species, and may reflect species differences in detectability by otters due to camouflage and behavior.

Bacular size, growth, and allometry in the largest extant otariid, the Steller sea lion (Eumetopias jubatus)

Abstract Bacula are relatively small in terrestrially mating species of pinnipeds (otariids and elephant seals, Mirounga), perhaps reflecting adaptive size reduction to minimize bacular fracture. Fur seals and sea lions (Otariidae) are a good group with which to investigate this question, because most species copulate solely on land and body size varies interspecifically. We studied bacular size and relative growth in the largest extant otariid, the Steller sea lion (Eumetopias jubatus). Bacula roughly tripled in length and increased 30-fold in mass between 1 and 8 years of age. Allometric relationships changed over development; bacular length and mass changed from being initially positively allometric to body length to negatively allometric and isometric, respectively; bacular mass and thickness were positively allometric to body length throughout life, and apical growth was isometric then was positively allometric to bacular length. In adults (>7 years of age), bacula averaged 18.1 cm length (6.2% of body length), 36.7 g mass, and 2.02 g/cm density (mass : length). The baculum of Eumetopias is about the same length relative to body length as in other adult male otariids but is about twice the density, presumably to increase strength. Information on small or aquatically mating species of otariid are needed to extend our findings and interpretations.

Variation and Integration of the Simple Mandibular Postcanine Dentition in Two Species of Phocid Seal

Abstract Pinnipeds generally swallow prey whole, and most have simple, homodont, nonoccluding cheek teeth. We investigated whether cheek teeth in seals are more variable and weakly integrated than in terrestrial Carnivora. We measured mandibular length and crown length of mandibular postcanines (PCs) in ringed seals (Pusa hispida; n = 912) from the Canadian Arctic, and harp seals (Pagophilus groenlandicus; n = 636) from Newfoundland and Labrador. PC size was uncorrelated or only weakly correlated with adult mandibular length. PC length and mandibular length were strongly bilaterally symmetrical (r ≥ 0.8 between left and right sides). PC size was moderately variable (coefficients of variation [CVs] ∼ 7–10%), and CV varied with position in the toothrow. Adjacent PCs were correlated more strongly in size (to r > 0.8) than PCs more distant from one another. In summary, PC size in ringed and harp seals was slightly more variable than cheek teeth in complex dentitions of fissipeds, and the 2 seals were similar to fissipeds in strong bilateral symmetry in mandibular and PC size, patterned variation along the toothrow, and correlated size between adjacent PCs.

Bacular variation and allometry in the western martenMartes caurina

Many sexually-selected structures are variable and positively allometric relative to body size. For the western martenMartes caurina Merriam, 1890 from Vancouver Island, British Columbia, we investigated variation in the baculum compared with other bones and allometry of bacular to body size. Carcass length did not differ, and humeral and mandibular lengths differed little (< 1 and 2%, respectively), between age groupings < 1 and ≥ 1 yr old. In contrast, bacular length increased by 16%, and thickness by 29% (mid shaft) and 86% (basal) between those groupings, and thickness and mass continued to grow after the second year of life. Controlling for body size, bacular size varied more than humeral or mandibular size (CV for linear variables ∼4–8% for baculum, ∼2% for humerus or mandible). Some positive static allometry of bacular size to body size was found, but correlations between bacular and body size were weak (r=0.3–0.4). So penile size as related to bacular size could be a reliable but imprecise quality indicator during copulation. Weak polygamy (serial promiscuity), complex copulatory mechanisms, and high energetic costs of reproduction, likely select for multiple cues in mate-choice by females, not just penile cues affected by bacular size or shape.

Seasonal energetics of northern phocid seals

The metabolic rate of harp (Pagophilus groenlandicus), harbor (Phoca vitulina), and ringed seals (Pusa hispida) was measured at various temperatures in air and water to estimate basal metabolic rates (BMRs) in these species. The basal rate and body composition of three harp seals were also measured throughout the year to examine the extent to which they vary seasonally. Marine mammalian carnivores generally have BMRs that are over three times the rates expected from body mass in mammals generally, both as a response to a cold-water distribution and to carnivorous food habits with the basal rates of terrestrial carnivores averaging about 1.8 times the mean of mammals. Phocid seals, however, have basal rates of metabolism that are 30% lower than other marine carnivores. Captive seals undergo profound changes in body mass and food consumption throughout the year, and after accounting for changes in body mass, the lowest rate of food intake occurs in summer. Contrary to earlier observations, harp seals also have lower basal rates during summer than during winter, but the variation in BMR, relative to mass expectations, was not associated with changes in the size of fat deposits. The summer reduction in energy expenditure and food consumption correlated with a reduction in BMR. That is, changes in BMR account for a significant portion of the seasonal variation in energy expenditure in the harp seal. Changes in body mass of harp seals throughout the year were due not only to changes in the size of body fat deposits, but also to changes in lean body mass. These results suggest that bioenergetics models used to predict prey consumption by seals should include time-variant energy requirements.

VARIATION, SEXUAL DIMORPHISM, AND ALLOMETRY IN MOLAR SIZE OF THE BLACK BEAR

Abstract We investigated molar-crown–size variation, sexual dimorphism, and allometry in the black bear (Ursus americanus), using hunter-shot specimens (n = 429) from the island of Newfoundland, and museum specimens from elsewhere in Canada and the continental United States (n = 502). We predicted higher variation in and weaker correlations among molar size in this omnivorous species than in other species of Carnivora with dentition more specialized for carnivory, because of relatively weak normalizing selection on food-processing mechanisms in U. americanus. Molar-size variation in Newfoundland bears (mean coefficient of variation ∼ 5.6%) was intermediate between species of Carnivora with simpler (e.g., pinnipeds) and more complex (e.g., canids) postcanine dentition. There was negligible size variation within the molar teeth, unlike some mammals. Bilateral symmetry was strong, especially in mandibular length (r ∼ 1.0 between left and right sides; r ∼ 0.95 for other mandibular and maxillary variables and molar size); symmetry in molar size was higher than in phocid seals. Size was positively correlated across molars, especially between adjacent (but not occluding) molars; patterns were similar between sexes and geographic regions, and correlation levels did not differ from other species of Carnivora. We also predicted (and found) smaller sexual differences (= 100[(male/female) − 1]) in molar size than in body size, because definitive molar size is attained early in life: differences in molar size averaged ∼5.5% in Newfoundland (higher in continental subsamples), which is less than differences in cranial size (7–9%) or body mass1/3 (21%). Sexes did not differ in relative molar size. Molar size was mainly isometric or positively allometric to adult body size (using mandibular and maxillary size as proxies).