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Watching Netflix's "Night on Earth", I discovered that Hippos, or at least some Hippos, have quite a weird appearance in infrared. They seem to be covered by IR bright spots all over their body, possibly excluding their heads.

Something similar can be seen in this video on YT starting around 0:12. The following picture is a screenshot from that video:

enter image description here

What's the reason for this weird pattern? Why are some spots on a hippo's skin warmer than the rest?


EDIT in response to a comment by @jamesfq:

As far as I can tell, these images were taken without the use of IR illumination. The crew in the video linked above were apparently using a repurposed military thermal imager by Selex. As far as I know, these operate without any additional illumination.


EDIT II:

@Harun suggested that this might be related to hippo sweat, which looks a bit like blood. But the images I could find (the one below is from here) do not seem to suggest that it forms spots comparable to those in the IR image. But maybe the secretion spreads around the glands it comes from and looks quite different in the IR. It also looks like they sweat on the head, which has a much more uniform appearance in the IR than the body. Of course these are all just single images, maybe not representative.

enter image description here

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  • $\begingroup$ I don't remember the explanation, but there's a good description in this documentary: Inside nature's Giants:Hippo $\endgroup$ – Harun Feb 6 at 10:19
  • $\begingroup$ @Harun Thanks, will try to get my hands on this. $\endgroup$ – user35915 Feb 6 at 12:44
  • $\begingroup$ As a comment in lieu of a speculative answer below: I can also imagine the spots are depositions from insects or secretions from glands of the hippo. I've heard of secretions that have a sunscreening function, i.e. appear black in UV. They could also attract or repel UV-sensitive insects that commonly spend time around these animals; this is not uncommon with coloration visible only in UV (e.g. dark spots that plants harbor to attract). Just expanding on the speculation... I have no good suggestion as to why there would be spots in the IR spectrum! $\endgroup$ – S Pr Feb 6 at 16:21
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    $\begingroup$ It would help to know just how the IR photos were taken. Are they using night vision cameras that project their own infrared light (in which case the might be just spots of different IR "colors"), or are they using the hippos' own thermal radiation? $\endgroup$ – jamesqf Feb 6 at 17:23
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    $\begingroup$ @uhoh Good question regarding the ground. I asked on physics SE: physics.stackexchange.com/q/529616/35915 $\endgroup$ – user35915 Feb 7 at 7:28
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There is research to support the idea that these are areas used by the animal to regulate its body temperature. Citation: Schneider, Marion & Kolter, Lydia & Zoo, Cologne & Germany, & De, (2003). Visualisation of Body Surfaces Specialized for Heat Loss by Infrared Thermography. J. Exp. Biol. Quart. J. Exp. Physiol. Medicine Comp. Biochem. Physiol. A J. Anim. Science. 2068. 1727-1738.

As the researchers found using a thermal camera and imaging software, there are distinct spots that can be observed in the hippo. the authors of the above mentioned paper infer that these higher temperature areas coincide with areas of rich blood flow and high nerve innervation, used to regulate blood flow and thus body temperature.

The authors also note that similar mechanisms are present in marine animals, such as seals.

Our findings demonstrate for the first time that the hippopotamus is able to open thermal windows. They appear in all body surfaces and vary in number, size and shape. Earlier histological examinations of hippopotamuses revealed a network of blood vessels deep within the dermis, and a thin epidermis, richly applied with nerve endings [3], suggesting an underlying thermoregulatory mechanism through nerval vasomotor control of the skin through connected arteriovenous anastomoses. These are referred to as specialized cutaneous vascular structures, densely innervated by sympathetic nerve fibres which permit local regulation of blood flow [4]. Arteriovenous anastomoses have been identified as thermoregulatory structures in the skin of seals [5] and presumably represent the underlying structures for thermal windows in the hippopotamus. Elephant, rhinoceros and hippopotamus are large herbivores, live in the same climatic region, and have almost hairless skin. Elephants show a great capacity for heat loss in the ears [6]. Contrary to the rhinoceros, the hippopotamus lack sweat glands but exhibits large sub dermal glands distributed over the body which release a red secret.

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  • $\begingroup$ Nice, thank you. I couldn't find the full text. If you have access to it, could you clarify the connection between these areas and the red Hippo sweat? It's mentioned in the last sentence of the abstract, but I don't quite get the connection. Are the glands for the "sweat" also the thermal windows or are they distinct? $\endgroup$ – user35915 Feb 7 at 16:26
  • $\begingroup$ Also: my girlfriend reminded me that there is also footage of a seal in Night on Earth that shows the same spots, as is mentioned in the abstract. I had forgotten about that. $\endgroup$ – user35915 Feb 7 at 16:28
  • $\begingroup$ Nice answer, but the reference appears to be garbled — it includes a partial email address and multiple journal abbreviations all lumped together. It would be great if you could edit that so that it is easier to find the actual paper (all I could find was a Research Gate link, which appears to be where the incorrect reference information came from ...). $\endgroup$ – tyersome Feb 8 at 22:10
  • $\begingroup$ Here is a link to the abstract for the paper cited, there is also a link to download the whole paper on the below web page: researchgate.net/publication/… $\endgroup$ – Matthew Martin Feb 9 at 11:10
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    $\begingroup$ @MatthewMartin It seems that researchgate is your primary source. Not many people have an account in researchgate. Does this paper have a doi or pubmed link (check the first page of the article)? The citation is not clear as tyersome already mentioned. $\endgroup$ – WYSIWYG Feb 9 at 14:23
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Following from Matthew Martin's citations, we can infer what kind of temperature management the "biological thermal windows" are for, as they travel through 4-5 inches of fat and leather:

If the hippos dwell in 25'C to 30'C water for much of the day and night, they can waste energy warming up, energy which can be spent on growing, rearing young, defending territory.

As changes in vascular perfusion are not energetically demanding, thermal windows broaden the temperature range in which endothermic organisms can regulate body temperature with minimum costs (the thermoneutral zone), particularly by extending its upper limit. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4843926/

Hippo's have spent millions of years as a water dwelling clade, their closest living relatives being dolphines, whales and porpoises. So the hippos are equipped with radiator-valves can let them stay warm in water for many hours and stay cool on on the riverbank.

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