A giraffe and a harbour porpoise look like unlikely partners for a brain comparison. Yet they belong to the same broad evolutionary group, and the specimens in one study had brains of similar mass. Looking inside those brains revealed a shared arrangement of signaling cells that body shape alone would never suggest.
Leigh-Anne Dell and colleagues reported the comparison in 2012. They mapped neurons containing orexin, a chemical messenger involved in several aspects of physiological regulation. Both species had a previously undescribed cluster of small orexin-containing cells, alongside three groups of larger ones. The porpoises had more orexin cells in the sampled brains; the giraffes’ cells were larger on average.
The finding concerns the organization of one signaling system. It is not a ranking of intelligence, a count of all brain cells, or a direct explanation of either species’ behavior.
A comparison with a useful starting point
The team examined two subadult male giraffes and two adult male harbour porpoises. The giraffe brains weighed 509 and 544 grams, while the porpoise brains weighed 486 and 503 grams. Their broadly similar brain masses made it interesting to ask whether a particular neural system was also organized on a similar scale.
The focus was the hypothalamus, a region involved in coordinating many bodily functions. Orexin-containing neurons had been studied in several mammals, but their arrangement in these two species was poorly known.
The researchers cut the relevant brain tissue into thin sections and used antibodies to make cells containing orexin-A visible. They compared those sections with other stains that help identify tissue boundaries. This allowed them to map a labeled cell’s position within the surrounding anatomy, rather than simply counting dark spots on a slide.
Because a new cell grouping was an important result, they also checked the staining. A second orexin antibody produced the same distribution, and an inhibition control removed the signal. These checks supported the interpretation that the unexpected cells were detecting the intended target rather than an unrelated staining effect.
Two cell sizes, four groupings
The labeled neurons fell into two recognizable size classes. The researchers called them parvocellular and magnocellular: terms meaning small-celled and large-celled. Measurements of cell-body size supported the distinction seen through the microscope, although the size distributions overlapped slightly.
The smaller cells occupied the medial, or inner, part of the hypothalamus. In the giraffe they extended along a region near the third ventricle, one of the brain’s fluid-filled spaces. Instead of forming several clearly separated islands, they made a continuous cluster through successive tissue sections.
The larger cells formed three spatial groups. One occupied a main region farther to the side, another lay near the optic tract, and a third extended toward an adjacent region called the zona incerta. The names are anatomical landmarks: the key finding is that both species showed the same broad arrangement of a medial small-cell group and three larger-cell groups.
The larger groups resembled patterns already described in other mammals. The small-cell cluster was the more surprising observation. Dell and colleagues proposed that it might be characteristic of the broader group containing even-toed hoofed mammals and cetaceans. That was a comparative hypothesis, not proof from these four brains that every member of the group possesses it or that no other mammal does.
More cells and larger cells are different results
To estimate neuron numbers, the researchers used stereology, a method that samples tissue systematically and uses the sampling proportions to estimate a total. This matters because simply counting every visible profile in selected sections can give misleading results, especially when cell sizes differ.
The estimated totals averaged about 15,000 orexin-containing neurons in the giraffes and 21,000 in the porpoises. In both species, roughly half belonged to the small-cell group and half to the large-cell groups. The porpoise difference therefore was not confined to just one of the two size classes.
Cell size went in the other direction. Average cell-body volume, area and length were greater in the giraffes for both classes. The paper’s comparison plots show the distinction clearly: the porpoise bars are higher for cell number, while the giraffe bars are higher for average cell size.
This is why a single number cannot summarize the comparison. A system with fewer, larger cells differs from one with more, smaller cells in ways that could affect its connections, but neither count nor size alone measures what the whole network can do. The study did not map all of those connections or quantify the signaling output of the cells.
A map leaves functional questions open
Orexin is associated with a wide range of functions in mammals, including arousal and the regulation of feeding and energy balance. The authors discussed possible links between the newly described anatomy and those functions. They also suggested that larger giraffe neurons might be related to maintaining long projections toward the spinal cord.
Those possibilities were not tested by manipulating the cells or recording their activity. The study did not determine what the small-cell cluster controls, where all its projections go, or whether cell-number differences explain a particular feature of either animal’s life.
The sample also remains small: two males per species, with a difference in age category. Many measured cells improve the description of those brains, but they do not turn four animals into a broad survey of variation within each species.
For our wider giraffe science coverage, the contribution is a concrete anatomical discovery and a set of better questions. Giraffes and harbour porpoises shared an unexpected small-cell component within a familiar signaling system. Explaining its role requires following the cells’ connections and activity, beyond knowing their location, size and number.
Sources
Leigh-Anne Dell, Nina Patzke, Adhil Bhagwandin, Faiza Bux, Kjell Fuxe, Grace Barber, Jerome M. Siegel and Paul R. Manger (2012). “Organization and number of orexinergic neurons in the hypothalamus of two species of Cetartiodactyla: A comparison of giraffe (Giraffa camelopardalis) and harbour porpoise (Phocoena phocoena).” Journal of Chemical Neuroanatomy 44, 98–109. Paper · Open author manuscript.
