A giraffe can spend much of its social life with the same companions, or divide that time among animals from several different circles. Counting companions or measuring group size can blur that distinction. For a parasite, however, the distinction could matter: repeated exposure to familiar animals and exposure to several different sets of animals may create different opportunities to spread.
Kimberly VanderWaal and colleagues investigated that possibility at Ol Pejeta Conservancy in Kenya. Their 2016 study paired months of individually identified giraffe sightings with a search for intestinal worm eggs in dung. The social connections most closely associated with a positive result were those reaching outside an animal’s usual circle.
That finding gives a more specific meaning to the possible costs of social life. The relevant question was not simply whether sociable giraffes carried more parasites. It was whether the arrangement of their relationships helped explain which animals carried them.
Familiar circles and outside connections
From January to August 2011, the team collected 1,089 sightings of giraffe groups in a population of 212 animals. Researchers identified individuals by their coat patterns and recorded which animals occurred together. They used these repeated sightings to give each pair an association strength: a pair seen together frequently had a stronger connection than one seen together rarely.
Earlier analysis of these records had identified social circles, called cliques in the paper. Members associated more often with one another than with outsiders. These were recurring patterns within a population whose visible groups changed, rather than permanent herds that always moved as a unit.
The researchers separated each animal’s connections into those within its circle and those outside it. They called the latter “weak ties.” Here that term described connections between circles; it did not mean that every outside encounter was inconsequential or that all links below an arbitrary strength were grouped together.
This separation made an informative comparison possible. An animal could have strong, repeated associations with familiar companions without having many connections into the rest of the population. Another could link several circles. A single total for social activity would mix those situations.
Nine positive samples carried the main result
In the month after the behavioural observations, the team collected dung from 96 giraffes across the conservancy. Laboratory examination detected worm eggs in nine animals, about 9% of those sampled. This was a test of whether eggs were present, rather than a measurement of how ill an animal was or how many adult worms it carried.
Giraffes with stronger combined outside-circle connections were more likely to have a positive sample. The three best-supported statistical models all included this measure. Repeated associations within an animal’s own circle did not show a clear relationship with detection.
Age and the size of the area an animal used also had relationships with parasite occurrence when considered separately. Subadults and animals with larger home ranges were more likely to test positive. But models incorporating outside connections received stronger support, including comparisons that considered age and home-range size together with social ties.
The result rests on nine positive animals, so it needs that scale attached. It identifies a pattern worth explaining; it does not supply a precise universal risk estimate for a giraffe with a particular social life. Nor does a negative dung sample establish that an animal never encounters parasites.
A social connection need not be a physical encounter
The worms studied can pass through the environment. Eggs or larvae shed in dung may be encountered later in contaminated water or vegetation. Two giraffes need not touch, or even be present at exactly the same moment, for their behaviour to connect their exposure.
One interpretation is that associations across several circles bring an animal into contact with a wider variety of contamination sources. But outside ties may also accompany other behaviours that matter. More mobile animals can meet more associates and encounter more places; age can influence both behaviour and susceptibility.
All positive giraffe samples came from the central part of the study area. The team therefore repeated an analysis using only giraffes sampled there, reducing the broad geographical contrast. Outside ties still predicted a positive result. Samples from other hoofed species did not show the same clear central concentration of the relevant parasites.
Those checks made a simple geographical explanation less convincing. They did not locate the actual transmission sites or remove every difference in environmental exposure. The animals’ social records and their surroundings remain connected parts of the explanation.
Following exposure through time
VanderWaal’s team also compared social associations with shared bacterial types in another study of this population. Giraffes with stronger social associations were more likely to share an Escherichia coli subtype. That work and the worm study used the same 2011 association programme, so they are complementary measurements rather than independent repetitions of the behavioural result.
The bacterial comparison asked whether a network of observed associations corresponded to a network built from microbes. This worm study asked which kinds of connections best explained parasite detection. Together they show why “social contact” needs to be defined before it can explain exposure.
Repeated dung sampling would help test whether changes in outside connections precede new detections in the same animals. Sampling potential environmental sources could then distinguish a broadly shared contamination site from exposure associated with particular social circles. Both comparisons would have to accommodate imperfect detection and the time between acquiring an infection and shedding detectable eggs.
The achievement here was to make a particular social arrangement testable. Knowing that giraffes form groups is only the beginning; knowing how animals connect those groups can reveal a different biological pattern.
Sources
- Kimberly L. VanderWaal, Vincent Obanda, George P. Omondi, Brenda McCowan, Hui Wang, Hsieh Fushing and Lynne A. Isbell (2016). “The ‘strength of weak ties’ and helminth parasitism in giraffe social networks.” Behavioral Ecology 27(4), 1190–1197. DOI · Author-hosted full paper.
- Kimberly L. VanderWaal, Edward R. Atwill, Lynne A. Isbell and Brenda McCowan (2014). “Linking social and pathogen transmission networks using microbial genetics in giraffe (Giraffa camelopardalis).” Journal of Animal Ecology 83, 406–414. DOI · Author-hosted full paper.
