Mammalian Scent Gland Marking and Social Behavior

1. What they did

Many mammals leave scent on objects, the ground and each other from special skin glands. The authors, psychologists at the University of Texas at Austin, reviewed what was known by the mid-1970s about this scent marking and what it does in social life, hoping to draw more behaviour researchers into the field.

They:

• Set out the concepts. What chemical signals (pheromones) are, how they are classed, and the main jobs they do: bringing animals together, recognition, and territory, status and spacing.
• Listed species with scent glands. Table 2 gives 55 mammal species in eight orders with specialised scent glands and marking behaviour, and Table 5 lists deer, antelope and one pig species with preorbital glands (in front of the eye).
• Proposed seven criteria for judging whether a scent gland matters socially and in evolution.
• Reviewed the best-studied species one by one: Columbian ground squirrel, Mongolian gerbil, golden hamster, guinea pig, pika, European rabbit, sugar glider, even-toed hoofed animals in general, pronghorn, black-tailed deer, Maxwell duiker, lemurs and marmosets.
• Compared them (Table 8) and drew out common patterns and gaps.

2. What they found

Where and how animals mark

• Glands sit where the body meets the things the animal touches: on the flanks and back in burrowing or runway-using rodents, on the chest in tree-climbing marsupials and monkeys, and between the toes in deer.
• Some marking is passive: the interdigital glands of the mule deer leave secretion on trails whenever the animal moves.
• Marks are often concentrated at territory edges. In the pika, 72% of cheek marks were along the territory perimeter. Columbian ground squirrels mark most on the edge of the territory, and pronghorn males rub the gland below the ear on bushes and grasses, especially along territory borders, on regular “marking patrols”.
• Other marks include urine, anal drags, fecal pellets on rabbit dung hills, and dung piles in some deer.

Who marks, and when

• In almost all species studied, males mark more than females, dominant animals mark more than subordinates, and marking starts at puberty and rises in the breeding season.
• Fights and strangers trigger marking. In the gerbil, the male who becomes dominant marks at high levels and the loser stops marking altogether. In the golden hamster, flank gland size was linked to the percentage of fights won.
• Female gerbils mark heavily while pregnant and nursing, and mothers recognise their own pups by the scent they put on them.
• Castration reduces marking and gland size in several species, and male hormones restore them, so the authors conclude that sex hormones control marking in nearly all mammals.

What the scent carries

• Individual, group and sex identity: sugar gliders share a community scent and attack a group member carrying a stranger’s scent; rabbits identify their own young by smell; marmosets can tell a male’s mark from a female’s.
• Confidence on home ground: a rabbit meeting another rabbit where its own odour is present wins fights more often (69% of the time for males, 66% for females). This is the one clear demonstration they found that scent marks change the outcome of fights.
• Scent marks do not keep intruders out. Neighbouring ground squirrels and pronghorn still enter marked territories. The authors found no evidence that scent creates a barrier of any kind; the marks may instead reassure owners and intimidate invaders.
• Active chemicals had been identified in only three or possibly four species: phenylacetic acid in the gerbil, a lactone in the black-tailed deer’s tarsal gland, isovaleric acid among eight compounds from the pronghorn’s gland, and studies of rabbit gland secretions. The chemistry differs between species even when the job is similar.

3. What they concluded

• In the broadest sense, scent marking is a reproductive activity, tied to dominance and breeding through the same hormones.
• Scent marking is not stereotyped and mechanical: it is variable and goal-directed, and depends on the social setting.
• Scent glands should matter most in animals that are active at night, live in thick cover or rough terrain, have small home ranges, or are desert species that must save water.
• Big gaps remain, especially in how the brain controls marking, the chemistry of the signals, and how marking affects breeding success. Laboratory studies can link field observations to body chemistry.

6. Other technologies, skills or innovations

• Chemical analysis of gland secretions (gas chromatography, thin layer chromatography and mass spectrometry).
• Ultraviolet light to show where house mice left urine on filter paper.
• Long-wave light to see the fluorescent Harderian gland secretion that gerbils groom onto their faces.

7. Implications for conservation (from the paper)

• Many wild even-toed hoofed animals, such as deer, antelope, giraffe and okapi, were endangered. The authors hoped more would be learned about their social adaptations before they disappear.